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.

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 and 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.

From the course of chemistry it is known that the mass fraction is the content of a certain element in some substance. It would seem that such knowledge is of no use to an ordinary summer resident. But do not rush to close the page, as the ability to calculate the mass fraction for a gardener can be very useful. However, in order not to get confused, let's talk about everything in order.

What is the meaning of the concept of "mass fraction"?

The mass fraction is measured as a percentage or simply in tenths. A little higher, we talked about the classic definition, which can be found in reference books, encyclopedias or school chemistry textbooks. But to understand the essence of what has been said is not so simple. So, suppose we have 500 g of some complex substance. Complex in this case means that it is not homogeneous in composition. By and large, any substances that we use are complex, even simple table salt, the formula of which is NaCl, that is, it consists of sodium and chlorine molecules. If we continue the reasoning on the example of table salt, then we can assume that 500 grams of salt contains 400 grams of sodium. Then its mass fraction will be 80% or 0.8.

Why does a gardener need this?

I think you already know the answer to this question. The preparation of all kinds of solutions, mixtures, etc. is an integral part of the economic activity of any gardener. In the form of solutions, fertilizers, various nutrient mixtures, as well as other preparations are used, for example, growth stimulants "Epin", "Kornevin", etc. In addition, it is often necessary to mix dry substances, such as cement, sand and other components, or ordinary garden soil with purchased substrate. At the same time, the recommended concentration of these agents and preparations in prepared solutions or mixtures in most instructions is given in mass fractions.

Thus, knowing how to calculate the mass fraction of an element in a substance will help the summer resident to properly prepare the necessary solution of fertilizer or nutrient mixture, and this, in turn, will necessarily affect the future harvest.

Calculation algorithm

So, the mass fraction of an individual component is the ratio of its mass to the total mass of a solution or substance. If the result obtained needs to be converted into a percentage, then it must be multiplied by 100. Thus, the formula for calculating the mass fraction can be written as follows:

W = Mass of substance / Mass of solution

W = (Mass of substance / Mass of solution) x 100%.

An example of determining the mass fraction

Let's assume that we have a solution, for the preparation of which 5 g of NaCl was added to 100 ml of water, and now it is necessary to calculate the concentration of table salt, that is, its mass fraction. We know the mass of the substance, and the mass of the resulting solution is the sum of two masses - salt and water and equals 105 g. Thus, we divide 5 g by 105 g, multiply the result by 100 and get the desired value of 4.7%. This is the concentration that the saline solution will have.

More practical task

In practice, the summer resident often has to deal with tasks of a different kind. For example, it is necessary to prepare an aqueous solution of a fertilizer, the concentration of which by weight should be 10%. In order to accurately observe the recommended proportions, you need to determine what amount of the substance will be needed and in what volume of water it will need to be dissolved.

The solution of the problem starts in the reverse order. First, you should divide the mass fraction expressed as a percentage by 100. As a result, we get W \u003d 0.1 - this is the mass fraction of the substance in units. Now let's denote the amount of substance as x, and the final mass of the solution - M. In this case, the last value is made up of two terms - the mass of water and the mass of fertilizer. That is, M = Mv + x. Thus, we get a simple equation:

W = x / (Mv + x)

Solving it for x, we get:

x \u003d W x Mv / (1 - W)

Substituting the available data, we obtain the following dependence:

x \u003d 0.1 x Mv / 0.9

Thus, if we take 1 liter (that is, 1000 g) of water to prepare the solution, then approximately 111-112 g of fertilizer will be needed to prepare the solution of the desired concentration.

Solving problems with dilution or addition

Suppose we have 10 liters (10,000 g) of a ready-made aqueous solution with a concentration in it of a certain substance W1 = 30% or 0.3. How much water will need to be added to it so that the concentration drops to W2 = 15% or 0.15? In this case, the formula will help:

Mv \u003d (W1x M1 / ​​W2) - M1

Substituting the initial data, we get that the amount of added water should be:
Mv \u003d (0.3 x 10,000 / 0.15) - 10,000 \u003d 10,000 g

That is, you need to add the same 10 liters.

Now imagine the inverse problem - there are 10 liters of an aqueous solution (M1 = 10,000 g) with a concentration of W1 = 10% or 0.1. It is necessary to obtain a solution with a mass fraction of fertilizer W2 = 20% or 0.2. How much starting material should be added? To do this, you need to use the formula:

x \u003d M1 x (W2 - W1) / (1 - W2)

Substituting the original value, we get x \u003d 1 125 g.

Thus, knowledge of the simplest basics of school chemistry will help the gardener to properly prepare fertilizer solutions, nutrient substrates from several elements, or mixtures for construction work.

Chemistry is definitely an interesting science. Despite all its complexity, it allows us to better understand the nature of the world around us. And what's more - at least elementary knowledge in this subject seriously helps in everyday life. For example, the determination of the mass fraction of a substance in a multicomponent system, that is, the ratio of the mass of any component to the total mass of the entire mixture.

Necessary:

- calculator;
- scales (if you first need to determine the masses of all components of the mixture);
is Mendeleev's periodic system of elements.

