JEE Main 2021 · 17 March · evening · Single correct · +4 / −1
For the coagulation of a negative sol, the species below, that has the highest flocculating power is :
- (a) Na⁺
- (b) Ba²⁺
- (c) PO₄³⁻
- (d) SO₄²⁻
Answer and solution
Answer (b) Ba²⁺
Solution
Chemistry ma’am · 3 steps
A negative sol is coagulated by positive ions, so first keep the cations, then pick the higher charge.
- Which ions act. The particles of a negative sol carry negative charge. Only ions of the opposite charge, that is cations, can neutralise them and make them settle.
- Hardy–Schulze rule. Among ions of the right sign, the one with the higher charge has the greater flocculating power. A 2+ ion neutralises far more charge on the sol than a 1+ ion.
Ion Sign Can coagulate a negative sol? Charge Na⁺ positive yes 1 Ba²⁺ positive yes 2 PO₄³⁻ negative no, same sign as the sol 3 SO₄²⁻ negative no, same sign as the sol 2 - So. Of the two cations, Ba²⁺ has the higher charge, so it has the highest flocculating power. The answer is (b).
The idea to keep. In coagulation, check the sign first: only the ion opposite to the sol's charge counts. Then the bigger the charge on that ion, the stronger its flocculating power.
Answer from the NTA final answer key. Solution by PrepWiser.