Question

Describe rast method of determination of relative molecular mass. Also write the equation used giving significance of the terms involved.

Answer 1

In this method molecular mass of an unknown solute is calculated by measuring depression of freezing point of pure solvent after addition of solute is to

depending on the method pf depression of freezing point(ΔT) is proportional to the concentration of the solution(m).

so the equation is

ΔTαm

And then

ΔT=$k_{f}$m

  m=$\frac{mass of solute(W)}{molecular weight of solute(M)*weight of solvent(kg)}$

Here depression of freezing point(ΔT)is experimentally determined mass of solute and also mass of the solvent is known.Therefore by using the above equation molecular weight of solute can easily determined.

Answer 2

Explanation:

Rast method:

• Rast method is used to identify the molecular mass of an unknown solute, in this method we have to identify the depression of ‘freezing point’ of pure solvent after addition of solute to it.

According to “Rast method: concentration of solute is directly proportional to the “depression of freezing point”.

$\Delta T=K_{f} m \ldots \ldots \ldots \ldots \ldots(1)$

Here, $K_{f}$ = “Molar freezing point depression constant”  

$m=\frac{\text { Mass of solute }}{\text {Molecular weight of solute} \times \text {Weight of solvent in kilograms.}}$

The value of depression freezing point is experimentally determined.

Hence, by using equation (1) we can determine the molecular weight of solute.