Determine the mass percentage of the elements of a complex substance. Mass fraction of a chemical element in a complex substance

Knowing the chemical formula, you can calculate the mass fraction of chemical elements in a substance. element in substances is denoted by the Greek. the letter "omega" - ω E / V and is calculated by the formula:

where k is the number of atoms of this element in the molecule.

What is the mass fraction of hydrogen and oxygen in water (H 2 O)?

Solution:

M r (H 2 O) \u003d 2 * A r (H) + 1 * A r (O) \u003d 2 * 1 + 1 * 16 \u003d 18

2) Calculate the mass fraction of hydrogen in water:

3) Calculate the mass fraction of oxygen in water. Since the composition of water includes atoms of only two chemical elements, the mass fraction of oxygen will be equal to:

Rice. 1. Formulation of the solution of problem 1

Calculate the mass fraction of elements in the substance H 3 PO 4.

1) Calculate the relative molecular weight of the substance:

M r (H 3 RO 4) \u003d 3 * A r (H) + 1 * A r (P) + 4 * A r (O) \u003d 3 * 1 + 1 * 31 + 4 * 16 \u003d 98

2) We calculate the mass fraction of hydrogen in the substance:

3) Calculate the mass fraction of phosphorus in the substance:

4) Calculate the mass fraction of oxygen in the substance:

1. Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. "Chemistry, Grade 8" / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. - M.: AST: Astrel, 2006.

2. Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M .: AST: Astrel: Profizdat, 2006. (p. 34-36)

3. Chemistry: 8th grade: textbook. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§15)

4. Encyclopedia for children. Volume 17. Chemistry / Chapter. edited by V.A. Volodin, leading. scientific ed. I. Leenson. - M.: Avanta +, 2003.

1. A single collection of digital educational resources ().

2. Electronic version of the journal "Chemistry and Life" ().

4. Video lesson on the topic "Mass fraction of a chemical element in a substance" ().

Homework

1. p.78 No. 2 from the textbook "Chemistry: 8th grade" (P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M .: AST: Astrel, 2005).

2. With. 34-36 №№ 3.5 from the Workbook in Chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.

You will need

• You need to determine which option your task belongs to. In the case of the first option, you will need a periodic table. In the case of the second, you need to know that the solution consists of two components: a solute and a solvent. And the mass of the solution is equal to the masses of these two components.

Instruction

In the case of the first version of the problem:
According to Mendeleev, we find the molar mass of a substance. The molar sum of the atomic masses that make up a substance.

For example, the molar mass (Mr) of calcium hydroxide Ca(OH)2: Mr(Ca(OH)2) = Ar(Ca) + (Ar(O) + Ar(H))*2 = 40 + (16 + 1) *2 = 74.

If there is no measuring vessel into which water can be poured, calculate the volume of the vessel in which it is located. The volume is always equal to the product of the area of ​​the base and the height, and there are usually no problems with vessels of a standing shape. Volume water in a jar will be equal to the area of ​​the round base to the height filled with water. Multiplying the density? per volume water V you will receive mass water m: m=?*V.

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note

You can determine the mass by knowing the amount of water and its molar mass. The molar mass of water is 18, since it consists of the molar masses of 2 hydrogen atoms and 1 oxygen atom. MH2O = 2MH+MO=2 1+16=18 (g/mol). m=n*M, where m is the mass of water, n is the quantity, M is the molar mass.

What is mass fraction element? From the name itself, you can understand that this is a value indicating the ratio of the mass element, which is part of the substance, and the total mass of this substance. It is expressed in fractions of a unit: percent (hundredths), ppm (thousandths), etc. How can you calculate the mass of a element?

Instruction

For clarity, consider carbon, well known to everyone, without which there would be no. If carbon is a substance (for example,), then its mass share can be safely taken as a unit or 100%. Of course, diamond also contains impurities of other elements, but in most cases, in such small quantities that they can be neglected. But in such modifications of carbon as or, the content of impurities is quite high, and neglect is unacceptable.

If carbon is part of a complex substance, you must proceed as follows: write down the exact formula of the substance, then, knowing the molar masses of each element included in its composition, calculate the exact molar mass of this substance (of course, taking into account the "index" of each element). After that, determine the mass share by dividing the total molar mass element on the molar mass of the substance.

For example, you need to find the mass share carbon in acetic acid. Write the formula for acetic acid: CH3COOH. To facilitate calculations, convert it to the form: C2H4O2. The molar mass of this substance is made up of the molar masses of the elements: 24 + 4 + 32 = 60. Accordingly, the mass fraction of carbon in this substance is calculated as follows: 24/60 = 0.4.

