What weight of the non-volatile solute urea' `(NH_(2)-CO-NH_(2))` needs to be dissolved in `100 g` of water in order to decrease the vapour pressure of water by `25%`? What will be the molality of the solution?

Answer

111

Explanation

`(P_(0)-P)/(P^(0)) =(w/m)/(w/m + W/M)`
Let the initial (normal pressure `(P^(@)=P)`
`therefore` Pressure of solution `=75/100 xx P = 3/4 P`
m=60, M=18, W= 100 g
`therefore (P-3/4P)/(P) =(w//60)/(w/60 + 100/18)`
`1/4 =(w//60)/((w//60) + 5.55)` or `(4w)/(60) = w/60 + 5.55`
or w= 111 g