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

Step 1: We want the incorrect statements.
Check each one and mark whether it is right or wrong.

Step 2: Statement-I.
Water ionises more as temperature rises, so at $310\,K$ the value $K_w$ is larger than at $298\,K$. That makes neutral pH a little below $7$. So Statement-I is correct.

Step 3: Statement-II.
For very dilute $10^{-8}\,M$ HCl, the $H^+$ from water cannot be ignored. Adding acid can never make a solution basic, so the pH stays just below $7$, around $6.96$, not $8$. So Statement-II is incorrect.

Step 4: Statement-III.
$CH_3COONa$ is a strong electrolyte and is almost fully ionised. Acetic acid only suppresses the ionisation of the acid through the common ion effect, not the ionisation of the salt. So Statement-III is incorrect.

Step 5: Statement-IV.
CuS has a much smaller $K_{sp}$ than ZnS, so in acidic medium only CuS precipitates while $Zn^{2+}$ stays in solution. So Statement-IV is correct.

Step 6: Collect the wrong ones.
The incorrect statements are II and III.
\[ \boxed{\text{II, III}} \]