Question

Which of the following cations has maximum coagulating power to cause precipitation of negatively charged arsenious sulfide ?

• A. \(\text{Na}^{+}\)
• B. \(\text{Zn}^{++}\)
• C. \(\text{Ca}^{++}\)
• D. \(\text{Al}^{+++}\)

Hint:

The coagulating ion carries a charge opposite to the sol, and a higher charge means greater power (Hardy-Schulze rule).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept
Hardy-Schulze rule: the coagulating power of an ion increases with its charge.
The effective ion has a charge opposite to that of the colloidal particles.

Step 2: Sign of the Sol
Arsenious sulfide sol, \(\text{As}_{2}\text{S}_{3}\), is negatively charged.
So cations cause its coagulation.

Step 3: Compare the Cations
The charges are \(\text{Na}^{+}(1)\), \(\text{Zn}^{2+}(2)\), \(\text{Ca}^{2+}(2)\), \(\text{Al}^{3+}(3)\).
Order of power: \(\text{Na}^{+}<\text{Zn}^{2+}\approx\text{Ca}^{2+}<\text{Al}^{3+}\).

Step 4: Conclusion
\(\text{Al}^{3+}\) has the highest charge, so it is the best coagulant.

Final Answer:
Aluminium ion has the greatest coagulating power, option (D). \[ \boxed{\text{(D) }\text{Al}^{+++}} \]