If you need to calculate it as a percentage, respectively, 0.4 * 100 = 40%. That is, each acetic acid contains (approximately) 400 grams of carbon.

Of course, the mass fractions of all other elements can be found in exactly the same way. For example, mass in the same acetic acid is calculated as follows: 32/60 \u003d 0.533 or approximately 53.3%; and the mass fraction of hydrogen is 4/60 = 0.666 or approximately 6.7%.

Sources:

• mass fractions of elements

The mass fraction of a substance shows its content in a more complex structure, for example, in an alloy or mixture. If the total mass of a mixture or alloy is known, then knowing the mass fractions of the constituent substances, one can find their masses. To find the mass fraction of a substance, you can know its mass and the mass of the entire mixture. This value can be expressed in fractional values ​​or percentages.

Task 3.1. Determine the mass of water in 250 g of a 10% sodium chloride solution.

Solution. From w \u003d m in-va / m solution find the mass of sodium chloride:
m in-va \u003d w m solution \u003d 0.1 250 g \u003d 25 g NaCl
Because the m r-ra = m in-va + m r-la, then we get:
m (H 2 0) \u003d m solution - m in-va \u003d 250 g - 25 g \u003d 225 g H 2 0.

Task 3.2. Determine the mass of hydrogen chloride in 400 ml of hydrochloric acid solution with a mass fraction of 0.262 and a density of 1.13 g/ml.

Solution. Because the w = m in-va / (V ρ), then we get:
m in-va \u003d w V ρ \u003d 0.262 400 ml 1.13 g / ml \u003d 118 g

Task 3.3. To 200 g of a 14% salt solution was added 80 g of water. Determine the mass fraction of salt in the resulting solution.

Solution. Find the mass of salt in the original solution:
m salt \u003d w m solution \u003d 0.14 200 g \u003d 28 g.
The same mass of salt remained in the new solution. Find the mass of the new solution:
m solution = 200 g + 80 g = 280 g.
Find the mass fraction of salt in the resulting solution:
w \u003d m salt / m solution \u003d 28 g / 280 g \u003d 0.100.

Task 3.4. What volume of a 78% sulfuric acid solution with a density of 1.70 g/ml should be taken to prepare 500 ml of a 12% sulfuric acid solution with a density of 1.08 g/ml?

Solution. For the first solution we have:
w 1 \u003d 0.78 And ρ 1 \u003d 1.70 g / ml.
For the second solution we have:
V 2 \u003d 500 ml, w 2 \u003d 0.12 And ρ 2 \u003d 1.08 g / ml.
Since the second solution is prepared from the first by adding water, the masses of the substance in both solutions are the same. Find the mass of the substance in the second solution. From w 2 \u003d m 2 / (V 2 ρ 2) we have:
m 2 \u003d w 2 V 2 ρ 2 \u003d 0.12 500 ml 1.08 g / ml \u003d 64.8 g.
m 2 \u003d 64.8 g. We find
the volume of the first solution. From w 1 = m 1 / (V 1 ρ 1) we have:
V 1 \u003d m 1 / (w 1 ρ 1) \u003d 64.8 g / (0.78 1.70 g / ml) \u003d 48.9 ml.

Task 3.5. What volume of a 4.65% sodium hydroxide solution with a density of 1.05 g/ml can be prepared from 50 ml of a 30% sodium hydroxide solution with a density of 1.33 g/ml?

Solution. For the first solution we have:
w 1 \u003d 0.0465 And ρ 1 \u003d 1.05 g / ml.
For the second solution we have:
V 2 \u003d 50 ml, w 2 \u003d 0.30 And ρ 2 \u003d 1.33 g / ml.
Since the first solution is prepared from the second by adding water, the masses of the substance in both solutions are the same. Find the mass of the substance in the second solution. From w 2 \u003d m 2 / (V 2 ρ 2) we have:
m 2 \u003d w 2 V 2 ρ 2 \u003d 0.30 50 ml 1.33 g / ml \u003d 19.95 g.
The mass of the substance in the first solution is also equal to m 2 \u003d 19.95 g.
Find the volume of the first solution. From w 1 = m 1 / (V 1 ρ 1) we have:
V 1 \u003d m 1 / (w 1 ρ 1) \u003d 19.95 g / (0.0465 1.05 g / ml) \u003d 409 ml.
Solubility coefficient (solubility) - the maximum mass of a substance soluble in 100 g of water at a given temperature. A saturated solution is a solution of a substance that is in equilibrium with the existing precipitate of that substance.

