Q.Predict if the solutions of the following salts are neutral, acidic or basic: NaCl, KBr, NaCN, NH4NO3, NaNO2 and KF

Concept: Salt hydrolysis – the cation and anion of a salt can react with water. A salt's pH depends on the relative strengths of the parent acid and base from which it derives.

Reasoning:

Each salt dissociates completely. We examine whether the ions hydrolyze:

1. NaCl: $\text{Na}^+$ (from strong base NaOH) and $\text{Cl}^-$ (from strong acid HCl) – neither hydrolyzes → neutral

2. KBr: $\text{K}^+$ (from KOH, strong base) and $\text{Br}^-$ (from HBr, strong acid) – neither hydrolyzes → neutral

3. NaCN: $\text{Na}^+$ (non-hydrolyzable) and $\text{CN}^-$ (conjugate base of weak acid HCN) – $\text{CN}^- + \text{H}_2\text{O} \rightleftharpoons \text{HCN} + \text{OH}^-$ → basic

4. NH₄NO₃: $\text{NH}_4^+$ (conjugate acid of weak base NH₃) hydrolyzes: $\text{NH}_4^+ + \text{H}_2\text{O} \rightleftharpoons \text{NH}_3 + \text{H}_3\text{O}^+$; $\text{NO}_3^-$ (from strong acid HNO₃) does not → acidic …

When a salt dissolves in water, it dissociates into its constituent ions. Whether the resulting solution is acidic, basic, or neutral depends on how these ions interact with water—a process called salt hydrolysis. The key insight comes from Brønsted–Lowry theory: ions derived from weak acids or bases can act as conjugate bases or acids, respectively, and will react with water to shift the pH.

The strategy is simple: identify the parent acid and base that would combine to form each salt, then apply this rule:

• Strong acid + Strong base → neutral solution (neither ion hydrolyzes)
• Weak acid + Strong base → basic solution (anion is a strong conjugate base)
• Strong acid + Weak base → acidic solution (cation is a strong conjugate acid)
• Weak acid + Weak base → depends on relative strengths (compare $K_a$ and $K_b$)

Let's analyze each salt systematically.

1. NaCl (Sodium chloride)

Parent acid: HCl (strong acid)

Parent base: NaOH (strong base)

When NaCl dissolves, we get $\text{Na}^+$ and $\text{Cl}^-$. Sodium ion is the conjugate acid of a strong base, so it has negligible acidity. Chloride ion is the conjugate base of a strong acid, so it has negligible basicity. Neither ion hydrolyzes.

Solution: Neutral (pH ≈ 7)

2. KBr (Potassium bromide)

Parent acid: HBr (strong acid)

Parent base: KOH (strong base)

Both $\text{K}^+$ and $\text{Br}^-$ come from strong parents. By the same reasoning as NaCl, neither ion affects the pH.

Solution: Neutral (pH ≈ 7)

3. NaCN (Sodium cyanide)

Parent acid: HCN (weak acid, $K_a \approx 6.2 \times 10^{-10}$)

Parent base: NaOH (strong base)

The cyanide ion $\text{CN}^-$ is the conjugate base of the weak acid HCN, so it will accept protons from water:

This produces hydroxide ions, raising the pH. Sodium ion remains a spectator.

Solution: Basic (pH > 7)

4. NH₄NO₃ (Ammonium nitrate)

Parent acid: HNO₃ (strong acid)

Parent base: NH₃ (weak base, $K_b \approx 1.8 \times 10^{-5}$)

The ammonium ion $\text{NH}_4^+$ is the conjugate acid of the weak base ammonia, so it donates protons to water:

This generates hydronium ions, lowering the pH. Nitrate ion $\text{NO}_3^-$ is the conjugate base of a strong acid and does not hydrolyze.

Solution: Acidic (pH < 7)

5. NaNO₂ (Sodium nitrite)

Parent acid: HNO₂ (weak acid, $K_a \approx 4.5 \times 10^{-4}$)

Parent base: NaOH (strong base)

The nitrite ion $\text{NO}_2^-$ is the conjugate base of weak nitrous acid: …