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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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ririkko ririkko
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11 years ago
I need to find the concentration of a carbonic acid solution that has a pH of 6.1.
I know that the pKa value is 6.3 for carbonic acid, how do I find the concentration?
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wrote...
#1
11 years ago
Write the acid dissociation of H2CO3 in water:

H2CO3 + H2O = HCO3- + H3O+

Ka = [HCO3-][H3O]/[H2CO3]

Since,
pH = -log [H3O]
6.1 = -log[H3O+]
[H3O+] = 7.943282347 x 10^-7

and
pKa = -logKa
6.3 = -logKa
Ka = 5.011872336 x 10^-7

Note that there is a 1:1 relationship between HCO3- and H3O+. Therefore [HCO3-]=[H3O+]
Plug in the values,

Ka = [HCO3-][H3O]/[H2CO3]
5.011872336 x 10^-7 = (7.943282347 x 10^-7)(7.943282347 x 10^-7 )/[H2CO3]
[H2CO3] = 1.25 x 10^-6

That's it!n_n
wrote...
#2
11 years ago
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