Problem 3.6. The solubility coefficient of potassium chlorate at 25 °C is 8.6 g. Determine the mass fraction of this salt in a saturated solution at 25 °C.

Solution. 8.6 g of salt dissolved in 100 g of water.
The mass of the solution is:
m solution \u003d m water + m salt \u003d 100 g + 8.6 g \u003d 108.6 g,
and the mass fraction of salt in the solution is equal to:
w \u003d m salt / m solution \u003d 8.6 g / 108.6 g \u003d 0.0792.

Problem 3.7. The mass fraction of salt in a potassium chloride solution saturated at 20 °C is 0.256. Determine the solubility of this salt in 100 g of water.

Solution. Let the solubility of the salt be X g in 100 g of water.
Then the mass of the solution is:
m solution = m water + m salt = (x + 100) g,
and the mass fraction is:
w \u003d m salt / m solution \u003d x / (100 + x) \u003d 0.256.
From here
x = 25.6 + 0.256x; 0.744x = 25.6; x = 34.4 g per 100 g of water.
Molar concentration With- the ratio of the amount of solute v (mol) to the volume of the solution V (in liters), c \u003d v (mol) / V (l), c \u003d m in-va / (M V (l)).
Molar concentration shows the number of moles of a substance in 1 liter of solution: if the solution is decimolar ( c = 0.1 M = 0.1 mol/l) means that 1 liter of the solution contains 0.1 mol of the substance.

Problem 3.8. Determine the mass of KOH required to prepare 4 liters of a 2 M solution.

Solution. For solutions with a molar concentration, we have:
c \u003d m / (M V),
Where With- molar concentration,
m- the mass of the substance,
M is the molar mass of the substance,
V- the volume of the solution in liters.
From here
m \u003d c M V (l) \u003d 2 mol / l 56 g / mol 4 l \u003d 448 g KOH.

Problem 3.9. How many ml of a 98% solution of H 2 SO 4 (ρ = 1.84 g / ml) must be taken to prepare 1500 ml of a 0.25 M solution?

Solution. The task of diluting the solution. For a concentrated solution we have:
w 1 \u003d m 1 / (V 1 (ml) ρ 1).
Find the volume of this solution V 1 (ml) \u003d m 1 / (w 1 ρ 1).
Since a dilute solution is prepared from a concentrated one by mixing the latter with water, the mass of the substance in these two solutions will be the same.
For a dilute solution we have:
c 2 \u003d m 2 / (M V 2 (l)) And m 2 \u003d s 2 M V 2 (l).
We substitute the found value of the mass into the expression for the volume of the concentrated solution and carry out the necessary calculations:
V 1 (ml) \u003d m / (w 1 ρ 1) \u003d (s 2 M V 2) / (w 1 ρ 1) \u003d (0.25 mol / l 98 g / mol 1.5 l) / (0, 98 1.84 g/ml) = 20.4 ml.

The mass fraction of a substance is the ratio of the mass of a certain substance to the mass of a mixture or solution in which this substance is located. It is expressed in fractions of a unit or as a percentage.

Instruction

1. The mass fraction of a substance is found by the formula: w \u003d m (c) / m (cm), where w is the mass fraction of the substance, m (c) is the mass of the substance, m (cm) is the mass of the mixture. If the substance is dissolved, then the formula looks like this: w \u003d m (c) / m (p-ra), where m (p-ra) is the mass of the solution. The mass of the solution, if necessary, can also be detected: m (p-ra) \u003d m (c) + m (p-la), where m (p-la) is the mass of the solvent. If desired, the mass fraction can be multiplied by 100%.

2. If the value of the mass is not given in the condition of the problem, then it can be calculated with the support of several formulas, the data in the condition will help to choose the appropriate one. The first formula for finding mass is: m = V*p, where m is mass, V is volume, p is density. The further formula looks like this: m = n * M, where m is the mass, n is the number of substance, M is the molar mass. The molar mass, in turn, is made up of the nuclear masses of the elements that make up the substance.

