Question

0.5 molal aqueous solution of a weak acid (HX) is 20% Ionized. If Kr of water is 1.86 K kg mol^-1, the lowering in freezing point of solution is ______.

Options

• 0.56 K

• 1.12 K

• - 1.12 K

• - 0.56 K

Answer 1

0.5 molal aqueous solution of a weak acid (HX) is 20% Ionized. If Kr of water is 1.86 K kg mol-1, the lowering in freezing point of solution is 1.12 K.

Explanation:

\[\ce{HX <=> H+ + X-}\]

For dissociation of an electrolyte,

`alpha = ("i - 1")/"n - 1"`

∴ 0.2 = `("i - 1")/"2 - 1"`

∴ i - 1 = 0.2

∴ i = 0.2 + 1 = 1.2 

Now, Δ T_f = i k_f m = 1.2 × 1.86 × 0.5 = 1.12 K