 # How to calculate a mass fraction of substance

The mass fraction of substance is the relation of mass of a certain substance to the mass of mix or solution in which there is this substance. Is expressed in shares of unit or as a percentage.

## Instruction

1. The mass fraction of substance is on a formula: w = m (v)/m (cm) where w is a mass fraction of substance, m (v) is the mass of substance, m (cm) is the mass of mix. If substance is dissolved, then the formula looks so: w = m (v)/m (solution) where m (solution) there is a mass of solution. The mass of solution if necessary too can be found: m (solution) = m (v) + m (r-la) where m (r-la) is the mass of solvent. If desired the mass fraction can be increased by 100%.

2. If in a statement of the problem it is not given value of weight, then it can be calculated by means of several formulas, sizes data in a condition will help to choose necessary. The first formula for finding of weight: m = V*p where m is weight, V – the volume, p – density. The following formula looks so: m = n*M where m is weight, n is amount of substance, M – molar weight. Molar weight in turn consists of atomic masses of the elements which are a part of substance.

3. For the best understanding of this material we will solve a problem. Mix of copper and magnesian sawdust weighing 1.5 g was processed excess of sulfuric acid. As a result of reaction, hydrogen of 0.56 l (normal conditions) was emitted. Calculate a mass fraction of copper in mix. In this task there takes place reaction, write down its equation. From two substances a lot of hydrochloric acid interacts only magnesium: Mg + 2HCl = MgCl2 + H2. To find a mass fraction of copper in mix, it is necessary to substitute values in the following formula: w(Cu) = m(Cu)/m (cm). The mass of mix is given, we will find the mass of copper: m(Cu) = m (cm) – m(Mg). We look for the mass of magnesium: m(Mg) = n(Mg) * M (Mg). The reaction equation will help to find amount of substance of magnesium. We find amount of substance of hydrogen: n = V/Vm = 0.56/22.4 = 0.025 mol. On the equation it is visible that n(H2) = n(Mg) = 0.025 mol. We count the mass of magnesium, knowing that the molar mass of magnesium is equal to 24 g/mol: m(Mg) = 0.025*24 = 0.6 g. We find the mass of copper: m(Cu) = 1.5 - 0.6 = 0.9 g. It was necessary to calculate a mass fraction: w(Cu) = 0.9/1.5 = 0.6 or 60%.

Author: «MirrorInfo» Dream Team