Question

A 2 molal solution of sodium chloride in water causes an elevation in the boiling point of water by 1.88 K. What is the value of Van’t Hoff factor? What does it signify? [K_b = 0.52 K kg mol⁻¹]

Answer 1

ΔT_b (expected) = 0.52 × 2

= 1.04 K

ΔT_b (observed) = 1.88

`i = (Delta T_b("observed"))/(Delta T_b("expected"))`

= `1.88/1.04`

i = 1.80

It signifies partial dissociation of sodium chloride in water.

Answer 2

Given: Molality, m = 2 mol/kg

Elevation in boiling point, ΔT_b = 1.88 K

Boiling point elevation constant, K_b = 0.52 K kg mol⁻¹

By using the formula

ΔT_b = i K_b m

`i = (Delta T_b)/(K_b * m)`

= `1.88/(0.52 xx 2)`

= `1.88/1.04`

i = 1.81

The van’t Hoff factor (i) = 1.81 indicates that NaCl partially dissociates in water.

• If NaCl dissociated completely, i would be 2 (Na⁺ and Cl⁻).
• Since i < 2, this shows incomplete or partial dissociation of NaCl in the solution.