The boiling point elevation constant (Kb) of a volatile liquid is given by the following expression
Kb=ΔHvRTb2M
Where Tb=boiling point of the liquid
M=molar mass of the liquid
ΔHv= Enthalpy of vaporisation
The boiling points of the two solvents are in the ratio 2:1, which means that the boiling point elevation constants are also in the ratio 2:1. Let's call the boiling point elevation constant of solvent X as (Tb)x, and the boiling point elevation constant of solvent Yas (Tb)y.
Given: Mx=My
(Tb)x=2(Tb)y
2(ΔHv)x=(ΔHv)y
∴(Kb)y(Kb)x=(12)2×12=18=m
∴m=8
Therefore, the value of m is 8(nearest integer).