3. For a better understanding of this material, let's solve the problem. A mixture of copper and magnesium filings weighing 1.5 g was treated with an excess of sulfuric acid. As a result of the reaction, hydrogen was released in a volume of 0.56 l (typical data). Calculate the mass fraction of copper in the mixture. In this problem, a reaction takes place, we write down its equation. Of the 2 substances, only magnesium interacts with an excess of hydrochloric acid: Mg + 2HCl = MgCl2 + H2. In order to find the mass fraction of copper in the mixture, you need to substitute the values ​​\u200b\u200binto the following formula: w (Cu) \u003d m (Cu) / m (cm). The mass of the mixture is given, we find the mass of copper: m (Cu) \u003d m (cm) - m (Mg). We are looking for the mass of magnesium: m (Mg) \u003d n (Mg) * M (Mg). The reaction equation will help to find the number of magnesium substance. We find the number of hydrogen substance: n \u003d V / Vm \u003d 0.56 / 22.4 \u003d 0.025 mol. The equation shows that n(H2) = n(Mg) = 0.025 mol. We calculate the mass of magnesium, knowing that the molar mass of magnesium is 24 g / mol: m (Mg) \u003d 0.025 * 24 \u003d 0.6 g. We find the mass of copper: m (Cu) \u003d 1.5 - 0.6 \u003d 0.9 g It remains to calculate the mass fraction: w (Cu) \u003d 0.9 / 1.5 \u003d 0.6 or 60%.

Mass fraction shows as a percentage or in fractions the content of the substance in any solution or element in the composition of the substance. Knowing how to calculate the mass fraction is beneficial not only in chemistry lessons, but also when you want to prepare a solution or mixture, say, for culinary purposes. Or change the percentage, in the composition that you already have.

Instruction

1. The mass fraction is calculated as the ratio of the mass of a given component to the total mass of the solution. To acquire the total as a percentage, you need to multiply the resulting quotient by 100. The formula looks like this:? = m (solute) / m (solution)?,% =? * 100

2. Let's consider for example the direct and inverse problems. Let's say you dissolved 5 grams of table salt in 100 grams of water. What percentage solution did you get? The solution is very primitive. You know the mass of the substance (salt), the mass of the solution will be equal to the sum of the masses of water and salt. Thus, you should divide 5 g by 105 g and multiply the result of the division by 100 - this will be the result: you will get a 4.7% solution. Now the inverse problem. You want to prepare 200 grams of a 10% aqueous solution of what is desired. How much substance to take to dissolve? We act in the reverse order, we divide the mass fraction expressed as a percentage (10%) by 100. We get 0.1. Now let's make a simple equation, where we denote the required number of substances x and, consequently, the mass of the solution as 200 g + x. Our equation will look like this: 0.1=x/200g+x. When we solve it, we get that x is approximately 22.2 g. The result is checked by solving the direct problem.

3. It is more difficult to find out what numbers of solutions of a known percentage must be taken to acquire a certain number of solutions with new given qualities. Here it is required to compose and solve a system of equations. In this system, the first equation is an expression of the famous mass of the resulting mixture, in terms of two unfamiliar masses of the initial solutions. Say, if our goal is to get 150 g of a solution, the equation will look like x + y \u003d 150 g. The second equation is the mass of the solute equal to the sum of the same substance, as part of 2 miscible solutions. Say, if you want to have a 30% solution, and the solutions that you mix are 100%, that is, a pure substance, and 15%, then the second equation will look like: x + 0.15y \u003d 45 g. for a little, solve the equation system and find out how much substance needs to be added to a 15% solution in order to get a 30% solution. Try it.

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To calculate quantity substances, find out its mass with the help of scales, express it in grams and divide by the molar mass, which can be detected with the support of the periodic table. To determine the number substances gas under typical conditions, apply Avogadro's law. If the gas is in other conditions, measure the pressure, volume and temperature of the gas, then calculate quantity substances in him.

You will need

• You will need scales, a thermometer, a manometer, a ruler or tape measure, the periodic table of Mendeleev.

Instruction

1. Definition of a number substances in a solid or liquid. Find the mass of the investigated body with the help of scales, express it in grams. Determine from which substances the body is composed, then with the support of the periodic table, detect the molar mass substances. To do this, find the elements that make up the molecule substances of which the body is made. According to the table, determine their nuclear masses, if the table indicates a fractional number, round it up to a whole number. Find the sum of the masses of all the atoms in the molecule substances, get the molecular weight, which is numerically equal to the molar mass substances in grams per mole. After this, divide the previously measured mass by the molar mass. As a result, you will get quantity substances in moles (?=m/M).

2. Number substances gas under typical conditions. If the gas is in typical conditions (0 degrees Celsius and 760 mmHg), detect its volume. To do this, measure the volume of the room, cylinder or vessel where it is located, from the fact that the gas occupies each volume provided to it. In order to get its value, measure the geometric dimensions of the vessel, where it is located with the support of a tape measure and with the support of mathematical formulas, find its volume. A particularly classic case is the parallelepiped-shaped room. Measure its length, width and height in meters, then multiply them and get the volume of gas that is in it in cubic meters. To discover quantity substances gas, divide the resulting volume by the number 0.0224 - the molar volume of gas under typical conditions.

