Concentration of Solution

There are several ways for describing concentration of solution. They include molarity, molality, normality, mole fraction and mass percentage.

Molarity

Molarity is defined as the number of moles of solute dissolved per litre of solution and is usually denoted by M. For example, a
2.0 molar solution of sulphuric acid would be labelled as 2.0 M H₂SO₄. It is prepared by adding 2.0 mol of H₂SO₄ to water to make a litre of solution.

M = n/v

Molality

It is defined as the number of moles of solute dissolved per kilogram of solvent. It is designated by the symbol m. For example, 2.0 m H₂SO₄ is read 2 molal sulphuric acid and is prepared by adding 2.0 mol of H₂SO₄ to 1 kg of solvent.

Normality

It is defined as the number of gram equivalent weights of solute dissolved per litre of the solution. Normality is denoted by the symbol N. For example, the label 0.5 N KMnO₄ is read 0.5 normal and represents a solution which contains 0.5 gram equivalent of KMnO₄ per litre of solution.

The number of parts by weight of a substance (element or compound) that will combine with or displace, directly or indirectly 1.008 parts by weight of hydrogen, 8 parts by weight of oxygen and 35.5 parts by weight of chlorine is known as equivalent weight. Like atomic weight and molecular weight, equivalent weight is also a number and hence no units are used to express it. However, when equivalent weight is expressed in grams, it is known as gram equivalent weight of the substance.

Mole Fraction

The mole fraction of a component in a solution is the ratio of its number of moles to the total number of moles of all the components in the solution. For example, if a solution contains 2 mol of alcohol and 3 mol of water, the mole fraction of alcohol is 2/5, and that of water 3/5.

Mass Percentage

Mass percentage is the mass of solute present in 100 g of solution. For example, 5% solution of KMnO₄ in water means that 5 g of KMnO₄ is present in 100 g of the aqueous solution of KMnO₄.