pH of mixed acid solutions

[Prodan]

Don't suppose anyone can give me any pointers on solving this problem?

"What will the approximate pH of a buffer prepared by mixing 100 mL of 0.1M NaHPO4 and 100 mL of 0.1M NaH2PO4? (For H3PO4, pKa1 = 2.1, pKa2 = 6.8, pKa3 = 12.5)"

I've been trying to figure it out for a while, but I can't remember enough about mixed solutions, and I'm not finding the examples in my textbook helpful.

[Cyrion]

Have you tried using the Henderson-Hasselbalch equation? Calculate the initial concentrations of your acid and base, choose the correct pKa (should be pKa2) and plug and chug.

[Nomrom]

Fro some reason it makes me feel better, knowing that people from all over say plug and chug.

[Mando Knight]

Who doesn't?

[Prodan]

Have you tried using the Henderson-Hasselbalch equation? Calculate the initial concentrations of your acid and base, choose the correct pKa (should be pKa2) and plug and chug.
Alright, I figured out the problem. Turns out I made an error when checking the answer and looked at the wrong problem...

[Superglucose]

Don't suppose anyone can give me any pointers on solving this problem?

"What will the approximate pH of a buffer prepared by mixing 100 mL of 0.1M NaHPO4 and 100 mL of 0.1M NaH2PO4? (For H3PO4, pKa1 = 2.1, pKa2 = 6.8, pKa3 = 12.5)"
NaHPO4 should be NaHPO4- because PO4 is 3- not 2-. That means we have a buffer solution of conjugate base and its conjugate acid, rather than a mixed solution.

Initial concentrations are all we really need. Also the pKa of H2PO4- is 7.2 in the value I found.

So pH = 7.2 + log [1]/[1] = 7.2

[Prodan]

The problem gives it as 6.8 for both the answer and the pKa.

[Deth Muncher]

Fro some reason it makes me feel better, knowing that people from all over say plug and chug.

Agreed. :smallsmile: