Answer:
Carbonic acid/bicarbonate; bicarbonate
Explanation:
H₂CO₃ + H₂O ⇌ H₃O⁺ + HCO₃⁻; pKₐ₁ = 6.35
HCO₃⁻ + H₂O ⇌ H₃O⁺ + CO₃²⁻; pKₐ₂ = 10.33
[tex]\text{pH}& = &\text{pK}_{\text{a}} + \log\dfrac{[\text{A}^{-}]}{\text{[HA]}}\\\\7.4 & = &\text{pK}_{\text{a}} + \log\dfrac{[\text{A}^{-}]}{\text{[HA]}}[/tex]
The best buffer is one for which pKₐ ≈ pH.
6.35 is closer to 7.4, so the carbonic acid/bicarbonate form of the buffer predominates
The pH of the blood is higher (more basic) than the pKₐ of carbonic acid, so its basic form (bicarbonate, HCO₃⁻) predominates.
