Thread: What would be the pH of a solution prepared by combining equal quantities of NaH2PO4 and Na2HPO4?

Explain with an equation.

Please help! I have been over thinking the question for the past half hour. There are no numbers included in the question; this is all of the information I have besides some equations. However, I feel as if they are unneeded.

Henderson-Hasselbalch Equation:
pH = pKa + log([A-]/[AH])

For the first question,
2nd pKa of phosphoric acid is 7.2, and [A-]/[AH] = 1 (equimolar quantities), so
pKa = 7.2 + log(1) = 7.2.

For the second equation,
3rd pKa of phosphoric acid is 12.35, and neutralizing half is the same as [A-]/[AH] = 1, so
pKa = 12.35 + log(1) = 12.35.