Every person knows that the bodies around us are made up of atoms and molecules. They have a different shape and structure. When solving problems in chemistry and physics, it is often required to find the mass of a molecule. Consider in this article several theoretical methods for solving this problem.
General information
Before considering how to find the mass of a molecule, you should get acquainted with the concept itself. The following are a few examples.
A molecule is called a set of atoms that are combined with each other by one or another type of chemical bond. Also, they should and can be considered as a single whole in various physicochemical processes. These bonds may be ionic, covalent, metallic, or van der Waals.
The well-known water molecule has the chemical formula H 2 O. The oxygen atom in it is connected via polar covalent bonds with two hydrogen atoms. Such a structure determines many of the physical and chemical properties of liquid water, ice, and steam.
Natural gas methane is another prominent representative of molecular matter. Its particles are formed by a carbon atom and four hydrogen atoms (CH 4 ). In space, the molecules are in the form of a tetrahedron with carbon in the center.
Air is a complex mixture of gases, which mainly consists of oxygen molecules O 2 and nitrogen N 2 . Both types are connected by strong double and triple covalent non-polar bonds, which leads to their high chemical inertness.
Determination of the mass of a molecule through its molar mass
The periodic table of chemical elements contains a large amount of information, among which there are atomic units of mass (amu). For example, a hydrogen atom has an amu equal to 1, and an oxygen atom has 16. Each of these numbers shows the mass in grams that a system containing 1 mol of atoms of the corresponding element will have. Recall that the unit of measurement of the amount of substance 1 mol represents the number of particles in the system corresponding to the Avogadro number N A , it is 6.02 * 10 23 .
When a molecule is considered, they use the concept of not molecular mass, but molecular mass. The latter is a simple amount of amu for the atoms that make up the molecule. For example, the molar mass for H 2 O will be 18 g / mol, and for O 2 - 32 g / mol. Having a general concept, then we can proceed to calculations.
The molar mass M is simply used to calculate the mass of the molecule m 1 . To do this, use the simple formula:
m 1 = M / N A.
In some problems, the mass of the system m and the amount of substance n in it can be given. In this case, the mass of one molecule is calculated as follows:
m 1 = m / (n * N A ).
Perfect gas
This concept refers to a gas whose molecules randomly move in different directions at high speeds and do not interact with each other. The distances between them far exceed their own dimensions. For such a model, the following expression is true:
P * V = n * R * T.
It is called the law of Mendeleev-Clapeyron. As can be seen, the equation relates pressure P, volume V, absolute temperature T and the amount of substance n to each other. In the formula, R is the gas constant numerically equal to 8.314. The written law is called universal because it does not depend on the chemical composition of the system.
If three thermodynamic parameters are known - T, P, V and the value m of the system, then the mass of the ideal gas molecule m 1 is not difficult to determine by the following formula:
m 1 = m * R * T / (N A * P * V).
This expression can also be written in terms of the gas density ρ and the Boltzmann constant k B :
m 1 = ρ * k B * T / P.
Task example
It is known that the density of some gas is 1.225 kg / m 3 at atmospheric pressure 101325 Pa and a temperature of 15 o C. What is the mass of the molecule? What kind of gas are we talking about?
Since we are given the pressure, density, and temperature of the system, we can use the formula obtained in the previous paragraph to determine the mass of one molecule. We have:
m 1 = ρ * k B * T / P;
m 1 = 1.225 * 1.38 * 10 -23 * 288.15 / 101325 = 4.807 * 10 -26 kg.
To answer the second question of the problem, we find the molar mass M of gas:
M = m 1 * N A ;
M = 4.807 * 10 -26 * 6.02 * 10 23 = 0.029 kg / mol.
The obtained molar mass value corresponds to gas air.