Q.11 Which one among the following mixtures gives a buffer solution in water?
(A) CH3COOH + CH3COONa
(B) CH3COOH + NaCl
(C) NaOH + NaCl
(D) NaOH + CH3COONa
CH3COOH + CH3COONa forms a buffer solution in water. This mixture of a weak acid and its conjugate base resists pH changes effectively. The correct answer is option (A).
Buffer Solution Basics
A buffer solution maintains stable pH when small amounts of acid or base are added. It requires either a weak acid with its conjugate base (acidic buffer) or a weak base with its conjugate acid (basic buffer).
Option Analysis
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(A) CH3COOH + CH3COONa: Acetic acid (weak acid, CH3COOH ⇌ CH3COO⁻ + H⁺) pairs with sodium acetate (provides CH3COO⁻ conjugate base). Added H⁺ reacts with CH3COO⁻ to form CH3COOH; added OH⁻ reacts with CH3COOH to form CH3COO⁻. This resists pH change, forming an acidic buffer.
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(B) CH3COOH + NaCl: Weak acid alone with neutral salt (NaCl → Na⁺ + Cl⁻, no buffering species). Added base consumes all CH3COOH without conjugate replenishment; pH shifts drastically.
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(C) NaOH + NaCl: Strong base (NaOH → Na⁺ + OH⁻) with neutral salt. No weak acid/base pair; fully dissociates, causing large pH swings with added acid/base.
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(D) NaOH + CH3COONa: Strong base reacts completely with CH3COO⁻ (hydrolyzes to basic solution). Excess OH⁻ dominates; lacks weak acid for buffering.
Introduction (with Keyphrase: “which mixture gives buffer solution in water”)
Searching for “which mixture gives buffer solution in water”? CH3COOH + CH3COONa stands out as the classic acidic buffer. This guide breaks down all options for CSIR NET aspirants, explaining why weak acid-salt pairs like acetic acid and sodium acetate maintain pH stability in aqueous solutions.
What Makes a Buffer Solution?
Buffer solutions resist pH changes via equilibrium between weak acids/bases and conjugates. Acidic buffers (pH < 7) use weak acid + conjugate base salt; pH ≈ pKa (Henderson-Hasselbalch: pH = pKa + log([salt]/[acid])). CH3COOH (pKa 4.76) + CH3COONa exemplifies this.
Why CH3COOH + CH3COONa Forms a Buffer
CH3COOH partially dissociates; CH3COONa supplies CH3COO⁻. Equilibrium shifts per Le Chatelier: H⁺ + CH3COO⁻ → CH3COOH (neutralizes acid); CH3COOH + OH⁻ → CH3COO⁻ + H2O (neutralizes base). pH stays near 4.76.
Why Other Mixtures Fail
| Mixture | Reason Not a Buffer |
|---|---|
| CH3COOH + NaCl | Neutral salt adds no conjugate; weak acid depletes easily |
| NaOH + NaCl | Strong base + neutral salt; no weak pair |
| NaOH + CH3COONa | Strong base hydrolyzes salt fully; excess OH⁻ |
CSIR NET Exam Tips
Focus on identifying weak acid/conjugate pairs. Practice: Buffer capacity peaks at [acid] = [salt]. Avoid strong acid/base mixes.
