In Henderson-Hasselbalch relation for bicarbonate buffering, pH equals pKa plus log of which ratio?

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Multiple Choice

In Henderson-Hasselbalch relation for bicarbonate buffering, pH equals pKa plus log of which ratio?

Explanation:
This question tests applying the Henderson-Hasselbalch equation to the bicarbonate buffering system. For this buffer, the base is bicarbonate (HCO3−) and the acid is carbonic acid formed from dissolved CO2 (CO2(aq)). The equation is pH = pKa + log([base]/[acid]), so here pH = pKa + log([HCO3−]/[CO2(aq)]). In practice, dissolved CO2 is proportional to the arterial CO2 partial pressure (PCO2) via Henry’s law, so we often see pH ≈ pKa + log([HCO3−]/(0.03 × PCO2)). Thus the correct ratio to use is [HCO3−]/[CO2], not the reciprocal or involving H+ directly.

This question tests applying the Henderson-Hasselbalch equation to the bicarbonate buffering system. For this buffer, the base is bicarbonate (HCO3−) and the acid is carbonic acid formed from dissolved CO2 (CO2(aq)). The equation is pH = pKa + log([base]/[acid]), so here pH = pKa + log([HCO3−]/[CO2(aq)]). In practice, dissolved CO2 is proportional to the arterial CO2 partial pressure (PCO2) via Henry’s law, so we often see pH ≈ pKa + log([HCO3−]/(0.03 × PCO2)). Thus the correct ratio to use is [HCO3−]/[CO2], not the reciprocal or involving H+ directly.

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