Calculate the pH of a blood plasma sample with a total CO 2 concentration of 25.6 mM and bicarbonate concentration of 24.5 mM. The relevant p K a of carbonic acid is 6.1. Enter the answer with three significant figures.

pH = 7.45

Explanation:

The equilibrium that takes place in the buffer system is:

CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻ [CO₂] = [H₂CO₃] + [HCO₃⁻]

We first calculate [H₂CO₃]:

[H₂CO₃] = [CO₂] - [HCO₃⁻] [H₂CO₃] = 25.6 mM - 24.5 mM = 1.1 mM

The pH of the buffer according to the Henderson-Hasselbach equation would be:

pH = pKa + log [HCO₃⁻]/[H₂CO₃] pH = 6.1 + log 24.5/1.1 pH = 7.45