Q. 0.5 molal aqueous solution of a weak acid (HX) is 20% ionised. If Kf for water is 1.86 K kg mol⁻¹, the lowering in freezing point of the solution is
A) 0.56 K
B) 1.12 K
C) – 0.56 K
D) – 1.12 K
Explanation
[Solution from source] For HX ⇌ H⁺ + X⁻ with α = 0.2, i = 1 + α = 1.2. ΔTf = iKf m = 1.2 × 1.86 × 0.5 = 1.116 ≈ 1.12 K. 0.56 K ignores the van 't Hoff factor and rounds wrongly; the depression itself is a positive quantity. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.