3g urea is dissolved in 45g of water. The relative lowering of vapour pressure is:

A) 0.05

B) 0.04

C) 0.02

D) 0.01

Solution

Urea = Mass/ mol mass. 3/60. = 0.05

Water= Mass/ mol mass. 45/60. = 2.5

By relative lowering of vapour pressure

∆P/Po. =. X(water)

0.05/0.05+2.5

= 0.0196

= 0.02 is the answer