Urea ( N H 2 - C O - N H 2 ) needs to be dissolved in 100g of water, in order to decrease the vapour pressure of water by 25%? What will be the molality of the solution?

Step-by-step solution

p 0 - p S p 0 = x S o l u t e s = W S o l u t e s M S o l u t e W S o l u t e s M S o l u t e s + W S o l v e n t M S o l v e n t P 0 - P S P 0 = w A m A × m B W B (A stands for solute and B for solvent) x - x × 75 100 x × 75 100 = w 60 × 18 100 [Let the vapour pressure of pure water, P 0 = x ] 0.25 x 0.75 x = w 60 × 18 100 ∴ w = 111.118 Molality = 111.11 60 × 1000 100 = 18.52