The pH of human blood is steady at a value of approximately 7.4 owing to the following equilibrium reactions:
CO₂(aq)+ H₂O(l) ⇌ H₂CO₃ (aq) ⇌ HCO₃⁻(aq)+ H⁺(aq)
Acids formed during normal cellular respiration react with the HCO₃⁻ to form carbonic acid, which is in equilibrium with CO₂(aq) and H₂O(l). During vigorous exercise, a person’s H₂CO₃ blood levels were 26.3 mM, whereas his CO₂ levels were 1.63 mM. On resting, the H₂CO₃ levels declined to 24.9 mM. What was the CO₂ blood level at rest?