Each student who carefully studied the periodic table, probably noticed that, in addition to the number of the chemical element, it contains information about the weight of its atom. In this article, we consider what the molar mass is and where it is used.
What is a mole?
Before answering the question "what is the molar mass", it is necessary to deal with such an important quantity in chemistry as mole.
In the 19th century, Amedeo Avogadro, carefully studying the Gay-Lussac law for ideal gases in an isochoric process, came to the conclusion that equal volumes of different substances under identical conditions (temperature and pressure) contain an equal number of atoms or molecules. Avogadro’s ideas contradicted the theories of the time about the chemical structure and behavior of gas substances, so they were accepted only after half a century.
At the beginning of the XX century, using more modern technologies, it was possible to determine the number of hydrogen molecules in 2 grams of this gas. This amount is called the "mole". The term itself was introduced by Wilhelm Ostwald, from Latin it is translated as "heap", "cluster".
In 1971, the mole became one of the 7 main units of measure in the SI system. Currently, under 1 mole is understood such a number of silicon atoms, which is contained in an ideal sphere with a mass of 0.028085 kg. The very number of particles, corresponding to 1 mol, is called the Avogadro number. Equally, it is approximately 6.02 * 10 23 .
What is the molar mass?
Now you can return to the topic of the article. Moth and molar mass are two interrelated quantities. Under the second understand the weight of one mole of any substance. Obviously, the type of chemical element or the composition of the molecule of a gas directly determines the molar mass. According to this definition, the following expression can be written:
M = m a * N A.
Where m a is the mass of one atom, N A is the Avogadro number. That is, to obtain the value of M, it is necessary to multiply the weight of one particle (molecule, atom, atomic cluster) by the Avogadro number.
As noted in the introduction of the article, each element in the periodic table contains information about its atomic mass. It represents the weight in grams per mole. Obviously, to get the molar mass in kg / mol, the tabular value should be divided by 1000. For example, for niobium at number 41 we see the number 92.9, that is, 1 mol of its atoms has a weight of 92.9 grams.
Where is the value of M used in chemistry?
Knowing now what the molar mass is, let us consider where it is used in chemistry.
The concept of the amount of substance and molar weight plays an important role in the preparation of chemical reactions, since they occur only with a strict ratio of reagents. For example, the reaction of hydrogen combustion with the formation of a water molecule is given below:
2H 2 + O 2 = 2H 2 O.
It is seen that 2 mol of hydrogen, which have a mass of 4 grams, react without residue with 1 mol of oxygen weighing 32 grams. As a result, 2 mol of water molecules are formed, with an indicator of 36 grams. From these figures it is clear that in the process of chemical transformations the mass is preserved. In reality, the weight of the reactants and conversion products is slightly different. This slight difference is due to the thermal effect of the reaction. The mass difference can be calculated using the Einstein formula to relate weight and energy.
In chemistry, the concept of molar mass is also closely related to the concentration of the same name. Typically, solids that are soluble in liquids are characterized by the number of moles in one liter, that is, the molar concentration.
It is important to understand that the value in question is constant only for a given chemical element or specific compound, for example, for H 2 it is 2 g / mol, and for O 3 - 48 g / mol. If its value for one compound is greater than for another, then this means that the elementary particle of the first substance itself has a larger mass than the second.
Gases and their molar volume
The molar mass is also associated with the physics of ideal gases. In particular, it is used when determining the volume of the gas system under specific external conditions, if the quantity of the substance is known.
Ideal gases are described by the Clapeyron-Mendeleev equation, which has the form:
P * V = n * R * T.
Here n is the amount of substance that is associated with the molar mass as follows:
n = m / M.
The gas volume can be determined if its m, temperature T and pressure P are known, according to the following formula:
V = m * R * T / (M * P).
The molar volume is called such that at 0 o C and a pressure of one atmosphere takes 1 mol of any gas. From the formula above, you can calculate this value, it is 22.4 liters.