1 molal aqueous solution of an electrolyte A₂B₃ is 60% ionised. The boiling point of the solution at 1 atm is _________ K. (Rounded off to the nearest integer)
[Given K b for (H₂O) = 0.52 K kg mol⁻¹]

Solution

Chemistry ma’am · 3 steps

First count the ions one formula unit gives, then let the 60% ionisation set the van't Hoff factor.

1. Count the ions. A₂B₃ → 2A³⁺ + 3B²⁻, so one formula unit gives n = 5 ions.
2. Van't Hoff factor. With degree of ionisation α = 0.6, i = 1 + (n - 1)α = 1 + 4 × 0.6 = 3.4.

   Δ T_b = i K_b m = 3.4 × 0.52 × 1 = 1.768 K

3. So. Water boils at 373.15 K at 1 atm, so T_b = 373.15 + 1.768 = 374.92 K, which rounds to 375.

The common slip: 2. Stopping at the elevation Δ T_b = 1.768 K gives 2, not 375. The question asks for the boiling point, so add 373.15 K.

The idea to keep. For a partly ionised electrolyte that splits into n ions, use i = 1 + (n-1)α. Then the elevation is i K_b m, and you add it to the pure solvent's boiling point.

Answer from the NTA final answer key. Solution by PrepWiser.