[orgo814]

To make a phosphate buffer at pH 6.82 starting with one liter of 10 mM phosphoric acid (pKs are 2.15, 6.82, and 12.38), you could add:

The answer is 15 millimoles KOH. I'm having trouble finding out why it is 15 millimoles.

I've been trying to use the Henderson hasselbalch equation but keep getting 0.005 M (so 5 millimoles base). Could use a little help.

[AWK]

Hint
Write down a stepwise reaction of neutralization of H₃PO₄