Question

If the weight of the non-volatile solute urea (NH₂–CO–NH₂) is to be dissolved in 100 g of water, in order to decrease the vapour-pressure of water by 25%, then the weight of the solute will be ______ g.

Options

• 111.1

• 289.5

• 336.2

• 429.5

Answer 1

If the weight of the non-volatile solute urea (NH₂–CO–NH₂) is to be dissolved in 100 g of water, in order to decrease the vapour pressure of water by 25%, then the weight of the solute will be 111.1 g.

Explanation:

Assume the vapour pressure of water (p⁰) = 100

∴ Vapour pressure of urea solution (p_s) = 75

Weight of urea = `w_1`

Molecular weight of urea = M₁

Weight of water = `w_2`

Molecular weight of water = M₂

By Raoult’s law,

`("p"^0 - "p"_"s")/"p"^0 = (w_1/"M"_1)/(w_2/"M"_2 + w_1/"M"_1)`

So, `(100 - 75)/100 = (w_1/60)/(100/18 + w_1/60)`

⇒ Weight of urea `(w_1)` = 111.1 g