CBSE Class 12 · 2017 · SolutionsLong Answer

(a) 30 g of urea (M = 60 g mol

^{- 1}

) is dissolved in 846 g of water. Calculate the vapour pressure of water for this solution if the vapour pressure of pure water at 298 K is 23.8 mm Hg.
(b) Write two differences between ideal solutions and non-ideal solutions.

Solution

(a) Moles of urea:

n_{2} = \frac{30}{60} = 0.5 mol

Moles of water:

n_{1} = \frac{846}{18} = 47 mol

Mole fraction of water:

x_{1} = \frac{47}{47.5} = 0.9895

By Raoult's law:

p = x_{1} p^{\circ} = 0.9895 \times 23.8

p = 23.55 mm Hg

(b) Ideal solutions obey Raoult's law over the entire concentration range; non-ideal solutions show positive or negative deviations.
For ideal solutions

Δ_{mi x} H = 0

and

Δ_{mi x} V = 0

; for non-ideal solutions both are non-zero (the A–B interactions differ in strength from the A–A and B–B interactions).

CBSE Class 12 Board · 2017 · Subjective

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