Question

How is the molar mass of a non-volatile substance determined from the lowering of vapour pressure?

Answer 1

The molar mass of a non-volatile solute can be determined using the relative lowering of vapour pressure as follows:

If w = mass of solute, W = mass of solvent, M = molecular mass of solvent, and M' = molecular mass of solute, we have

`(P^circ - P)/P^circ = (w/(M^'))/(W/M + w/(M^'))`

If the solution is very dilute, we have

`(P^circ - P)/P^circ = (w//M)/(W//M)`

`(P^circ - P)/P^circ = (wM)/(W * M^')`