How to find a mass fraction of a substance

In the process, the study of molecular chemical formulas is sooner or later the moment comes when it is necessary to study the method to determine the mass fraction of a particular element. This is a relatively simple process, if you first examine all the subtleties of the calculation.

1

What is a mass fraction?

• The student during the study of chemical reactions with various elements, mixtures and systems, has to be encountered with the need to calculate the concentration of the selected substance in multicomponent mixtures. By itself, the mass fraction is the ratio of the mass of a particular element, which is part of the mixture, to the mass of the mixture itself.
• In equations, it is denoted by the Greek letter "Omega" and is calculated in percent. That is why if at the calculations, it will be more than 100, recommended to double-check the calculation. The minimum value can be zero.
• Based on the Law on the preservation of the mass of substances involved in the chemical reaction, the concentration of each individual component in the mixture or substance is calculated. Calculation of the concentration is important not only in chemistry lessons - the manufacture of drugs, the calculation of the number of fertilizers and the preparation of axles without it is simply impossible.

2

Methods of calculation

Since the mass fraction of a substance is the ratio of the mass of the element in the selected particle of the mixture with its molecular weight, the calculation occurs according to the scheme:
w \u003d (m · m) / 100%
The total concentration of all components of the mixture will be equal to one or a hundred percents - depending on the calculated quantity selected. Depending on whether the concentration is necessary to know, the formula changes slightly:

1. The proportion of the chemical element. It is calculated by the formula reflecting its dependence with respect to the atomic mass of its element, multiplied by the number of its atoms in the compound and to the mass of the substance molecules. Similar method calculate the mass fraction of any component in crystallohydrates and other water mixtures, since most elements in minerals are in this form.
   W (O) \u003d (Mr · 2) / m) x 100%
   To facilitate the calculation of the mass fractions of all components, a part of the substance of this size is used to be equal to its molar mass. This will help the level of element in its composition to its molar mass, multiplied by the number of its atoms in the molecule.
2. Share of substances in a mixture or solution. In this case, the scheme will change not strongly: there will be a mass of the substance of the solution on the site of the numerator and as a denominator - the mass of the solution itself entirely.
   W \u003d (M (V-BA) · M (P-RG)) / 100%
   In the calculation process, it is recommended to remember that to determine the mass concentration you need to take the ratio with all the solution, and not only with the amount of solvent.
3. Impurities. In this case, to calculate the concentration of any compound, out of 100% it will be necessary to subtract the share of all impurities. For example, if we are talking about the iron obtained from minerals and the amount of impurities does not exceed 80%, then the calculation of the content of pure iron in the source material will be as follows:
   100 – 80= 20%.

The calculation of the mass fraction of the substance is quite simple, if you know the index of the elements, their molar mass and carefully monitor the calculations.