Question

Which of the following electrolytes is most effective in the coagulation of a negative sol like \(As_2S_3\)?

• A. \(NaCl\)
• B. \(MgCl_2\)
• C. \(AlCl_3\)
• D. \(KCl\)

Hint:

Since $As_2S_3$ is a negative sol, only the cation of each salt matters for coagulation; the common anion (chloride) can be ignored right away. Compare the cations by their charge, and remember coagulating power grows very steeply with valency, not in simple proportion to it.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question relates to the coagulation (precipitation) of colloidal solutions by the addition of electrolytes.
Step 2: Key Formula or Approach:
The Hardy-Schulze Rule states:
1. Coagulation is caused by ions having a charge opposite to that of the colloidal particles.
2. The greater the valency of the flocculating ion, the greater is its power to cause coagulation.
Step 3: Detailed Explanation:
The given sol \( As_2S_3 \) (Arsenious sulphide) is a negative sol.
To coagulate a negative sol, we need cations (positively charged ions).
Let's look at the cations provided by the options:
(A) \( NaCl \rightarrow Na^+ \) (Valency = +1)
(B) \( MgCl_2 \rightarrow Mg^{2+} \) (Valency = +2)
(C) \( AlCl_3 \rightarrow Al^{3+} \) (Valency = +3)
(D) \( KCl \rightarrow K^+ \) (Valency = +1)
According to the rule, higher the charge, higher the effectiveness.
Since \( Al^{3+} \) has the highest charge among the choices, it will be the most effective.
Step 4: Final Answer:
\( AlCl_3 \) is the most effective electrolyte because it provides the trivalent \( Al^{3+} \) ion.