home Audience 1 Mass fraction of a chemical element in a substance formula. How to find mass fraction

Mass fraction of a chemical element in a substance formula. How to find mass fraction

Instructions

The mass fraction of a substance is found by the formula: w = m(in)/m(cm), where w is the mass fraction of the substance, m(in) is the mass of the substance, m(cm) is the mass of the mixture. If dissolved, then it looks like this: w = m(in)/m(solution), where m(solution) is the mass of the solution. If necessary, the mass of the solution can also be found: m(solution) = m(in) + m(solution), where m(solution) is the mass of the solvent. If desired, the mass fraction can be multiplied by 100%.

If the problem statement does not give a mass value, then it can be calculated using several formulas; the values given in the statement will help you choose the right one. The first formula for: m = V*p, where m is mass, V is volume, p is density. The following formula looks like this: m = n*M, where m is mass, n is the amount of substance, M is molar mass. The molar mass, in turn, consists of the atomic masses of the elements that make up the substance.

To better understand this material, let’s solve the problem. A mixture of copper and magnesium filings weighing 1.5 g was treated with excess . As a result of the reaction, hydrogen volume is 0.56 l (). Calculate the mass fraction of copper in the mixture.
In this problem, we write down its equation. Of the two substances with excess hydrochloric acid, only magnesium: Mg + 2HCl = MgCl2 + H2. To find the mass fraction of copper in the mixture, you need to substitute the values into the following formula: w(Cu) = m(Cu)/m(cm). The mass of the mixture is given, let’s find the mass of copper: m(Cu) = m(cm) – m(Mg). We are looking for mass: m(Mg) = n(Mg)*M(Mg). The reaction equation will help you find the amount of magnesium. We find the amount of hydrogen substance: n = V/Vm = 0.56/22.4 = 0.025 mol. The equation shows that n(H2) = n(Mg) = 0.025 mol. We calculate the mass of magnesium, knowing that the molar is 24 g/mol: m(Mg) = 0.025*24 = 0.6 g. Find the mass of copper: m(Cu) = 1.5 – 0.6 = 0.9 g. Remaining calculate the mass fraction: w(Cu) = 0.9/1.5 = 0.6 or 60%.

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note

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

Sources:

"Chemistry Manual", G.P. Khomchenko, 2005.
Calculation of the share of sales by region

The mass fraction shows, as a percentage or in fractions, the content of a substance in a solution or an element in the composition of a substance. The ability to calculate mass fraction is useful not only in chemistry lessons, but also when you want to prepare a solution or mixture, for example, for culinary purposes. Or change the percentage in your existing composition.

Instructions

For example, you need at least 15 cubic meters for the winter. meters of birch firewood.
Look for the density of birch firewood in the reference book. This is: 650 kg/m3.
Calculate the mass by substituting the values into the same specific gravity formula.

m = 650*15 = 9750 (kg)

Now, based on the carrying capacity and capacity of the body, 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 concept of specific gravity. The specific density of a substance is the same as specific gravity.

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, their masses can be found. You can find the mass fraction of a substance by knowing its mass and the mass of the entire mixture. This value can be expressed in fractions or percentages.

You will need

scales;
periodic table of chemical elements;
calculator.

Instructions

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, use a scale to determine the masses that make up the mixture or. Then fold them. 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% (ω%=(m/M)∙100%). For example, 20 g of table salt is dissolved in 140 g of water. To find the mass fraction of salt, add the masses of these two substances M = 140 + 20 = 160 g. Then find the mass fraction of the substance ω% = (20/160)∙100% = 12.5%.

If you need to find the mass fraction of an element in a substance with a known formula, use the periodic table of elements. Using it, find the atomic masses of the elements that are in the substance. If one is in the formula several times, multiply its atomic mass by that number and add the results. This will be the molecular weight of the substance. To find the mass fraction of any element in such a substance, divide its mass number in a given chemical formula M0 by the molecular mass of a given substance M. Multiply the result by 100% (ω%=(M0/M)∙100%).

For example, determine the mass fraction of chemical elements in copper sulfate. Copper (copper II sulfate), has the chemical formula CuSO4. The atomic 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 atoms. Calculate the molecular mass of the 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 (ω%=(64/160)∙100%)=40%. Using the same principle, one can determine the mass fractions of all elements in this substance. Mass fraction of sulfur (S) ω%=(32/160)∙100%=20%, oxygen (O) ω%=(64/160)∙100%=40%. Please note that the sum of all mass fractions of the substance must be 100%.

