Question

A 1.0945 g sample of an unknown nonelectrolyte is dissolved in 10.0054 g of benzophenone and produces a solution that freezes at 36.8 oC. If the pure benzophenone melted at 48.1 oC, what is the molecular weight of the unknown compoumd? The Kf for benzophenone is 9.8 oC/m.

Answer 1

Depression in freezing point= freezing point- melting point of pure compound= 36.8-48.1 =-11.3 deg.c

we know that depression in freezing point = kf*m where Kf= 9.8deg.c/m and m= molality

11.3= 9.8*m

m= 11.3/9.8=1.5m =1.5 moles of solute/ kg of solvent Benzephenone

1 kg solvent contains 1.5 moles but the amount of solvent = 10.054= 10.054/1000=0.010054 kg

0.010054 kg contains 1.5*0.010054/1=0.015081 moles of unknown compound

but Moles of solute= 1.0945/M, where M= Molecular weight of unknown compound

1.0945/M= 0.015081

M=1.0945/0.015081=72 g/mole