Q. A 0.1 molal aqueous solution of a weak acid is 30% ionized. If Kf for water is 1.86°C/m, the freezing point of the solution will be
A) −0.36°C
B) −0.24°C
C) −0.18°C
D) −0.54°C
Explanation
[Solution from source] HA ⇌ H⁺ + A⁻ with α = 0.3 gives i = 1 + α = 1.3. ΔTf = iKf m = 1.3 × 1.86 × 0.1 = 0.24°C, so the freezing point is −0.24°C. Ignoring ionisation gives −0.18°C, and taking full dissociation gives −0.36°C. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.