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How to find out relative molecular weight. How to find molar mass

The relative molecular weight of a substance shows how many times a molecule of a given substance is heavier than 1/12 of a pure carbon atom. It can be found if its chemical formula is known using Mendeleev's periodic table of elements. Otherwise, use other methods to find molecular mass, keeping in mind that it is numerically equal to the molar mass of the substance expressed in grams per mole.

You will need

  • - periodic table of chemical elements;
  • - sealed container;
  • - scales;
  • - pressure gauge;
  • - thermometer.

Instructions

  • If the chemical formula of a substance is known, determine its molecular mass using Mendeleev’s periodic table of chemical elements. To do this, determine the elements that are included in the formula of the substance. Then, find their relative atomic masses, which are written in the table. If the atomic mass in the table is presented fractional number, round it to the nearest integer. If a chemical formula contains several atoms of a given element, multiply the mass of one atom by their number. Add the resulting atomic masses and get the relative molecular mass of the substance.
  • For example, to find the molecular weight of sulfuric acid H2SO4, find the relative atomic masses of the elements that are included in the formula, respectively, hydrogen, sulfur and oxygen Ar(H)=1, Ar(S)=32, Ar(O)=16. Considering that there are 2 atoms of hydrogen in a molecule, and 4 atoms of oxygen, calculate the molecular mass of the substance Mr(H2SO4)=2 1+32+4∙16=98 atomic mass units.
  • If you know the amount of a substance in moles ν and the mass of the substance m, expressed in grams, determine its molar mass; for this, divide the mass by the amount of the substance M=m/ν. It will be numerically equal to its relative molecular weight.
  • If the number of molecules of a substance N and mass m are known, find its molar mass. It will be equal to the molecular mass by finding the ratio of the mass in grams to the number of molecules of the substance in this mass, and multiply the result by Avogadro’s constant NА=6.022^23 1/mol (M=m∙N/NA).
  • To find the molecular mass of an unknown gas, find its mass in a sealed container of known volume. To do this, pump the gas out of it, creating a vacuum there. Weigh the cylinder. Then pump the gas back in and find its mass again. The difference in mass of the empty and inflated cylinder will be equal to the mass of the gas. Measure the pressure inside the cylinder using a pressure gauge in Pascals and the temperature in Kelvin. To do this, measure the temperature of the surrounding air, it will be equal to the temperature inside the cylinder in degrees Celsius, to convert it to Kelvin, add 273 to the resulting value. Determine the molar mass of the gas by finding the product of temperature T, gas mass m and the universal gas constant R (8, 31). Divide the resulting number by the values ​​of pressure P and volume V, measured in m³ (M=m 8.31 T/(P V)). This number will correspond to the molecular weight of the gas being tested.

Instructions

If you carefully examine the table of Dmitry Ivanovich Mendeleev, you can see that it has the appearance of an apartment high-rise building in which there are “tenants” - elements. Each of them has a surname () and a chemical name. Moreover, each of the elements lives in its own apartment, and therefore has. This information is presented in all cells of the table.

However, there is another figure there, which at first glance is completely incomprehensible. Moreover, it is indicated with several values ​​after the decimal point, which is done for greater accuracy. It is this number that you need to pay attention to, because this is the relative atomic mass. Moreover, this is a constant value that does not need to be memorized and can be found in the table. By the way, even on the Unified State Examination according to D.I. Mendeleev is reference material, available for use, and everyone has it in an individual package - KIM.

The molecular mass, or rather the relative mass of a substance, is denoted by the letters (Mr) and is made up of the relative atomic masses (Ar) of the elements that form the molecule. Relative atomic mass is exactly that mysterious number that appears in every cell of the table. For calculations, these values ​​must be rounded to a whole number. The only exception is the chlorine atom, whose relative atomic mass is 35.5. This characteristic has no units of measurement.