Instructions:

• So, it became necessary for you to determine the mass fraction of a substance. Where to begin? First of all, it depends on the specific task and on the tools at hand for the job. But in any case, in order to determine the content of a component in a mixture, you need to know its mass and the total mass of the mixture. You can do this either on the basis of known data, or on the basis of your own research. To do this, you will need to weigh the added component on a laboratory scale. After the mixture is ready, weigh it also.
• Write the mass of the desired substance as " m«, total mass systems put under the designation " M". In this case, the formula for the mass fraction of a substance will take the following form: W=(m/M)*100. The result obtained is recorded as a percentage.
• Example: calculate the mass fraction of 15 grams of salt dissolved in 115 grams of water. Solution: the total mass of the solution is determined by the formula M=m to +m c, where m in- mass of water mc- mass of salt. From simple calculations, it can be determined that the total mass of the solution is 130 grams. According to the above definition formula, we obtain that the content of table salt in the solution will be equal to W=(15/130)*100=12%.
• A more particular situation is the need to define mass fraction of a chemical element in a substance . It is defined exactly the same. The main calculation principle will remain the same, only instead of the mass of the mixture and the specific component, you will have to deal with the molecular weights of chemical elements.
• All the necessary information can be found in the periodic system of Mendeleev. Break down the chemical formula of a substance into its main components. Using the periodic table, determine the mass of each element. Summing them up, get the molecular weight of your substance ( M). Similarly to the previous case, the mass fraction of a substance, and to be more precise, an element will be determined by the ratio of its mass to the molecular mass. The formula will take the following form W=(m a /M)*100. Where m a is the atomic mass of the element, M is the molecular weight of the substance.
• Let's consider this case with a specific example. Example: determine the mass fraction of potassium in potash. Potash is potassium carbonate. Its formula K2CO3. The atomic mass of potassium is 39 , carbon - 12 , oxygen - 16 . The molecular weight of carbonate will be determined as follows - M \u003d 2m K + m C + 2m O \u003d 2 * 39 + 12 + 2 * 16 \u003d 122. A potassium carbonate molecule contains two potassium atoms with an atomic mass equal to 39 . The mass fraction of potassium in the substance will be determined by the formula W \u003d (2m K / M) * 100 \u003d (2 * 39 / 122) * 100 \u003d 63.93%.

Mass fraction of an element in substance- This is one of the topics that is included in the course of chemistry. The skills and abilities to determine this parameter can be useful when testing knowledge during control and independent work, as well as at the exam in chemistry.

You will need

• - periodic system of chemical elements D.I. Mendeleev

Instruction

• In order to calculate the mass share, you must first find the relative atomic mass (Ar) of the desired element, as well as the relative molecular mass (Mr) of the substance. Next, apply the formula by which the mass fraction of the element is determined (W) W \u003d Ar (x) / Mr x 100%, in which W is the mass fraction of the element (measured in fractions or%); Ar (x) is the relative atomic mass of the element; Mr is the relative molecular mass of a substance. To determine the relative atomic and molecular mass, use the periodic system of chemical elements of D.I. Mendeleev. When calculating, be sure to take into account the number of atoms of each element.
• Example #1: Determine the Mass share hydrogen in water. Find according to the table D.I. Mendeleev relative atomic mass of hydrogen Ar (H) = 1. Since there are 2 hydrogen atoms in the formula, therefore, 2Ar (H) = 1 x 2 = 2 Calculate the relative molecular mass of water (H2O), which is made up of 2 Ar (H) and 1 Ar (O). Mr (H2O) \u003d 2Ar (H) + Ar (O)Ar (O) \u003d 16, therefore Mr (H2O) \u003d 1 x 2 + 16 \u003d 18
• Write down the general formula for determining the mass fraction of an element W \u003d Ar (x) / Mr x 100% Now write down the formula, in relation to the condition of the problem W (H) \u003d 2 Ar (H) / Mr (H2O) x 100% Make calculations W (H) \u003d 2 / 18 x 100% = 11.1%
• Example #2: Determine the Mass share oxygen in copper sulfate (CuSO4). Find according to the table D.I. Mendeleev, the relative atomic mass of oxygen Ar (O) \u003d 16. Since there are 4 oxygen atoms in the formula, therefore, 4 Ar (O) \u003d 4 x 16 \u003d 64 Calculate the relative molecular weight of copper sulfate (CuSO4), which consists of 1 Ar (Cu), 1 Ar (S) and 4 Ar (O).Mr (CuSO4) = Ar (Cu) + Ar (S) + 4 Ar (O).Ar (Cu) = 64 Ar (S) = 324 Ar (O) = 4 x 16 \u003d 64, therefore Mr (CuSO4) \u003d 64 + 32 + 64 \u003d 160
• Write down the general formula for determining the mass fraction of an element W \u003d Ar (x) / Mr x 100% Now write down the formula, in relation to the condition of the problem W (O) \u003d 4 Ar (O) / Mr (CuSO4) x 100% Make calculations W (O) \u003d 64 / 160 x 100% = 40%