An important component of blood is the buffer combination ofbicarbonate ion and

Question

An important component of blood is the buffer combination ofbicarbonate ion and carbonic acid. Consider blood with a pH of7.42. (a) What is the ratio of[H2CO3] to [HCO3 ]?


(b) What does the pH become if 18% of thebicarbonate ions are converted to carbonic acid?


(c) What does the pH become if 20.% of thecarbonic acid molecules are converted to bicarbonateions?
(a) What is the ratio of[H2CO3] to [HCO3 ]?


(b) What does the pH become if 18% of thebicarbonate ions are converted to carbonic acid?


(c) What does the pH become if 20.% of thecarbonic acid molecules are converted to bicarbonateions?

Explanation / Answer

a) The ratio is .089 to 1
The pH of a buffer can be calculated with this equation:
pH = pKa + log([A-]/[HA])
Where
[A-] = conjugate base
[HA] = weak acid
In this case
[A-] = HCO3-
[HA] = H2CO3
In the example:
7.42 = pKa + log(1/0.089)
pKa = 6.37
So if 18% is covnerted to carbonic acid, then you have 82%HCO3-
pH = 6.37 + log(.82/(.18+.089)) = 6.85 = 6.9 (2 sigfigs)
What does the pH become if 20.% of the carbonic acid molecules areconverted to bicarbonate ions?
pH = 6.37 + log(1.0178/(.089-.0178)) = 7.53 = 7.5 (2 sigfigs)
It's convenient to think of this system as H2CO3 and HCO3, butthat's not what's happening on the chemical level.
If you want to impress your teacher, tell him/her that in aqueoussolution carbonic acid only exists in equilibrium with carbondioxide, and the concentration of H2CO3 there is much lower thanthe CO2 concentration:
CO2 + H2O <-----> HCO3 + H+

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