Example 1. Find the molecular mass(KOH)
A potassium hydroxide molecule consists of one potassium atom (K), one oxygen atom (O), and one hydrogen atom (H). Therefore, we find:
Mr (KOH) = Ar (K) + Ar (O) + Ar (H)


Hence: Mr (KOH) = 39 + 16 + 1 = 56

Example 2. Find the molecular mass sulfuric acid (H2SO4 ash-two-es-o-four)
A sulfuric acid molecule consists of two hydrogen atoms (H), one sulfur atom (S) and four oxygen atoms (O). Therefore, we find:
Mr (H2SO4) = 2Ar (H) + Ar (S) + 4Ar (O)
According to the table D.I. Mendeleev we find the values ​​of the relative atomic masses of elements:
Ar (K) = 39, Ar (O) = 16, Ar (H) = 1
Hence: Mr (H2SO4) = 2 x 2 + 32 + 4 x 16 = 98

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note

When performing calculations, multiplication or division is performed first, and only then addition or subtraction.

Helpful advice

When determining relative atomic mass, round off the values ​​found in the D.I. table. Mendeleev to an integer

Sources:

  • how to calculate molecular weight
  • Molecular mass determination

To find the molecular mass, find the molar mass substances in grams per mole, since these quantities are numerically equal. Or find mass particles of a molecule in atomic mass units, add their values ​​and get the molecular mass. To find the molecular mass of a gas, you can use the Clapeyron-Mendeleev equation.

You will need

  • For calculations you will need Mendeleev's periodic table, scales, thermometer, pressure gauge.

Instructions

Calculation using the periodic table. Determine the chemical formula of the substance under study. In the periodic table, find the chemical elements that make up the molecule. In the appropriate cells, find their atomic mass. If the table is a fraction, round it to the nearest whole number. If the same element occurs several times in a molecule, multiply it mass by the number of occurrences. Add up all the atoms. The result will be substances.

Calculation of molecular weight when converting from grams. If given the mass of one molecule in grams, multiply it by Avogadro's constant, which is 6.022 10^(23) 1/mol. The result will be substances in grams per mole. Its numerical value coincides with the molecular mass in atomic mass units.

Calculation of the molecular mass of an arbitrary gas. Take a cylinder of known volume measured in cubic meters, pump out the air from it and weigh it on a scale. Then pump gas into it, molecular mass which needs to be determined. Find again mass balloon. The difference between a gas cylinder and an empty cylinder will be equal to the mass of the gas, enter in grams. Measure the pressure with a pressure gauge (in) and the temperature with a thermometer, turning it to . To do this, add the number 273 to the degrees Celsius obtained as a result of the measurement. To find the molar mass gas, its mass multiply by temperature and the number 8.31 (universal gas constant). Sequentially divide the obtained result by the value of the gas pressure and its volume M=m 8.31 T/(P V). This measure, expressed in grams per mole, is the numerical molecular mass of a gas expressed in atomic mass units.

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Sources:

  • molecular weight calculation

The relative molecular mass of a substance (or simply molecular mass) is the ratio of the mass of a given substance to 1/12 of the mass of one carbon atom (C). Find the relative molecular mass mass very easy.

You will need

  • Periodic table and molecular weight table

Instructions

The relative mass of a substance is the sum of its atomic masses. In order to learn atomic mass one way or another, just look at the periodic table. It can be found on the cover of any book, or purchased separately at a bookstore. A pocket version or an A4 sheet is quite suitable for this. Any modern chemistry equipped with a full-scale wall-mounted periodic table.

Having learned atomic mass element, you can begin to calculate the molecular mass of the substance. This is easiest to show with an example:
It is required to calculate the molecular mass water (H2O). From the molecular formula it is clear that a water molecule consists of two H atoms and one O atom. Therefore, the calculation of the molecular mass of water can be reduced to the action:
1.008*2 + 16 = 18.016

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note

Atomic mass as a concept appeared in 1803, thanks to the works of the then famous chemist John Dalton. In those days, the mass of any atom was compared with the mass of a hydrogen atom. This concept was further developed in the works of another chemist, Berzelius, in 1818, when he proposed using an oxygen atom instead of a hydrogen atom. Since 1961, chemists in all countries have adopted the mass of 1/16 of an oxygen atom or the mass of 1/12 of a carbon atom as a unit of atomic mass. The latter is precisely indicated in the periodic table of chemical elements.

