In blood (pH = 7.4), carbon dioxide is present as a buffer of carbonic acid and bicarbonate. Neglecting free CO2, what fraction will be present as carbonic acid? (pKa of carbonic acid = 6.3).
I used Henderson-Hasselbach equation and got H2CO3 = 0.079HCO3. For fraction, I assumed HCO3 = 1M.
So then the fraction is .079/(1+.079) = .074, which is 7.4 percent.
But the answer key states that it is 8.5 percent. Did I do something wrong?
1 Answer
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Just so you’ll know, there are no undissociated molecules of H2CO3 in any kind of aqueous solution solution. Molecular H2CO3 may exist in interstellar space, but not on Earth and certainly not in water. What we call “carbonic acid” is actually CO2 dissolved in water and in equilibrium with small amounts of H+ and HCO3^- ions.
CO2(aq) + H2O(l) <==> H+ + HCO3^- ………… Ka = 5.01×10^-7