Instructions

Determine the chemical form of the substance whose mass fractions of elements need to be found. Take the periodic table of Mendeleev and find in it the cells of elements corresponding to the atoms that make up the molecule of a given substance. In the cell, find the mass number of each element. If the found value of the mass number element fractional, round it to the nearest .

In the case where atoms of the same type occur several times in a molecule, multiply their atomic mass by this number. Add the masses of all the elements that make up the molecule to get the value in atomic mass units. For example, if you need to find the mass of a salt molecule, which is sulfate (Na2SO4), determines the atomic mass of sodium Ar(Na) = 23, sulfur Ar(S) = 32 and Ar(O) = 16. Since the molecule contains 2 sodium, take the value 23*2=46 for it, and 16*4=64, which has 4 atoms. Then the mass of the molecule of sodium sulfate will be Mr(Na2SO4)=46+32+64=142.

To calculate the mass fractions of the elements that make up the molecule of a given substance, find the ratio of the masses of the atoms included in the molecule of the substance to the mass of the molecule, and multiply the result by 100%. For example, if we consider sodium sulfate Na2SO4, calculate the mass fractions of its elements in this way: - the mass fraction of sodium will be ω(Na)= 23 2 100%/142=32.4%;
- the mass fraction of sulfur will be ω(S)= 32 100%/142=22.5%;
- the mass fraction of oxygen will be ω(O)= 16 4 100%/142=45.1%.

Mass fractions show the relative elements in a given molecule of a substance. Check the correctness of the calculation by adding the mass fractions of the substance. Their sum should be 100%. In the example under consideration, 32.4%+22.5%+45.1%=100%, the calculation is made.

It is perhaps impossible to find an element as essential for life as oxygen. If a person can live without food for several weeks, without water for several days, then without oxygen - only a few minutes. This substance is widely used in various fields of industry, including the chemical industry, and also as a component of rocket fuel (oxidizer).

Instructions

Often there is a need to determine the mass of oxygen located in some closed volume, or as a result of a chemical reaction. For example: 20 grams of permanganate were subjected to thermal decomposition, the reaction was completed. How many grams of oxygen were released?

First of all, remember that potassium - aka - has the chemical formula KMnO4. When heated, it decomposes, forming potassium manganate - K2MnO4, the main one - MnO2, and O2. Having written the reaction equation and selected the coefficients, you get:

2KMnO4 = K2MnO4 + MnO2 + O2

Given that the approximate molecular weight of two molecules of potassium permanganate is 316, and the molecular weight of an oxygen molecule is, respectively, 32, by solving the proportion, calculate:

20 * 32 /316 = 2,02
That is, with the thermal decomposition of 20 grams of potassium permanganate, approximately 2.02 grams of oxygen are obtained. (Or rounded 2 grams).

Or, for example, it is necessary to determine the mass of oxygen located in a closed volume if its temperature and pressure are known. Here the universal Mendeleev-Clapeyron equation, or in other words the “equation of state of an ideal gas,” comes to the rescue. It looks like this:

PVm = MRT
P – gas pressure,

V is its volume,

m is its molar mass,

M – mass,

R – universal gas constant,

T – temperature.

You see that the required value, that is, the mass of gas (oxygen), after bringing all the initial data into one system of units (pressure - , temperature - in degrees Kelvin, etc.), can be easily calculated using the formula:

Of course, real oxygen is not the ideal gas to describe which this equation was introduced. But at pressure and temperature values close to , the deviations of the calculated values from the actual ones are so insignificant that they can be safely neglected.

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

Instructions

For clarity, consider the well-known carbon, without which there would be no . If carbon is a substance (for example), then its mass share can be safely taken as one 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 carbon modifications such as or, the impurity content 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 this, determine the mass share, dividing the total molar mass element per molar mass of the substance.

