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Chemistry Question on Solutions

The depression in the freezing point observed for a formic acid solution of concentration 0.5 mL L-1 is 0.0405°C. Density of formic acid is 1.05 g mL-1. The Van't Hoff factor of the formic acid solution is nearly (Given for water kf = 1.86 k kg mol-1)

A

0.8

B

1.1

C

1.9

D

2.4

Answer

1.9

Explanation

Solution

ΔTf of formic acid = 0.0405°C
Concentration = 0.5 mL/L
and density = 1.05 g/mL
∴ Mass of formic acid in solution = 1.05 × 0.5 g
= 0.525 g
∴ According to Van’t Hoff equation,
ΔT_f=ik_f.m
0.0405=i×1.86×0.52546×1\frac{i×1.86×0.525}{46×1}
(Assuming mass of 1 L water = kg)
i=0.0405×461.86×0.525\frac{0.0405×46}{1.86×0.525}=1.9