Question

Which among the following salts forms basic solution when dissolved in water?

• A. $NaNO_3$
• B. $CH_3COONH_4$
• C. KCN
• D. $NH_4F$

Hint:

To quickly assess a salt, identify the cation (which comes from the base) and the anion (which comes from the acid). The "stronger" parent dictates the final pH. Strong base wins = basic. Strong acid wins = acidic.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify which of the four given salts will undergo hydrolysis in water to produce a solution with a pH greater than 7 (a basic solution).

Step 2: Key Formula or Approach:
The acid-base nature of an aqueous salt solution depends entirely on the relative strengths of the parent acid and base from which the salt was derived:
- Strong Acid + Strong Base $\rightarrow$ Neutral solution ($pH \approx 7$).
- Strong Acid + Weak Base $\rightarrow$ Acidic solution ($pH < 7$).
- Weak Acid + Strong Base $\rightarrow$ Basic solution ($pH > 7$).

Step 3: Detailed Explanation:
Let's trace each salt back to its parent acid and base:
- (a) $NaNO_3$: Derived from $NaOH$ (Strong Base) and $HNO_3$ (Strong Acid). Neither ion undergoes hydrolysis. Solution is Neutral.
- (b) $CH_3COONH_4$: Derived from $NH_4OH$ (Weak Base) and $CH_3COOH$ (Weak Acid). Because their dissociation constants are roughly equal ($K_a \approx K_b$), they neutralize each other. Solution is Neutral.
- (d) $NH_4F$: Derived from $NH_4OH$ (Weak Base) and $HF$ (Weak Acid). HF is a stronger acid than $NH_4OH$ is a base, so this solution would be slightly Acidic.
- (c) KCN: Derived from $KOH$ (Strong Base) and $HCN$ (Very Weak Acid). The strong base dominates the character of the salt. The weak acid's conjugate base, the cyanide ion ($CN^-$), undergoes anion hydrolysis to produce free $OH^-$ ions in the water: $CN^- + H_2O \rightleftharpoons HCN + OH^-$. Solution is strongly Basic.

Step 4: Final Answer:
KCN forms a basic solution, matching option (c).