Chemistry

Chemistry

Problem

Four 0.50 L acetate buffer solutions are prepared, each having a combined buffer component concentration of 1.0 M. After 0.020 mol of solid NaOH is dissolved in each beaker, which buffer will experience the smallest change in pH? a) [CH3COOH]=0.75 M;  [CH3COO−]=0.25 M[CH_{3}COOH] = 0.75\,\text{M};\; [CH_{3}COO^{-}] = 0.25\,\text{M} b) [CH3COOH]=0.60 M;  [CH3COO−]=0.40 M[CH_{3}COOH] = 0.60\,\text{M};\; [CH_{3}COO^{-}] = 0.40\,\text{M} c) [CH3COOH]=0.50 M;  [CH3COO−]=0.50 M[CH_{3}COOH] = 0.50\,\text{M};\; [CH_{3}COO^{-}] = 0.50\,\text{M} d) [CH3COOH]=0.10 M;  [CH3COO−]=0.90 M[CH_{3}COOH] = 0.10\,\text{M};\; [CH_{3}COO^{-}] = 0.90\,\text{M}
Intuitive explanation

The key is recognizing that buffer capacity—how much acid or base a buffer can neutralize before its pH shifts significantly—depends on the absolute amount of the weak acid component, not just the ratio of acid to conjugate base. When NaOH is added, it consumes the weak acid (CH3COOH\text{CH}_3\text{COOH}), so buffers with more weak acid can absorb more base before the ratio becomes too skewed. Since all solutions have the same total concentration (1.0 M) and volume (0.50 L), the buffer with the highest proportion of acetic acid—closest to pure weak acid rather than mostly its salt—will have the most weak acid available to neutralize the added OH−^−, resulting in the smallest pH change.

Intuitive explanation

The key is recognizing that buffer capacity—how much acid or base a buffer can neutralize before its pH shifts significantly—depends on the absolute amount of the weak acid component, not just the ratio of acid to conjugate base. When NaOH is added, it consumes the weak acid (CH3COOH\text{CH}_3\text{COOH}), so buffers with more weak acid can absorb more base before the ratio becomes too skewed. Since all solutions have the same total concentration (1.0 M) and volume (0.50 L), the buffer with the highest proportion of acetic acid—closest to pure weak acid rather than mostly its salt—will have the most weak acid available to neutralize the added OH−^−, resulting in the smallest pH change.

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