Question

A solution containing 18 g of a non-volatile solute in 200 g of water freezes at 272.07 K. Calculate the molecular mass of the solute.
(Given: K_f = 1.86 K/m)

Answer 1

Given: Mass of solute (w₂) = 18 g

Mass of water (solvent) (w₁) = 200 g = 0.200 kg

Freezing point of solution = 272.07 K

Freezing point of pure water = 273.00 K

Depression in freezing point (ΔT_f) = 273.00 − 272.07 = 0.93 K

Freezing point depression constant K_f = 1.86 K kg mol⁻¹

`DeltaT_f = K_f * w_2/(M * w_1)`

`M = (K_f * w_2)/(Delta T_f * w_1)`

= `(1.86 xx 18)/(0.93 xx 0.200)`

= `33.48/0.186`

M = 180 g/mol

∴ The molecular mass of the solute is 180 g/mol.