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prepare a liter of "carbonate buffer" at a pH of 10.10

How would you prepare a liter of "carbonate buffer" at a pH of 10.10?
Ka = 4.2 times ten to the negative 7(carbonic acid)
Ka = 4.8 times ten to the negative 11(bicarbonate ion)
It says the key to solving the problem is picking the correct Ka value. How do I know which one to work with?

Solution Preview

First of all, we need to understand the equilibrium reactions for the dissociation of carbonic acid.
Carbonic acid's formula is H2CO3. It has two acidic protons. They dissociate in series like this:

H2CO3 + H2O <-----> HCO3- + H3O+

HCO3- + H2O <-----> CO3-2 + H3O+

The Ka for the first reaction is 4.2x10^-7.
The Ka for the second reaction is 4.8x10^-11.

To make a "carbonate buffer" at pH 10.10, do we want to have a mixture of H2CO3 (carbonic acid) and HCO3- (bicarbonate), OR do we want a mixture of HCO3- (bicarbonate) and CO3-2(carbonate)?

That's the question, isn't it? In other words, which of the two chemical reactions are we interested in exploiting, the first one or the second one?

How ...

Solution Summary

It shows how to prepare a liter of "carbonate buffer" at a pH of 10.10.