K4Fe(CN)6⇌4K++Fe(CN)64−
Initialconc.1m00
Final conc. (1−0.4)m4×0.4m0.4m
=0.6m=1.6m=0.4m
Effective molality =0.6+1.6+0.4=2.6m
For same boiling point, the molality of another solution should also be 2.6m.
Now, 18.1 weight percent solution means
18.1gm solute is present in 100gm solution and hence, (100−18.1=)81.9gm water. Now, 2.6=81.9/100018.1/M
∴ Molar mass of solute, M=85