Q. The freezing point depression constant for water is −1.86°C m⁻¹. If 5.00 g Na₂SO₄ is dissolved in 45.0 g H₂O, the freezing point is changed by −3.82°C. Calculate the van't Hoff factor for Na₂SO₄
A) 0.381
B) 2.05
C) 2.63
D) 3.11
Explanation
[Solution from source] Moles of Na₂SO₄ = 5.00/142 = 0.0352 mol; molality = 0.0352/0.0450 kg = 0.782 m. i = ΔTf/(Kf·m) = 3.82/(1.86 × 0.782) = 3.82/1.455 = 2.63. Ideal complete dissociation would give 3, so 3.11 is too high. Answer (3).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.