Question

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.

Answer 1

pH = 7.45

Step-by-step 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