Question

Which salt from following forms aqueous solution having pH less than 7?

• A. $CH_3COONa$
• B. $Na_2SO_4$
• C. $CuSO_4$
• D. $Na_2CO_3$

Hint:

Logic Tip: Alkali and alkaline earth metal cations ($Na^+, K^+, Ca^{2+}$) generally do not hydrolyze. Transition metal cations ($Cu^{2+}, Fe^{3+}, Zn^{2+}$) heavily hydrolyze to produce $H^+$, turning their solutions acidic unless countered by a weak acid anion.

Answer 1

The Correct Option is C

Concept:
The pH of an aqueous salt solution depends on the hydrolysis of its constituent ions.

• Salt of Strong Acid + Strong Base $\rightarrow$ Neutral (pH $\approx$ 7)
• Salt of Weak Acid + Strong Base $\rightarrow$ Basic (pH > 7)
• Salt of Strong Acid + Weak Base $\rightarrow$ Acidic (pH < 7)

Step 1: Analyze the parent acid and base for $CH_3COONa$ and $Na_2CO_3$. 
$CH_3COONa$ (Sodium acetate) is formed from: - Weak acid: Acetic acid ($CH_3COOH$) - Strong base: Sodium hydroxide ($NaOH$) Hydrolysis of acetate ions produces $OH^-$. Solution is basic (pH $> 7$). $Na_2CO_3$ (Sodium carbonate) is formed from: - Weak acid: Carbonic acid ($H_2CO_3$) - Strong base: Sodium hydroxide ($NaOH$) Solution is basic (pH $> 7$). 
Step 2: Analyze the parent acid and base for $Na_2SO_4$. 
$Na_2SO_4$ (Sodium sulfate) is formed from: - Strong acid: Sulfuric acid ($H_2SO_4$) - Strong base: Sodium hydroxide ($NaOH$) Neither ion undergoes significant hydrolysis. Solution is neutral (pH $\approx$ 7). 
Step 3: Analyze the parent acid and base for $CuSO_4$. 
$CuSO_4$ (Copper(II) sulfate) is formed from: - Strong acid: Sulfuric acid ($H_2SO_4$) - Weak base: Copper(II) hydroxide ($Cu(OH)_2$) The $Cu^{2+}$ ions undergo cationic hydrolysis: $Cu^{2+} + 2H_2O \rightleftharpoons Cu(OH)_2 + 2H^+$. This releases $H^+$ ions into the solution, making it acidic. Therefore, its pH is less than 7.