3. Number substances gas with arbitrary parameters. Measure the gas pressure with a pressure gauge in pascals, its temperature in kelvins, for which add the number 273 to the degrees Celsius in which the thermometer measures. Also determine the volume of gas in cubic meters. To discover quantity substances divide the product of pressure and volume by temperature and the number 8.31 (universal gas continuous), ? = PV / (RT).

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Many liquids are solutions. These are, in particular, human blood, tea, coffee, sea water. The basis of the solution is the solute. There are tasks to find the mass fraction of this substance.

Instruction

1. Solutions are called homogeneous homogeneous systems, which consist of 2 or more components. They are divided into three categories: - liquid solutions; - solid solutions; - gaseous solutions. Liquid solutions include, say, dilute sulfuric acid, solid solutions include an alloy of iron and copper, and gaseous solutions include any mixture of gases. Regardless of the state of aggregation of the solution, it consists of a solvent and a solute. The most common solvent is usually water, with which the substance is diluted. The composition of solutions is expressed in different ways, especially often the value of the mass fraction of the solute is used for this. The mass fraction is a dimensionless quantity, and it is equal to the ratio of the mass of the solute to the total mass of each solution:? in = m in / m Mass fraction is expressed as a percentage or decimal fractions. To calculate this parameter as a percentage, use the following formula: w (substances) \u003d mv / m (solution) 100%. To find the same parameter in the form of a decimal fraction, do not multiply by 100%.

2. The mass of each solution is the sum of the masses of water and solute. Consequently, occasionally the above formula is written in a slightly different way: solute is an acid. It follows from this that the mass of the solute is calculated as follows:? in \u003d mHNO3 / mHNO3 + mH2O

3. If the mass of the substance is unknown, and only the mass of water is given, then in this case the mass fraction is found according to a slightly different formula. When the volume of a solute is known, find its mass using the following formula: mv \u003d V *? From this it follows that the mass fraction of the substance is calculated as follows:? v \u003d V *? / V *?

4. Finding the mass fraction of a substance is repeatedly carried out for utilitarian purposes. Say, when bleaching some material, you need to know the concentration of perhydrol in a peroxide solution. In addition, the exact calculation of the mass fraction is occasionally required in medical practice. In addition to formulas and an approximate calculation of the mass fraction in medicine, they also use experimental verification with the help of instruments, which can reduce the likelihood of errors.

5. There are several physical processes during which the mass fraction of a substance and the composition of the solution change. The first of these, called evaporation, is a process inverse to the dissolution of a substance in water. In this case, the solute remains, and the water is completely evaporated. In this case, the mass fraction cannot be measured - there is no solution. The exact opposite process is the dilution of a concentrated solution. The more it is diluted, the more strongly the mass fraction of the substance dissolved in it decreases. Concentration is a partial evaporation, in which not all water evaporates, but only part of it. The mass fraction of the substance in the solution increases in this case.

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What is mass fraction element? From the name itself, it is possible to realize that this is a value indicating the ratio of the mass element, which is part of the substance, and the total mass of this substance. It is expressed in fractions of a unit: percent (hundredths), ppm (thousandths), etc. How is it possible to calculate the mass of any element ?

Instruction

1. For clarity, take a look at carbon, well-known to everyone, without which there would be no organics. If carbon is a pure substance (say, diamond), then its mass share it is allowed to bravely take it as a unit or for 100%. Of course, diamond also contains impurities of other elements, but in most cases, in such small numbers that they can be neglected. But in such modifications of carbon as coal or graphite, the content of impurities is quite high, and such ignoring is unacceptable.

2. If carbon is part of a difficult substance, you need to do it in the following way: write down the exact formula of the substance, after that, knowing the molar masses of any element included in its composition, calculate the exact molar mass of this substance (of course, taking into account the "index" of any element). Later this determine the mass share by dividing the total molar mass element on the molar mass of the substance.

3. Let's say we need to find a mass share carbon in acetic acid. Write the formula for acetic acid: CH3COOH. To simplify the calculations, convert it to the form: С2Н4О2. The molar mass of this substance is made up of the molar masses of the elements: 24 + 4 + 32 = 60. Accordingly, the mass fraction of carbon in this substance is calculated as follows: 24/60 = 0.4.

4. If you need to calculate it as a percentage, respectively, 0.4 * 100 = 40%. That is, every kilogram of acetic acid contains (approximately) 400 grams of carbon.

5. Of course, in exactly the same way it is possible to detect mass fractions of all other elements. Say, the mass fraction of oxygen in the same acetic acid is calculated as follows: 32/60 \u003d 0.533, or approximately 53.3%; and the mass fraction of hydrogen is 4/60 = 0.666 or approximately 6.7%.