For example, you need to find a mass share carbon in acetic acid. Write the formula of acetic acid: CH3COOH. To make calculations easier, convert it to the form: C2H4O2. The molar mass of this substance is the sum 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 a completely similar way. For example, the mass in the same acetic acid is calculated as follows: 32/60 = 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

A chemical formula is a record made using generally accepted symbols that characterizes the composition of the molecule of a substance. For example, the formula of the well-known sulfuric acid is H2SO4. It can be easily seen that each sulfuric acid molecule contains two hydrogen atoms, four oxygen atoms and one atom. It must be understood that this is only an empirical formula; it characterizes the composition of the molecule, but not its “structure,” that is, the arrangement of atoms relative to each other.

You will need

- Mendeleev table.

Instructions

First, find out the elements that make up the substance and theirs. For example: what will be the nitric oxide level? Obviously, this molecule contains two elements: nitrogen and . Both of them are gases, that is, pronounced gases. So what valence do nitrogen and oxygen have in this compound?

Remember a very important rule: non-metals have higher and lower valencies. The highest corresponds to the group number (in this case, 6 for oxygen and 5 for nitrogen), and the lowest corresponds to the difference between 8 and the group number (that is, the lowest valence for nitrogen is 3, and for oxygen is 2). The only exception to this rule is fluorine, which in all its forms exhibits one valency equal to 1.

So what valency – higher or lower – do nitrogen and oxygen have? Another rule: in compounds of two elements, the one that is located to the right and higher in the Periodic Table exhibits the lowest valence. It is quite obvious that in your case it is oxygen. Therefore, in combination with nitrogen, oxygen has a valence of 2. Accordingly, nitrogen in this compound has a higher valence of 5.

Now remember valence itself: this is the ability of an atom of any element to attach to itself a certain number of atoms of another element. Each nitrogen atom in this compound has 5 oxygen atoms, and each oxygen atom has 2 nitrogen atoms. What is nitrogen? That is, what indices does each element have?

Another rule will help answer this question: the sum of the valences of the elements included in the compound must be equal! What is the least common multiple of the numbers 2 and 5? Naturally, 10! By dividing it into the valence values of nitrogen and oxygen, you will find the indices and the final formula compounds: N2O5.

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You will need

scales;
periodic table of chemical elements;
calculator.

Instructions

Knowing the chemical formula, you can calculate the mass fraction of chemical elements in a substance. element in substance is denoted in Greek. letter “omega” - ω E/V and is calculated using 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) = 2*A r (H) + 1*A r (O) = 2*1 + 1* 16 = 18

2) Calculate the mass fraction of hydrogen in water:

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

Rice. 1. Formulation of the solution to problem 1

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

1) Calculate the relative molecular mass of the substance:

M r (H 3 PO 4) = 3*A r (N) + 1*A r (P) + 4*A r (O) = 3*1 + 1* 31 +4*16 = 98

2) 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 problems and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry, 8th grade” / 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. 8th grade” / 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 education institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§15)

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

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

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

The concept of mass fraction of a substance reflects its content and concentration in a complex chemical structure, be it a mixture or an alloy. Knowing the total mass of an alloy or mixture, you 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: mass fraction of a substance mass of a substance / mass of the entire mixture.

Let's do a little experiment! To do this, we will need a 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. First, we put the substance itself on the scale. We obtained a mass of substance. 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. Let's divide the mass of cherry juice to the total mass of the mixture: 30/200=0.15 or 15%.

How to find the mass fraction of a solution

A solution to this problem may be needed when determining the concentration of food solutions (vinegar) or medications. The mass of a KOH solution, also known as potassium hydroxide, weighing 400 grams is given. KOH (mass of the substance itself) is 80 grams. It is necessary to find the mass fraction of bile in the resulting solution. Formula for finding the solution: KOH (mass of potassium hydroxide solution) 300 g, mass of dissolved substance (KOH) 40 g. Find KOH (mass fraction of alkali) in the resulting solution, t-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 look for substances in the table. The table shows the atomic mass of the elements. 6 carbons with atomic mass 12 and 12 hydrogens with atomic mass equal to 1. m (C6H12) = 6 x 12 + 12 x 1 = 84 g/mol, ω (C) = 6 m1(C) / m (C6H12) = 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 and various chemical reactions are tested. Or they introduce litmus tests that can show the presence of one or another component. After determining the initial structure of the substance, the isolation of components can begin. This is achieved through simple chemical reactions, when one substance comes into contact with another and a new one is obtained, a precipitate is possible. There are also 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.