Helpful advice

When using the periodic table in the form in which it is presented in most chemistry textbooks and other reference books, one must understand that this table is a shortened version of the original periodic table. In its most complete version, a separate line is devoted to each chemical element.

The molecular mass of a substance means the total atomic mass of all chemical elements that are part of this substance. To calculate the molecular mass substances, no special effort is required.

You will need

  • Mendeleev table.

Instructions

Now you need to take a closer look at any of the elements in this table. Under the name of any of the elements indicated in the table there is numeric value. This is precisely the atomic mass of this element.

Now it is worth looking at several examples of molecular mass calculations, based on the fact that atomic masses are now known. For example, you can calculate the molecular weight of a substance such as water (H2O). A water molecule contains one oxygen atom (O) and two hydrogen atoms (H). Then, having found the atomic masses of hydrogen and oxygen using the periodic table, we can begin to calculate the molecular mass:2*1.0008 (after all, there are two hydrogens) + 15.999 = 18.0006 amu (atomic mass units).

Another . The next substance, molecular mass which can be calculated, let it be ordinary table salt (NaCl). As can be seen from the molecular formula, a molecule of table salt contains one Na atom and one Cl atom. In this case, it is calculated as follows: 22.99 + 35.453 = 58.443 a.m.u.

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note

I would like to note that the atomic masses of isotopes of various substances differ from the atomic masses in the periodic table. This is due to the fact that the number of neutrons in the nucleus of an atom and inside an isotope of the same substance is different, therefore the atomic masses are also noticeably different. Therefore, it is customary to denote isotopes of various elements by the letter of the given element, adding its mass number in the upper left corner. An example of an isotope is deuterium (“heavy hydrogen”), the atomic mass of which is not one, like an ordinary atom, but two.

Molar is weight one mole of a substance, that is, an amount that contains the same number of atoms as 12 grams of carbon. In another way, such a quantity is called Avogadro's number (or constant), in honor of the Italian scientist who first put forward the hypothesis. According to it, in equal volumes ideal gases(at the same temperatures and pressures) must contain the same number of molecules.

We must firmly remember that one mole of any substance is approximately 6.022 * 1023 molecules (either atoms or ions) of this substance. Consequently, any quantity of any substance can be represented by elementary calculations in the form of a certain number of moles. Why was mola introduced at all? To make calculations easier. After all, the number of elementary (molecules, atoms, ions) even in the smallest sample of matter is simply colossal! Agree, it is much more convenient to express the amount of substances in moles rather than in huge ones with endless rows of zeros! Molar weight a substance is determined by adding the molar masses of all elements included in it, taking into account the indices. For example, you need to determine the molar mass of anhydrous sodium sulfate. First of all, write its chemical formula: Na2SO4. Do the calculations: 23*2 + 32 + 16*4 = 142 grams/mol. This will be the molar weight this salt. What if you need to determine the molar mass of a simple substance? The rule is absolutely the same. For example, molar weight oxygen O2 = 16*2 = 32 grams/mol, molar weight N2 = 14*2 = 28 grams/mol, etc. It is even easier to determine the molar mass of a molecule whose molecule consists of one atom. For example, molar weight sodium is 23/mol, silver is 108 grams/mol, etc. Of course, rounded values ​​are used here to simplify calculations. If there is greater accuracy, it is necessary for the same sodium to consider its relative atomic mass to be not 23, but 22.98. We must also remember that the molar mass of a substance depends on its quantitative and qualitative composition. Therefore, different substances with the same number of moles have different molar masses.

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Tip 6: How to Determine Relative Molecular Weight

The relative molecular mass of a substance is a value that shows how many times the mass of one molecule of a given substance is greater than 1/12 of the mass of the carbon isotope. In another way, it can be simply called molecular weight. How can you find the relative molecular mass?

You will need

  • Mendeleev table.

