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?

Explanation

Here, `(P^o-P_s)/(P^o) =25/100or (P^o-P_s)/P_s=25/75`
`W_A=100g,W_B=?`
For urea, `M_B=60//mol.`
for water, `M_A =18g//mol.`
`(P^o-P_s)/P_s=n_B/n_A=(W_A//M_B)/(W_A//M_A)`
`25/75=(W_B//60)/(100//18)`
`or W_B=111g`
Molality of the silution `=(111g)/(60gmol^(-1))xx1/(100g)xx100gkg^(-1)` ot 18.5 m.