Solution
I found the solution this morning at lunch (glucose is always the best!). When posted my message, I made a mistake because I didn't read the text of the problem carefully.
In fact, I have just a solution (which is not a buffer one), that is [H3PO4] = 0.2 M.
Then I've to add HPO4-- in order to make the pH=7.30.
Here we are:
[H3PO4]=0.2 M
We have two different kind of reactions. The former is a neutralisation
H3PO4 + HPO4-- = 2H2PO4- => [H2PO4-]=0.4M and
[HPO4--]'=0.2M
The latter is the buffer solution
[H+] = Ka2 * [H2PO4-]/[HPO4--]
5.012E-8 = 6.32E-8 * 0.4/x => [HPO4--]'' = 0.504M
So, in the end, the total amount of HPO4-- required is
[HPO4--] = [HPO4--]' + [HPO4--]'' = 0.2 + 0.504 = 0.704M
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