Question

A solution is prepared by dissolving 1.50 g of an unknown nonelectrolyte in 20.0 g of the organic solvent carbon tetrachloride (CCl4). If the boiling point of the solution is 79.4 °C, what is the molar mass of the unknown nonelectrolyte? The boiling point of pure CCl4 is 76.5 °C. ?I(CCl4) = 5.03 °C/?.

Answer 1

Boiling point is the temperature at which liquid and gas phases of a substance are in equilibrium with each other.

Boiling point of a pure solvent decreases by the addition of a non-volatile substance as the vapour pressure of pure solvent decreases by the addition of a non volatile substance.

there fore Boiling point of solution (Solvent having non volatile substance) is always greater than the boiling point of pure solvent,

Let T0 be the boiling point of Pure solvent and T be boiling point of solution

Hence T > T_{​​​​​​0}

The difference in the boiling points of solution and pure solvent is called Elevation in boiling point.

T - T_{​​​​​​0} is called elevation in boiling point (∆T_b )

Experimentally it is observed that elevation in boiling point is directly proportional to the molality of the solution.

∆T_b is directly proportional to m(molality of the solution)

∆T_b = K_fm