A buffer is prepared by dissolving 0.025 M phenol (C_6H_5OH) and 0.015 M sodium phenoxide (C_6H_5ONa) in water at 25°C. Given $K_{a} (p h e n o l) = 1.3 \times 1 0^{- 10}$ , calculate the pH of the buffer.

hard 5 marks Class 12 · Chemistry · Alcohols Phenols and Ethers

Answer

9.67

The correct answer is 9.67 — 5. $p H = 9.89 + (- 0.22) \approx 9.67$ .

Step-by-step solution

1. Use the Henderson–Hasselbalch equation: $p H = p K_{a} + lo g \frac{[ A ^{-} ]}{[ H A ]}$ .
2. $p K_{a} = - lo g_{10} (1.3 \times 1 0^{- 10}) \approx 9.89$ .
3. Ratio $[A^{-}] / [H A] = 0.015/0.025 = 0.60$ .
4. $lo g (0.60) \approx - 0.22$ .
5. $p H = 9.89 + (- 0.22) \approx 9.67$ .

Common mistake: Using the wrong ratio or forgetting the negative sign in the log term

Concept tested: Buffer pH calculation (Henderson–Hasselbalch)