Question

What will be the value of relative lowering of vapour pressure when 3 g urea is dissolved in 45 g of water

Answer 1

Given What will be the value of relative lowering of vapour pressure when 3 g urea is dissolved in 45 g of water

According to Raoult's law, the mole fraction of solute is equal to relative lowering of vapour pressure.

Molar mass of urea = 60 g/mole

Molar mass of water = 18 g/mole

So Δp/p = n₂ / n₂ + n₁ = 3/60 / 3/60 + 45/18

                              = 0.019 ≅ 0.02

So mole fraction of solute is 0.02

Answer 2

According to law of Raoult, the relative lowering of the Vapour pressure is equal to the mole fraction of the solute.

As per the given question, the Relative lowering of the Vapour pressure is 0.2.

The answer based on the formula comes to 0.197.

But as per the rule of maths, it is rounded off to the next higher decimal figure which is 0.2.