6. To check the accuracy of the calculations, add up the percentages of all elements: 40% (carbon) + 53.3% (oxygen) + 6.7% (hydrogen) = 100%. The account settled.

You have a two hundred liter barrel. You plan to completely fill it with diesel fuel, which you use to heat your mini-boiler room. And how much will it weigh, filled with solarium? Now let's calculate.

You will need

• - table of specific density of substances;
• – knowledge to make the simplest mathematical calculations.

Instruction

1. In order to find the mass of a substance by its volume, use the formula for the specific density of a substance. p \u003d m / vhere p is the specific density of the substance; m is its mass; v is the volume occupied. We will consider the mass in grams, kilograms and tons. Volumes in cubic centimeters, decimeters and measures. And specific gravity, respectively, in g/cm3, kg/dm3, kg/m3, t/m3.

2. It turns out, according to the conditions of the problem, you have a two-hundred-liter barrel. This means: a barrel with a capacity of 2 m3. It is called a two-hundred-liter one, since water, with its specific gravity equal to one, enters 200 liters into such a barrel. You are concerned about the mass. Therefore, bring it to the first place in the presented formula. m \u003d p * v On the right side of the formula, the value of p is unfamiliar - the specific gravity of diesel fuel. Find it in the directory. Even easier is to search the Internet with a query “specific gravity of diesel fuel”.

3. Found: the density of summer diesel fuel at t = +200 C - 860 kg / m3. Substitute the values ​​​​in the formula: m = 860 * 2 = 1720 (kg) 1 ton and 720 kg - 200 liters of summer diesel fuel weigh so much. Having hung the barrel in advance, it is allowed to calculate the total weight and estimate the capacity of the rack under the barrel with a solarium.

4. In rural areas, it can be useful to pre-calculate the mass of firewood needed by cubic capacity in order to determine the carrying capacity of the transport on which this firewood will be delivered. For example, you need at least 15 cubic meters for the winter. meters of birch firewood. Look in the reference literature for the density of birch firewood. This is: 650 kg / m3. Calculate the mass by substituting the values ​​\u200b\u200binto the same specific density formula. m \u003d 650 * 15 \u003d 9750 (kg) Now, based on the carrying capacity and body capacity, you can decide on the type of vehicle and the number of trips.

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Note!
Older people are more familiar with the representation of specific gravity. The specific gravity of a substance is the same as specific gravity.

The mass fraction of a substance shows its table of contents in a more difficult structure, say, in an alloy or mixture. If the total mass of a mixture or alloy is known, then knowing the mass fractions of the constituent substances, it is possible to detect their masses. To detect the mass fraction of a substance, it is possible to know its mass and the mass of each mixture. This value can be expressed in fractional units or percentages.

You will need

• scales;
• periodic table of chemical elements;
• calculator.

Instruction

1. Determine the mass fraction of the substance that is in the mixture through the masses of the mixture and the substance itself. To do this, with the support of weights, determine the masses of substances that make up a mixture or alloy. Then fold them up. Take the resulting mass as 100%. To find the mass fraction of a substance in a mixture, divide its mass m by the mass of the mixture M, and multiply the result by 100% (?%=(m/M)?100%). Let's say that 20 g of table salt are dissolved in 140 g of water. In order to find the mass fraction of salt, add the masses of these 2 substances M=140+20=160 g. After that, find the mass fraction of the substance?%=(20/160)?100%=12.5%.

2. If you need to find the table of contents or the mass fraction of an element in a substance with a known formula, use the periodic table of chemical elements. Use it to find the nuclear masses of the elements that make up the substance. If one element occurs several times in the formula, multiply its nuclear mass by this number and add up the totals. This will be the molecular weight of the substance. In order to find the mass fraction of any element in such a substance, divide its mass number in the given chemical formula M0 by the molecular weight of the given substance M. Multiply the result by 100% (?%=(M0/M)?100%).

3. Say, determine the mass fraction of chemical elements in copper sulphate. Copper sulfate (copper II sulfate) has the chemical formula CuSO4. The nuclear masses of the elements included in its composition are equal to Ar(Cu)=64, Ar(S)=32, Ar(O)=16, the mass numbers of these elements will be equal to M0(Cu)=64, M0(S)=32, M0(O)=16?4=64, taking into account that the molecule contains 4 oxygen atoms. Calculate the molecular weight of a substance, it is equal to the sum of the mass numbers of the substances that make up the molecule 64+32+64=160. Determine the mass fraction of copper (Cu) in the composition of copper sulfate (?%=(64/160)?100%)=40%. According to the same thesis, it is possible to determine the mass fractions of all elements in this substance. Mass fraction of sulfur (S) ?%=(32/160)?100%=20%, oxygen (O) ?%=(64/160)?100%=40%. Please note that the sum of all mass fractions of the substance must be 100%.

