Question

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 0.1 mol dm^–3 Ca3(PO₄)₂
• B. 10 mL of 0.2 mol dm^–3 AlCl₃
• C. 10 mL of 0.1 mol dm^–3 Na₂SO₄
• D. 10 mL of 0.15 mol dm^–3 CaCl₂

Hint:

When determining coagulation rates:
• Apply the Schulze-Hardy rule: the higher the charge of the counter-ion, the stronger its coagulating power.
• For negatively charged colloids, focus on the cation with the highest charge and concentration.

Answer 1

The Correct Option is B

- The colloidal solution formed when FeCl\(_3\) is added to NaOH is negatively charged.
- The coagulation of colloidal solutions is governed by the Schulze-Hardy rule: \textbf{the higher the valence of the oppositely charged ion, the greater its coagulating power.
Analysis:
   Among the given options:
   AlCl\(_3\) contains Al\(^{3+}\) ions, which have the highest coagulating power compared to other cations such as Ca\(^{2+}\) or Na\(^{+}\).
   Therefore, the solution containing Al\(^{3+}\) ions (AlCl\(_3\)) will coagulate the colloidal solution at the fastest rate.
Final Answer: (1) 10 mL of 0.2 mol dm\(^{-3}\) AlCl\(_3\).