JEE Main 2023 · 29 January · morning · Single correct · +4 / −1
Which of the following salt solutions would coagulate the colloid solution formed when FeCl₃ is added to NaOH solution, at the fastest rate?
- (a) 10 mL of 0.15 mol dm⁻³ CaCl₂
- (b) 10 mL of 0.2 mol dm⁻³ AlCl₃
- (c) 10 mL of 0.1 mol dm⁻³ Na₂SO₄
- (d) 10 mL of 0.1 mol dm⁻³ Ca₃(PO₄)₂
Answer and solution
Answer (b) 10 mL of 0.2 mol dm⁻³ AlCl₃
Solution
Chemistry ma’am · 3 steps
Order of mixing tells you the sol's charge; then pick the opposite ion with the highest charge.
- Charge on the sol. When FeCl₃ is added to NaOH, the NaOH is in excess. The Fe(OH)₃ particles adsorb the common ion OH⁻, so the sol is negatively charged.
- Hardy-Schulze rule. A negative sol is coagulated by cations. The higher the charge on the cation, the greater its coagulating power. The anions of the salt play no part here.
Option Salt Coagulating cation Cation charge (a) CaCl₂ Ca²⁺ +2 (b) AlCl₃ Al³⁺ +3 (c) Na₂SO₄ Na⁺ +1 (d) Ca₃(PO₄)₂ Ca²⁺ +2 - So. Al³⁺ carries the highest charge, and AlCl₃ also has the highest concentration, 0.2 mol dm⁻³. Option (b) coagulates the sol fastest.
The idea to keep. Order of mixing decides the sol's charge: the sol adsorbs the ion of the reagent in excess. Then only the oppositely charged ion coagulates it, and higher charge means faster coagulation.
Answer from the NTA final answer key. Solution by PrepWiser.