Mass fraction is the percentage content of a component in a mixture or an element in a substance. Not only schoolchildren and students face the tasks of calculating the mass fraction. The knowledge to calculate the percentage concentration of a substance finds absolutely utilitarian use in real life - where the preparation of solutions is required - from construction to cooking.

You will need

• - Mendeleev table;
• - formulas for calculating the mass fraction.

Instruction

1. Calculate the mass share a-priory. Because the mass of a substance is made up of the masses of the elements that make it up, then share of any constituent element, a certain part of the mass of the substance is brought. The mass fraction of a solution is equal to the ratio of the mass of the solute to the mass of each solution.

2. The mass of the solution is equal to the sum of the masses of the solvent (traditionally water) and the substance. The mass fraction of the mixture is equal to the ratio of the mass of the substance to the mass of the mixture containing the substance. Multiply the result by 100%.

3. detect mass share output with the support of the formula?=md/mp, where mp and md are the value of the supposed and actual obtained yield of the substance (mass), respectively. Calculate the estimated mass from the reaction equation using the formula m=nM, where n is the chemical number of the substance, M is the molar mass of the substance (the sum of the nuclear masses of all elements included in the substance), or the formula m=V?, where V is the volume of the substance, ? is its density. In turn, if necessary, replace the number of substances with the formula n \u003d V / Vm, or also find it from the reaction equation.

4. Mass share calculate the element of a difficult substance using the periodic table. Add up the nuclear masses of all the elements that make up the substance, multiplying by the indices if necessary. You will get the molar mass of the substance. Find the molar mass of an element from the periodic table. Calculate the mass share by dividing the molar mass of the element by the molar mass of the substance. Multiply by 100%.

Helpful advice
Pay attention to the physical process, the one that takes place. When evaporating, do not calculate the mass fraction, because there is no solution (water or any other liquid). Do not forget that during concentration, on the contrary, called partial evaporation, the mass fraction of the substance increases. If you dilute a concentrated solution, the mass fraction decreases.

The mass fraction of any component in a substance shows which part of the total mass is brought to the atoms of this particular element. Using the chemical formula of a substance and the periodic table of Mendeleev, it is possible to determine the mass fraction of all of the elements included in the formula. The resulting value is expressed as an ordinary fraction or percentage.

Instruction

1. If you want to determine the mass fraction of any element that makes up a chemical formula, start by calculating the number of atoms that is brought to all of the elements. Let's say the chemical formula of ethanol is written like this: CH?-CH?-OH. And the chemical formula of dimethyl ether is CH?-O-CH?. The number of oxygen atoms (O) in any of the formulas is one, carbon (C) - two, hydrogen (H) - six. Note that these are different substances because the identical number of atoms of the entire element in their molecules are arranged differently. However, the mass fractions of the entire element in dimethyl ether and ethanol will be identical.

2. Using the periodic table, determine the nuclear mass of each element included in the chemical formula. Multiply this number by the number of atoms of each element calculated in the previous step. In the example used above, the formula contains one oxygen atom each, and its atomic mass from the table is 15.9994. There are two carbon atoms in the formula, its atomic mass is 12.0108, which means that the total weight of the atoms will be 12.0108*2=24.0216. For hydrogen these numbers are 6, 1.00795 and 1.00795*6=6.0477, respectively.

3. Determine the total atomic mass of the entire molecule of the substance - add the numbers obtained in the previous step. For dimethyl ether and ethanol, this value should be equal to 15.9994+24.0216+6.0477=46.0687.

4. If you want to get the total in fractions of a unit, make up an individual fraction for each element included in the formula. Its numerator should contain the value calculated for this element in the second step, and put the number from the third step in the denominator of the whole fraction. The resulting ordinary fraction can be rounded to the required degree of accuracy. In the example used above, the mass fraction of oxygen is 15.9994/46.0687?16/46=8/23, carbon is 24.0216/46.0687?24/46=12/23, hydrogen is 6.0477/46, 0687?6/46=3/23.

5. To get the total as a percentage, convert the resulting ordinary fractions to decimal format and increase by a hundred times. In the example used, the mass fraction of oxygen in percent is expressed by the number 8/23 * 100? 34.8%, carbon - 12/23 * 100? 52.2%, hydrogen - 3/23 * 100? 13.0%.

