Arrange the following solutions in order of their increasing boiling points.
(i) 10⁻⁴ M NaCl
(ii) 10⁻⁴ M Urea
(iii) 10⁻³ M NaCl
(iv) 10⁻² M NaCl

Solution

Chemistry ma’am · 3 steps

Boiling point rise depends on particle count, so multiply each concentration by its i factor and compare.

1. What decides it. Elevation in boiling point is a colligative property: Δ T_b = i K_b m. The solvent is the same water for all four, so only the effective particle concentration i × c matters.
2. Find i. Urea does not dissociate, so i = 1. NaCl splits into Na⁺ and Cl⁻, so i = 2 in these dilute solutions.

   Solution	i	Particle concentration i × c
   (ii) 10⁻⁴ M urea	1	1 × 10⁻⁴, lowest
   (i) 10⁻⁴ M NaCl	2	2 × 10⁻⁴
   (iii) 10⁻³ M NaCl	2	2 × 10⁻³
   (iv) 10⁻² M NaCl	2	2 × 10⁻², highest

3. So. More particles give a higher boiling point, so the order is (ii) < (i) < (iii) < (iv), which is option (b).

The idea to keep. For any colligative property, compare i × c, not just c. An electrolyte at the same concentration always beats a non-electrolyte because it makes more particles.

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