How to find the mass of an element in a substance. Calculation of the mass fraction of chemical elements according to the formula of a 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)?

Decision:

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.

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)?

Decision:

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.

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).

Even one gram of a substance can contain up to a thousand different compounds. Each compound is responsible for a certain property of a substance, and it happens that this is not a certain substance, but a mixture. In any case, the situation often arises in the production of chemical waste disposal and the task of using secondary raw materials. It is the chemical reactions that make it possible to find and isolate a certain substance are dominant. But for this you must first learn how to find the mass fraction.

The concept of the mass fraction of a substance reflects its content and concentration in a complex chemical structure, whether it be a mixture or an alloy. Knowing the total mass of an alloy or mixture, one can find the masses of their constituent substances, provided that their mass fractions are known. How to find the mass fraction, the formula is usually expressed as a fraction: the mass fraction of a substance is the mass of the substance / the mass of the entire mixture.

Let's do a little experiment! To do this, we need the periodic table of chemical elements. Mendeleev, scales and calculator.

How to find the mass fraction of a substance

It is necessary to determine the mass fraction of the substance, the substance is in the form of a mixture. Initially, we put the substance itself on the scales. Got a lot of stuff. Knowing a certain mass of a substance in a mixture, we can easily obtain its mass fraction. For example, there is 170g. water. They contain 30 grams of cherry juice. Total weight=170+30=230 grams. Divide the mass of cherry juice by the total mass of the mixture: 30/200=0.15 or 15%.

How to find the mass fraction of a solution

The solution to this problem may be needed when determining the concentration of food solutions (vinegar) or drugs. Given the mass of a solution of KOH, also known as potassium hydroxide, weighing 400 grams. KOH (the mass of the substance itself) is 80 grams. It is necessary to find the mass fraction of bile in the resulting solution. Solution formula: KOH (mass of potassium hydroxide solution) 300 g, mass of solute (KOH) 40 g. Find KOH (mass fraction of alkali) in the resulting solution, t is mass fraction. m- mass, t (substance) = 100% * m (substance) / m (solution (substance). Thus KOH (mass fraction of potassium hydroxide solution): t (KOH) = 80 g / 400 g x 100% = 20 %.

How to find the mass fraction of carbon in a hydrocarbon

To do this, we use the periodic table. We are looking for substances in the table. The table shows the atomic mass of the elements. 6 carbons with an atomic mass of 12 and 12 hydrogens with an atomic mass of 1. m (C6H12) \u003d 6 x 12 + 12 x 1 \u003d 84 g / mol, ω (C) \u003d 6 m1 (C) / m (C6H12) \u003d 6 x 12/84 = 85%

Determination of the mass fraction in production is carried out in special chemical laboratories. To begin with, a small sample is taken, on which various chemical reactions are tested. Or they introduce litmus tests, which can show the presence of a particular component. After clarifying the initial structure of the substance, you can begin to isolate the components. This is achieved through simple chemical reactions, when one substance comes into contact with another and a new one is obtained, precipitation is possible. There are more advanced methods, such as electrolysis, heating, cooling, evaporation. Such reactions require large industrial equipment. Production, of course, can hardly be called environmentally friendly, however, modern waste treatment technologies make it possible to minimize the burden on nature.

Fractions of solute
ω = m1 / m,
where m1 is the mass of the solute and m is the mass of the entire solution.

If the mass fraction of the solute is needed, multiply the resulting number by 100%:
ω \u003d m1 / m x 100%

In tasks where you need to calculate the mass fractions of each of the elements included in the chemical, use the table D.I. Mendeleev. For example, find out the mass fractions of each of the elements that make up the hydrocarbon, which C6H12

m (C6H12) \u003d 6 x 12 + 12 x 1 \u003d 84 g / mol
ω (C) \u003d 6 m1 (C) / m (C6H12) x 100% \u003d 6 x 12 g / 84 g / mol x 100% \u003d 85%
ω (H) \u003d 12 m1 (H) / m (C6H12) x 100% \u003d 12 x 1 g / 84 g / mol x 100% \u003d 15%

Helpful advice

Solve the problems of finding the mass fraction of a substance after evaporation, dilution, concentration, mixing of solutions using formulas obtained from the determination of the mass fraction. For example, the problem of evaporation can be solved using the following formula
ω 2 \u003d m1 / (m - Dm) \u003d (ω 1 m) / (m - Dm), where ω 2 is the mass fraction of the substance in one stripped off solution, Dm is the difference between the masses before and after heating.

Sources:

how to determine the mass fraction of a substance

There are situations when it is necessary to calculate mass liquids contained in any container. This can be during a training session in the laboratory, and in the course of solving a household problem, for example, when repairing or painting.

Instruction

The easiest method is to resort to weighing. First, weigh the container together with, then pour the liquid into another container that is suitable in size and weigh the empty container. And then it remains only to subtract the smaller value from the larger value, and you get. Of course, this method can be resorted to only when dealing with non-viscous liquids, which, after overflow, practically do not remain on the walls and bottom of the first container. That is, the quantity will remain then, but it will be so small that it can be neglected, it will hardly affect the accuracy of the calculations.

And if the liquid is viscous, for example,? How then her mass? In this case, you need to know its density (ρ) and occupied volume (V). And then everything is elementary. Mass (M) is calculated from M = ρV. Of course, before calculating it is necessary to convert the factors into a single system of units.