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Note!
The mass fraction cannot be greater than one or, if it is expressed as a percentage, greater than 100%.

Since the 17th century Chemistry is no longer a descriptive science. Chemists began to widely use methods for measuring various parameters of a substance. The design of balances was improved more and more, making it possible to determine the masses of samples for gaseous substances, in addition to mass, volume and pressure were also measured. The use of quantitative measurements made it possible to understand the essence of chemical transformations, to determine the composition of complex substances.

As you already know, the composition of a complex substance includes two or more chemical elements. Obviously, the mass of all matter is composed of the masses of its constituent elements. This means that each element accounts for a certain part of the mass of matter.

The mass fraction of an element in a substance is denoted by the Latin small letter w (double-ve) and shows the share (part of the mass) attributable to this element in the total mass of the substance. This value can be expressed in fractions of a unit or as a percentage (Fig. 69). Of course, the mass fraction of an element in a complex substance is always less than unity (or less than 100%). After all, a part of the whole is always less than the whole, just as a slice of an orange is less than an orange.

Rice. 69.
Elemental composition diagram of mercury oxide

For example, mercury oxide HgO contains two elements - mercury and oxygen. When 50 g of this substance is heated, 46.3 g of mercury and 3.7 g of oxygen are obtained. Calculate the mass fraction of mercury in a complex substance:

The mass fraction of oxygen in this substance can be calculated in two ways. By definition, the mass fraction of oxygen in mercury oxide is equal to the ratio of the mass of oxygen to the mass of mercury oxide:

Knowing that the sum of the mass fractions of elements in a substance is equal to one (100%), the mass fraction of oxygen can be calculated by the difference:

In order to find the mass fractions of elements by the proposed method, it is necessary to conduct a complex and time-consuming chemical experiment to determine the mass of each element. If the formula of a complex substance is known, the same problem is solved much easier.

To calculate the mass fraction of an element, multiply its relative atomic mass by the number of atoms of a given element in the formula and divide by the relative molecular mass of the substance.

For example, for water (Fig. 70):

Let's practice in solving problems for calculating the mass fractions of elements in complex substances.

Task 1. Calculate the mass fractions of elements in ammonia, the formula of which is NH 3.

Task 2. Calculate the mass fractions of elements in sulfuric acid having the formula H 2 SO 4.

More often, chemists have to solve the inverse problem: to determine the formula of a complex substance by mass fractions of elements.

How such problems are solved, we will illustrate with one historical example.

Task 3. Two compounds of copper with oxygen (oxides) were isolated from natural minerals - tenorite and cuprite (Fig. 71). They differed from each other in color and mass fractions of elements. In black oxide (Fig. 72), isolated from tenorite, the mass fraction of copper was 80%, and the mass fraction of oxygen was 20%. In red copper oxide isolated from cuprite, the mass fractions of elements were 88.9% and 11.1%, respectively. What are the formulas for these complex substances? Let's solve these two simple problems.

Rice. 71. Mineral cuprite
Rice. 72. Black copper oxide isolated from tenorite mineral

3. The resulting ratio must be reduced to the values ​​of integers: after all, the indices in the formula, showing the number of atoms, cannot be fractional. To do this, the resulting numbers must be divided by the smaller of them (in our case, they are equal).

And now let's complicate the task a little.

Task 4. According to elemental analysis, calcined bitter salt has the following composition: mass fraction of magnesium 20.0%, mass fraction of sulfur - 26.7%, mass fraction of oxygen - 53.3%.

Questions and tasks

1. What is the mass fraction of an element in a compound called? How is this value calculated?
2. Calculate the mass fractions of elements in substances: a) carbon dioxide CO 2; b) calcium sulfide CaS; c) sodium nitrate NaNO 3; d) aluminum oxide A1 2 O 3.
3. In which of the nitrogen fertilizers is the mass fraction of the nitrogen nutrient the largest: a) ammonium chloride NH 4 C1; b) ammonium sulfate (NH 4) 2 SO 4; c) urea (NH 2) 2 CO?
4. In the mineral pyrite, 7 g of iron accounts for 8 g of sulfur. Calculate the mass fractions of each element in this substance and determine its formula.
5. The mass fraction of nitrogen in one of its oxides is 30.43%, and the mass fraction of oxygen is 69.57%. Determine the formula of the oxide.
6. In the Middle Ages, a substance called potash was extracted from the ashes of a fire and was used to make soap. The mass fractions of elements in this substance are: potassium - 56.6%, carbon - 8.7%, oxygen - 34.7%. Determine the formula for potash.