What mass of non-volatile solute (urea) needs to be dissolved in `100 g` of water in order to decrease the vapour pressure of water by `30%`. What will be the molality of solution?

Explanation

if `P^(@) = 100 mm` , then `P_(S) = 70 mm`
`100 - 70/70 = W_(2) xx 18 /100 xx 60`
`0.43 = W_(2) xx 18/ 100 xx 60 rArr W_(2) = 143.33`
Molality = `W_(2) xx 1000 /Mw_(2) xx W_(1) = 143.33 xx 1000 /60 xx 100 = 23.88`