Density liquids can be found in a physical or chemical reference book. But it is better to use a measuring device - a density meter (densitometer). And the volume can be calculated, knowing the shape and overall dimensions of the container (if it has the correct geometric shape). For example, if the same glycerin is in a cylindrical barrel with a base diameter d and a height h, then the volume

Mass fraction is one of the important parameters that is actively used for calculations and not only in chemistry. Preparation of syrups and brines, calculation of fertilizer application per area for a particular crop, preparation and administration of medicines. All these calculations require a mass fraction. The formula for finding it will be given below.

In chemistry, it is calculated:

for a component of a mixture, solution;
for an integral part of a compound (chemical element);
for impurities in pure substances.

A solution is also a mixture, only homogeneous.

Mass fraction is the ratio of the mass of a mixture component (substance) to its entire mass. Expressed in ordinary numbers or as a percentage.

The formula for finding is:

𝑤 \u003d (m (component parts) m (mixtures, in-va)) / 100%.

Mass fraction of a chemical element in a substance is the ratio of the atomic mass of a chemical element, multiplied by the number of its atoms in this compound, to the molecular weight of the substance.

For example, to define w oxygen (oxygen) in a molecule of carbon dioxide CO2, first we find the molecular weight of the entire compound. It is 44. The molecule contains 2 oxygen atoms. Means w oxygen is calculated as follows:

w(O) = (Ar(O) 2) / Mr(CO2)) x 100%,

w(O) = ((16 2) / 44) x 100% = 72.73%.

Similarly, in chemistry one defines, for example, w water in a crystalline hydrate - a complex compound with water. In this form in nature many substances are found in minerals.

For example, the formula for copper sulphate is CuSO4 5H2O. To determine w water in this crystalline hydrate, you need to substitute in the already known formula, respectively, Mr water (in the numerator) and the total m crystalline hydrate (to the denominator). Mr water 18, and total crystalline hydrate - 250.

w(H2O) = ((18 5) / 250) 100% = 36%

Finding the mass fraction of a substance in mixtures and solutions

The mass fraction of a chemical compound in a mixture or solution is determined by the same formula, only the numerator will be the mass of the substance in the solution (mixture), and the denominator will be the mass of the entire solution (mixture):

𝑤 \u003d (m (in-va) m (r-ra)) / 100%.

Attention should be paid that mass concentration is the ratio of the mass of a substance to the mass whole solution and not just a solvent.

For example, 10 g of table salt is dissolved in 200 g of water. You need to find the percentage concentration of salt in the resulting solution.

To determine the salt concentration, we need m solution. It is:

m (solution) \u003d m (salt) + m (water) \u003d 10 + 200 \u003d 210 (g).

Find the mass fraction of salt in the solution:

𝑤 = (10 210) / 100% = 4.76%

Thus, the concentration of sodium chloride in the solution will be 4.76%.

If the condition of the problem does not m, and the volume of the solution, then it must be converted to mass. This is usually done through the formula for finding the density:

where m is the mass of the substance (solution, mixture), and V is its volume.

This concentration is used most often. It is she who is meant (if there are no separate indications) when they write about the percentage of substances in solutions and mixtures.

In problems, the concentration of impurities in a substance or a substance in its minerals is often given. It should be noted that the concentration (mass fraction) of a pure compound will be determined by subtracting the impurity fraction from 100%.

For example, if it is said that iron is obtained from a mineral, and the percentage of impurities is 80%, then pure iron in the mineral is 100 - 80 = 20%.

Accordingly, if it is written that the mineral contains only 20% iron, then it is these 20% that will participate in all chemical reactions and in chemical production.

for example, for the reaction with hydrochloric acid, 200 g of a natural mineral was taken, in which the zinc content is 5%. To determine the mass of zinc taken, we use the same formula:

𝑤 \u003d (m (in-va) m (r-ra)) / 100%,

from which we find the unknown m solution:

m (Zn) = (w 100%) / m (mineral)

m (Zn) \u003d (5 100) / 200 \u003d 10 (g)

That is, 200 g of the mineral taken for the reaction contains 5% zinc.

Task. A sample of copper ore weighing 150 g contains monovalent copper sulfide and impurities, the mass fraction of which is 15%. Calculate the Mass of Copper Sulfide in the Sample.

Decision tasks can be done in two ways. The first is to find the mass of impurities from a known concentration and subtract it from the total m ore sample. The second way is to find the mass fraction of pure sulfide and use it to calculate its mass. Let's solve it both ways.

I way

First we find m impurities in the ore sample. To do this, we use the already known formula:

𝑤 = (m (impurities) m (sample)) / 100%,

m(impurity) \u003d (w m (sample)) 100%, (A)

m (impurity) \u003d (15 150) / 100% \u003d 22.5 (g).

Now, by the difference, we find the amount of sulfide in the sample:

150 - 22.5 = 127.5 g

II way

First we find w connections:

100 — 15 = 85%

And now, using it, using the same formula as in the first method (formula A), we find m copper sulfide:

m(Cu2S) = (w m (sample)) / 100% ,

m(Cu2S) = (85 150) / 100% = 127.5 (g).

Answer: the mass of monovalent copper sulfide in the sample is 127.5 g.

Video

From the video you will learn how to correctly calculate chemical formulas and how to find the mass fraction.

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