Question

In a 0.2 molal aqueous solution of a weak acid HX, the degree of ionisation is 0.3. Taking K_f for water as 1.85, the freezing point of the solution will be nearest to ______.

विकल्प

• −0.480°C

• −0.360°C

• −0.260°C

• +0.480°C

Answer 1

In a 0.2 molal aqueous solution of a weak acid HX, the degree of ionisation is 0.3. Taking K_f for water as 1.85, the freezing point of the solution will be nearest to −0.480°C.

Explanation:

Given: Molality (m) = 0.2 mol/kg

degree of ionisation (α) = 0.3

K_f = 1.85

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

⇒ n = 2 particles upon complete ionisation

Van’t hoff factor (i) = 1 + α(n − 1)

i = 1 + 0.3(2 − 1)

i = 1 + 0.3

i = 1.3

We know that

ΔT_f ​= i ⋅ K_f​ ⋅ m

= 1.3 × 1.85 × 0.2

= 0.481

T_f ​= 0°C − 0.481°C

T_f ​= −0.481°C ≈ −0.480°C