Question

Select the set of incorrect statements: Statement-I : The pH of neutral water at \(310\,K\) is lower than 7. Statement-II : The pH of an aqueous solution of \(1\times10^{-8}\,M\) HCl is 8. Statement-III : The ionization of \(CH_3COONa\) is suppressed in presence of \(CH_3COOH\). Statement-IV : CuS is precipitated when \(H_2S\) is passed through a solution of \(Cu^{2+}\) and \(Zn^{2+}\) ions in presence of dilute HCl since \(K_{sp}(CuS)\lt K_{sp}(ZnS)\).

• A. I, II, III
• B. II, III
• C. II, IV
• D. III, IV

Hint:

• Neutral pH is not always 7; it depends on temperature.

• Common ion effect suppresses ionization of weak electrolytes.

• For very dilute acids, autoionization of water must be considered.

• Sulphides with very small \(K_{sp}\) precipitate first.

Answer 1

The Correct Option is B

Concept: The correctness of each statement can be checked using ionic equilibrium, common ion effect and solubility product principles.

Step 1: Analyze Statement-I. At \(310\,K\), \[\begin{aligned} K_w \gt 10^{-14} \end{aligned}\] Hence, \[\begin{aligned} pK_w \lt 14 \end{aligned}\] For neutral water, \[\begin{aligned} pH=\frac{pK_w}{2} \end{aligned}\] Therefore, \[\begin{aligned} pH\lt 7 \end{aligned}\] Thus, \[\begin{aligned} \text{Statement-I is Correct} \end{aligned}\]

Step 2: Analyze Statement-II. For very dilute HCl (\(10^{-8}M\)), the contribution of \(H^+\) from water cannot be neglected. Hence, \[\begin{aligned} pH \neq 8 \end{aligned}\] In fact, the pH is slightly less than 7. Therefore, \[\begin{aligned} \text{Statement-II is Incorrect} \end{aligned}\]

Step 3: Analyze Statement-III. \(CH_3COONa\) is a strong electrolyte and undergoes complete ionization. The common ion effect suppresses the ionization of weak electrolytes, not strong electrolytes. Therefore, \[\begin{aligned} \text{Statement-III is Incorrect} \end{aligned}\]

Step 4: Analyze Statement-IV. \[\begin{aligned} K_{sp}(CuS) \ll K_{sp}(ZnS) \end{aligned}\] Hence CuS precipitates first in acidic medium while ZnS remains dissolved. Thus, \[\begin{aligned} \text{Statement-IV is Correct} \end{aligned}\]

Step 5: Prepare the truth table. \[ \begin{aligned} \text{Statement I} &:\quad \text{Correct} \\ \text{Statement II} &:\quad \text{Incorrect} \\ \text{Statement III} &:\quad \text{Incorrect} \\ \text{Statement IV} &:\quad \text{Correct} \end{aligned} \] Hence, the incorrect statements are \[\begin{aligned} \boxed{\text{II, III}} \end{aligned}\] Therefore, option \(\mathbf{(B)}\) is correct.