The Rast method, one of the older techniques for finding the molecular mass of an unknown, measures
Channel: Doubtnut
Date: 16/08/2020

The Rast method, one of the older techniques for finding the molecular mass of an unknown, measures the freezing point depression of solution in camphor. Camphor is used because its freezing point is very sensitive to added solution solute (It has highest `K_(f))`. A solution of `2.342 g` of an unknown substancein `49.88 g` of camphor freezes at `441.2 K`. If `K_(f)` for camphor is `40.0 Kg mol^(-1)`, calculate the molecular mass of the unknown.  Strategy: Find the molality of the solution from the freezing point of the pure camphor, the freezing point of the solution and the value of `K_(f)` by using the relationship `DeltaT_(f)=K_(f)m`. From the molality, find the no. of moles of solute in given mass of solvent. Finally find the molar mass (Melting point of pure camphar is 453.0 K)