Instructions

All you need for this is the Periodic Table and basic ability to do calculations. After all, relative molecular weight is the sum of the atomic masses of the elements that make up the composition of which you are interested. Of course, taking into account the indices of each element. The atomic mass of each element is indicated in the Periodic Table along with other important information, and with very high accuracy. Rounded values ​​are also quite suitable for these purposes.

Now take the Periodic Table and determine the atomic masses of each element included in its composition. There are three such elements: , sulfur, . Atomic mass (H) = 1, atomic mass of sulfur (S) = 32, atomic mass of oxygen (O) = 16. Taking into account the indices, sum up: 2 + 32 + 64 = 98. This is exactly the relative molecular mass of sulfuric acid. Please note that these are approximate, rounded results. If for some reason accuracy is required, then you will have to take into account that the atomic mass of sulfur is not exactly 32, but 32.06, hydrogen is not exactly 1, but 1.008, etc.

note

If you don’t have the Periodic Table at hand, find out the relative molecular mass of a particular substance using chemistry reference books.

Helpful advice

The mass of a substance in grams, which is numerically equal to its relative molecular mass, is called a mole.

The relative molecular weight of a substance shows how many times a molecule of a given substance is heavier than 1/12 of a pure carbon atom. It can be found if its chemical formula is known using Mendeleev's periodic table of elements. Otherwise, use other methods to find molecular mass, keeping in mind that it is numerically equal to the molar mass of the substance expressed in grams per mole.

You will need

  • - periodic table of chemical elements;
  • - sealed container;
  • - scales;
  • - pressure gauge;
  • - thermometer.

Instructions

If a substance is known, determine its molecular mass using Mendeleev’s periodic table of chemical elements. To do this, determine the elements that are in the formula of the substance. Then, find their relative atomic masses, which are written in the table. If the atomic mass in the table is represented as a fraction, round it to the nearest whole number. If it contains several atoms of a given element, multiply the mass of one atom by their number. Add the resulting atomic masses and get the relative molecular mass of the substance.

For example, to find the molecular weight of sulfuric H2SO4, find the relative atomic masses of the elements that are included in the formula, respectively, sulfur and oxygen Ar(H)=1, Ar(S)=32, Ar(O)=16. Considering that there are 2 atoms of hydrogen in a molecule, and 4 atoms of oxygen, calculate the molecular mass of the substance Mr(H2SO4)=2 1+32+4∙16=98 atomic mass units.

If you know the amount of a substance in moles ν and the mass of the substance m, expressed in grams, determine its molar mass; for this, divide the mass by the amount of the substance M=m/ν. It will be numerically equal to its relative molecular weight.

If the number of molecules of a substance N and mass m are known, find its molar mass. It will be equal to the molecular mass by finding the ratio of the mass in grams to the number of molecules of the substance in this mass, and multiply the result by Avogadro’s constant NА=6.022^23 1/mol (M=m∙N/NA).

To find the molecular mass of an unknown gas, find its mass in a sealed container of known volume. To do this, pump the gas out of it, creating a vacuum there. Weigh it. Then pump the gas back in and find its mass again. The difference in mass of the empty and inflated cylinder will be equal to the mass of the gas. Measure the pressure inside the cylinder using a pressure gauge in Pascals and in Kelvins. To do this, measure the ambient air temperature, it will be equal inside the cylinder in degrees Celsius, to convert it to Kelvin, add 273 to the resulting value.

Determine the molar mass of the gas by finding the product of temperature T, gas mass m and the universal gas constant R (8.31). Divide the resulting number by the values ​​of pressure P and volume V, measured in m³ (M=m 8.31 T/(P V)). This number will correspond to the molecular weight of the gas being tested.

Hydrogen is the first element of the periodic table and the most common in the Universe, since it is what stars are mainly made of. It is part of a substance vital for biological life - water. Hydrogen, like any other chemical element, has specific characteristics, including molar mass.

Instructions

Remember, molar mass? This is the mass of one mole, that is, the amount of it that contains approximately 6.022*10^23 elementary particles substances (atoms, molecules, ions). This number is called the “Avogadro number”, and is named after the famous scientist Amedeo Avogadro. The molar mass of a substance is numerically the same as its molecular mass, but has a different dimension: not atomic mass units (amu), but gram/mol. Knowing this, determine the molar mass hydrogen as easy as pie.

