The colligative properties are also called the democrative properties. These are discussed as follows:-

1. Relative lowering of vapor pressure:-

According to Raoult’s Law the vapour pressure of a solution containing non-volatile solute is given by the formula

p_S = x_B X P⁰_B ---------------(1)

where x_B = mole fraction of solvent and p⁰_B = vapour pressure of solvent in pure state. If x_A is the mole fraction of solute.

                x_A + x_B = 1                            or x_B = 1 – x_A

Substituting in equation (1)

                p_S = P⁰_B (1 – x_A)

Where W_A and M_A are the weight and molar mass of solute and W_B and M_B are the weight and molar mass of solvent.

Since the law is applicable to dilute solutions for which W_A << W_B, hence W_A can be

Or