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

111g, 18.5 m

Explanation

Let the initial pressure of solvent , `p_(A)^(@) = p`
Pressure of solution ` = p - (25)/(100) p = (3)/(4) p`
` (p-3//4p)/(p) = ((w_(B))/(60))/((w_(B))/(60) +(100)/(18))`
or `" " w_(B) = 111 g`
Now , molality `" " m = (111//60 xx 1000)/(100) = 18.5m`