What kind of molecule does it have? hydrogen? It is diatomic, with the formula H2. Immediately: a molecule is considered consisting of two atoms of the lightest and most common hydrogen isotope, protium, and not of the heavier

Atoms and molecules are the smallest particles of matter, so you can choose the mass of one of the atoms as a unit of measurement and express the masses of other atoms in relation to the chosen one. So what is molar mass, and what is its dimension?

What is molar mass?

The founder of the theory of atomic masses was the scientist Dalton, who compiled a table of atomic masses and took the mass of the hydrogen atom as one.

Molar mass is the mass of one mole of a substance. A mole, in turn, is an amount of substance that contains a certain number of tiny particles that participate in chemical processes. The number of molecules contained in one mole is called Avogadro's number. This value is constant and does not change.

Rice. 1. Formula for Avogadro's number.

Thus, the molar mass of a substance is the mass of one mole, which contains 6.02 * 10^23 elementary particles.

Avogadro's number got its name in honor of the Italian scientist Amedeo Avagadro, who proved that the number of molecules in equal volumes of gases is always the same

Molar mass in the International SI System is measured in kg/mol, although this value is usually expressed in grams/mol. This value is designated English letter M, and the molar mass formula is as follows:

where m is the mass of the substance, and v is the amount of the substance.

Rice. 2. Calculation of molar mass.

How to find the molar mass of a substance?

The table of D.I. Mendeleev will help you calculate the molar mass of a particular substance. Let's take any substance, for example, sulfuric acid. Its formula is as follows: H 2 SO 4. Now let's turn to the table and see what the atomic mass of each of the elements included in the acid is. Sulfuric acid consists of three elements - hydrogen, sulfur, oxygen. The atomic mass of these elements is respectively 1, 32, 16.

It turns out that the total molecular mass is equal to 98 atomic mass units (1*2+32+16*4). Thus, we found out that one mole of sulfuric acid weighs 98 grams.

The molar mass of a substance is numerically equal to the relative molecular mass if the structural units of the substance are molecules. The molar mass of a substance can also be equal to the relative atomic mass if the structural units of the substance are atoms.

Until 1961, an oxygen atom was taken as an atomic mass unit, but not a whole atom, but 1/16 of it. At the same time, chemical and physical units the masses were not the same. Chemical was 0.03% more than physical.

Currently accepted in physics and chemistry one system measurements. As standard e.a.m. 1/12 of the mass of a carbon atom is selected.

Rice. 3. Formula for the unit of atomic mass of carbon.

The molar mass of any gas or vapor is very easy to measure. It is enough to use control. Same volume gaseous substance equal in quantity to another substance at the same temperature. A well-known way to measure the volume of steam is to determine the amount of displaced air. This process is carried out using a side branch leading to a measuring device.

The concept of molar mass is very important for chemistry. Its calculation is necessary for the creation of polymer complexes and many other reactions. In pharmaceuticals, the concentration of a given substance in a substance is determined using molar mass. Also, molar mass is important when conducting biochemical research (the metabolic process in an element).

Nowadays, thanks to the development of science, the molecular masses of almost all components of blood, including hemoglobin, are known.

Most secondary school students consider chemistry to be one of the most difficult and unpleasant subjects for them. In fact, chemistry is no more complicated than physics or mathematics, and in some cases it is much more interesting than them. Many students, having not yet started studying chemistry, are already subconsciously afraid of it, having heard a lot of reviews from high school students about all the “horrors” of this subject and the “tyranny” of his teacher.

Another reason why chemistry is difficult is that it uses some specific key concepts and terms that the student has never encountered before and for which it is difficult to find an analogy in ordinary life. Without an appropriate explanation from the teacher, these terms remain misunderstood by students, which complicates the entire subsequent process of studying chemistry.

One of these terms is the concept of the molar mass of a substance and the task of finding it. This is the basis of the fundamentals of the entire subject of chemistry.

