Every student who carefully studied the periodic table, probably noticed that, in addition to the number of a chemical element, it contains information about the weight of its atom. In this article, we will look at what 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 understand such an important quantity in chemistry as the 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 that time about the chemical structure and behavior of gaseous substances, so they were accepted only half a century later.
At the beginning of the 20th century, with the help of more modern technologies, it was possible to determine the number of hydrogen molecules in 2 grams of this gas. This quantity is called"mol". The term itself was introduced by Wilhelm Ostwald, from Latin it translates as "heap", "cluster".
In 1971, the mole became one of the 7 basic units of measurement in the SI system. Currently, 1 mole is understood as the number of silicon atoms that is contained in an ideal sphere with a mass of 0.028085 kg. The very number of particles corresponding to 1 mole is called Avogadro's number. It is approximately 6.021023.
What is molar mass?
Now we can return to the topic of the article. Mole and molar mass are two interrelated quantities. The second is the weight of one mole of any substance. It is obvious that the type of chemical element or the composition of the molecule of a particular gas directly determines the molar mass. According to this definition, the following expression can be written:
M=ma NA.
Where ma is the mass of one atom, NA is Avogadro's 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 is 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 mole of its atoms has a weight of 92.9 grams.
Where is M used in chemistry?
Knowing nowwhat is molar mass, 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 go only with a strict ratio of reagents. For example, the reaction of hydrogen combustion with the formation of a water molecule is shown below:
2H2+ O2=2H2O.
It can be seen that 2 moles of hydrogen, which have a mass of 4 grams, react without residue with 1 mole of oxygen weighing 32 grams. As a result, 2 moles 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 conserved. In reality, the weight of the reactants and conversion products is slightly different. This small difference is due to the thermal effect of the reaction. The mass difference can be calculated by using Einstein's formula to relate weight and energy.
In chemistry, the concept of molar mass is also closely related to the concentration of the same name. Usually, 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 under consideration is constant only for a given chemical element or a specific compound, for example, for H2it 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 greater mass than the second.
Gases and their molar volume
Molar mass is also related to ideal physicsgases. In particular, it is used when determining the volume of a gas system under specific external conditions, if the amount of the substance is known.
Ideal gases are described by the Clapeyron-Mendeleev equation, which looks like:
PV=nRT.
Here n is the amount of substance that is related to the molar mass as follows:
n=m / M.
The volume of a gas can be determined if its m, temperature T and pressure P are known, using the following formula:
V=mRT / (MP).
A molar volume is one that, at 0 oC and a pressure of one atmosphere, occupies 1 mole of any gas. From the formula above, you can calculate this value, it is 22.4 liters.
