Question

Explain how the measurement of depression of freezing point is useful in the determination of molecular mass of a solute.

Answer 1

The measurement of depression of freezing point is useful in determining the molecular mass of a solute because the decrease in freezing point (ΔT_f) is directly proportional to the molality (m) of the solution, which involves the number of moles of solute. Since molality depends on the mass and molecular mass of the solute, we can rearrange the freezing point depression formula to calculate the molecular mass.

The relation is ΔT_f = K_f . m

= `K_f * w_2/(M_2 * w_1)`

∴ `M_2 = (K_f * w_2)/(M_2 * w_1)`

By measuring ΔT_f and knowing the values of K_f, w₂​, and w_1​, we can accurately determine the molecular mass M₂​ of the solute.