What is the molar mass of a substance
The classic definition is that molar mass is the mass of one mole of a substance. Everything seems simple, but it remains unclear what “one moth” is and whether it has any connection with insects.

Mole- this is the amount of a substance that contains a certain number of molecules, to be precise, 6.02 ∙ 10 23. This number is called a constant or Avogadro's number.

All chemicals have different compositions and molecular sizes. Therefore, if you take one portion consisting of 6.02 ∙ 10 23 molecules, then different substances will have their own volume and mass of this portion. The mass of this portion will be the molar mass of a particular substance. Molar mass is traditionally denoted in chemistry by the letter M and has the dimensions g/mol and kg/mol.

How to find the molar mass of a substance
Before you begin calculating the molar mass of a substance, you need to clearly understand the key concepts associated with it.

  1. Molar mass of a substance is numerically equal to the relative molecular mass if the structural units of the substance are molecules. The molar mass of a substance can also be equal to the relative atomic mass if the structural units of the substance are atoms.
  2. Relative atomic mass shows how many times the mass of a particular atom chemical element greater than a predetermined constant value for which the mass of 1/12 of a carbon atom is taken. The concept of relative atomic mass is introduced for convenience, since it is difficult for a person to operate with such small numbers as the mass of one atom.
  3. If a substance consists of ions, then in this case we talk about its relative formula mass. For example, the substance calcium carbonate CaCO 3 consists of ions.
  4. The relative atomic mass of a substance of a particular chemical element can be found in the periodic table. For example, for the chemical element carbon, the relative atomic mass is 12.011. Relative atomic mass has no units of measurement. The molar mass of carbon will be equal, as mentioned above, to the relative atomic mass, but at the same time it will have units of measurement. That is, the molar mass of carbon will be 12 g/mol. This means that 6.02 ∙ 10 23 carbon atoms will weigh 12 grams.
  5. Relative molecular mass can be found as the sum of the atomic masses of all chemical elements that form a molecule of a substance. Let's look at this using the example of carbon dioxide, or as everyone else calls it. carbon dioxide, having the formula CO 2.

    A carbon dioxide molecule contains one carbon atom and two oxygen atoms. Using the periodic table, we find that the relative molecular mass of carbon dioxide will be equal to 12 + 16 ∙ 2 = 44 g/mol. This is exactly the mass that a portion of carbon dioxide will have, consisting of 6.02 ∙ 10 23 molecules.

  6. The classic formula for finding the molar mass of a substance in chemistry is as follows:

    M = m/n



    where, m is the mass of the substance, g;
    n is the number of moles of a substance, that is, how many portions of 6.02 ∙ 10 23 molecules, atoms or ions it contains, moles.

    Accordingly, the number of moles of a substance can be determined by the formula:

    n = N/Na



    where, N is the total number of atoms or molecules;
    N a is Avogadro’s number or constant, equal to 6.02 ∙ 10 23.

    Most problems in finding the molar mass of a substance in chemistry are based on these two formulas. Using two related relationships is unlikely to be an insurmountable difficulty for most people. The main thing is to understand the essence of basic concepts such as mole, molar mass and relative atomic mass, and then solving problems in chemistry will not cause you any difficulties.

As an aid to finding the molar mass of a substance and solving most typical chemistry problems related to it, we suggest using our calculator. It's very easy to use. Below the line chemical formula of the compound in the drop-down list, select the first chemical element included in the formula of the chemical substance. In the box next to the list, enter the number of atoms of the chemical substance. If the number of atoms is one, leave the field blank. If you need to add a second and subsequent elements, then press the green plus and repeat the above action until you get the full formula of the substance. Check the correctness of the input using the updated chemical formula of the compound. Click the button Calculate to obtain the molar mass of the desired substance.

To solve most typical chemistry problems, you can also add one of the known conditions: the number of molecules, the number of moles, or the mass of the substance. Below the button Calculate after clicking it will be given complete solution tasks based on the input input data.

