Question

Which of the following exhibits the minimum coagulating power for precipitating of positively charged ferric oxide sol ?

• A. \(\text{KNO}_3\)
• B. \(\text{K}_2\text{SO}_4\)
• C. \(\text{K}_3\text{PO}_4\)
• D. \(\text{K}_4\text{Fe(CN)}_6\)

Hint:

For a positive sol the coagulating ion is the anion; higher charge means higher power (Hardy-Schulze rule).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
Hardy-Schulze rule: the coagulating ion is the one with charge opposite to the sol, and its power rises sharply with its charge. Ferric oxide sol is positive, so anions coagulate it.

Step 2: Detailed Explanation
The anions in the options are \(\text{NO}_3^-\) (charge 1), \(\text{SO}_4^{2-}\) (2), \(\text{PO}_4^{3-}\) (3) and \([\text{Fe(CN)}_6]^{4-}\) (4).
Order of power: \([\text{Fe(CN)}_6]^{4-} > \text{PO}_4^{3-} > \text{SO}_4^{2-} > \text{NO}_3^-\).
So \(\text{KNO}_3\), with the monovalent nitrate, has the minimum coagulating power.

Final Answer:
\(\text{KNO}_3\) has the least coagulating power, option (A). \[ \boxed{\text{KNO}_3} \]