Question

The vapour pressure of water at 20°C is 17 mm Hg. The vapour pressure of solution containing 2.8 g urea in 50 g of water is ____________.

Options

• 17.71 mm Hg

• 16.71 mm Hg

• 17.28 mm Hg

• 16.28 mm Hg

Answer 1

The vapour pressure of water at 20°C is 17 mm Hg. The vapour pressure of solution containing 2.8 g urea in 50 g of water is 16.71 mm Hg.

Explanation:

Molar mass of urea (NH₂CONH₂) = 14 + 2 + 12 + 16 + 14 + 2 = 60 g mol⁻¹

Molar mass of water = 18 g mol⁻¹

Now, using formula,

`("P"_1^0 - "P"_1)/("P"_1^0) = ("W"_2"M"_1)/("M"_2"W"_1)`

`(17  "mm Hg" - "P"_1)/(17  "mm Hg") = (2.8  "g" xx 18  "g mol"^-1)/(50  "g" xx 60  "g mol"^-1)`

∴ `(17  "mm Hg" - "P"_1)/(17  "mm Hg")` = 0.0168

∴ 17 mm Hg = 0.0168 × 17 mm Hg

∴ 17 mm Hg − P₁ = 0.2856 mm Hg

∴ P₁ = 17 mm Hg − 0.2856 mm Hg

= 16.71 mm Hg