If there are brackets in the chemical formula of a substance, then expand them by adding the corresponding index to each element. For example, instead of the classic formula for calcium hydroxide Ca(OH) 2, use the following formula for the chemical substance CaO 2 H 2 in the calculator.

In practical and theoretical chemistry there are and have practical significance two concepts such as molecular (it is often replaced by the concept of molecular weight, which is not correct) and molar mass. Both of these quantities depend on the composition of a simple or complex substance.

How to determine or molecular? Both of these physical quantities cannot (or almost cannot) be found by direct measurement, for example, by weighing a substance on a scale. They are calculated based on the chemical formula of the compound and the atomic masses of all elements. These quantities are numerically equal, but differ in dimension. expressed in atomic mass units, which are a conventional quantity and are designated a. e.m., as well as another name - “dalton”. The units of molar mass are expressed in g/mol.

The molecular masses of simple substances, the molecules of which consist of one atom, are equal to their atomic masses, which are indicated in the periodic table of Mendeleev. For example, for:

  • sodium (Na) - 22.99 a. eat.;
  • iron (Fe) - 55.85 a. eat.;
  • sulfur (S) - 32.064 a. eat.;
  • argon (Ar) - 39.948 a. eat.;
  • potassium (K) - 39.102 a. eat.

Also, the molecular weights of simple substances, the molecules of which consist of several atoms of a chemical element, are calculated as the product of the atomic mass of the element by the number of atoms in the molecule. For example, for:

  • oxygen (O2) - 16. 2 = 32 a. eat.;
  • nitrogen (N2) - 14.2 = 28 a. eat.;
  • chlorine (Cl2) - 35. 2 = 70 a. eat.;
  • ozone (O3) - 16. 3 = 48 a. eat.

Molecular masses are calculated by summing the product of the atomic mass and the number of atoms for each element included in the molecule. For example, for:

  • (HCl) - 2 + 35 = 37 a. eat.;
  • (CO) - 12 + 16 = 28 a. eat.;
  • carbon dioxide (CO2) - 12 + 16. 2 = 44 a. eat.

But how to find the molar mass of substances?

This is not difficult to do, since it is the mass of a unit amount of a particular substance, expressed in moles. That is, if the calculated molecular mass of each substance is multiplied by a constant value equal to 1 g/mol, then its molar mass will be obtained. For example, how do you find the molar mass (CO2)? It follows (12 + 16.2).1 g/mol = 44 g/mol, that is, MCO2 = 44 g/mol. For simple substances, molecules that contain only one atom of the element, this indicator, expressed in g/mol, numerically coincides with the atomic mass of the element. For example, for sulfur MS = 32.064 g/mol. How to find the molar mass of a simple substance, the molecule of which consists of several atoms, can be considered using the example of oxygen: MO2 = 16. 2 = 32 g/mol.

Examples have been given here for specific simple or complex substances. But is it possible and how to find the molar mass of a product consisting of several components? Like the molecular mass, the molar mass of a multicomponent mixture is an additive quantity. It is the sum of the products of the molar mass of the component and its share in the mixture: M = ∑Mi. Xi, that is, both the average molecular and average molar mass can be calculated.

Using the example of air, which contains approximately 75.5% nitrogen, 23.15% oxygen, 1.29% argon and 0.046% carbon dioxide (the remaining impurities, which are contained in smaller quantities, can be neglected): Mair = 28. 0.755 + 32. 0.2315 + 40 . 0.129 + 44 . 0.00046 = 29.08424 g/mol ≈ 29 g/mol.

How to find the molar mass of a substance if the accuracy of determining the atomic masses indicated in the periodic table is different? For some elements it is indicated with an accuracy of tenths, for others with an accuracy of hundredths, for others to thousandths, and for such elements as radon - to whole ones, for manganese to ten-thousandths.

When calculating the molar mass, it makes no sense to carry out calculations with greater accuracy than to tenths, since they have practical use when the purity of ourselves chemical substances or reagents will introduce a large error. All these calculations are approximate. But where chemists require greater accuracy, appropriate corrections are made using certain procedures: the titer of the solution is established, calibrations are made using standard samples, etc.