What weight of the non-volatile urea (`NH_2- CO - NH_2`) needs to be dissolved in 100g of water in order to decrease the vapour pressure of water by 25%? What will be molality of the solution?

Options

A. `11.1gm`
B. `111.1gm`
C. `9.82gm`
D. `982 gm`

Answer

2

Explanation

According to Raoult's law `(P^(@)-P_(S))/(P_(S))=(w)/(m)xx(M)/(W)" "[`Approx. `(P^(@)-P_(S^(@)))/(P_(0))`, which is true only for all soln.`]`
Given if `P^(@)=100`, then `P_(S)=75mm,m=60,w=100g,M=18`
`=(100-75)/(75)=(wxx18)/(60xx100)`
`:." "w=111.11g`