Be able to calculate the pH of a solution prepared by mixing 50.0mL of 0.200M NaH2PO4 (pKa2=7.20) with 50.0mL of 0.120M NaOH. I did work out a pH of 7.38 but I am not sure if i need to take into account any dilution of the mixing of solutions and do you need to know pKa1 to use the Henderson-Hasselbalch equation?

Answer 1

For this problem you do not need pKa1.(Note that pK1 has H3PO4 in it and that isn't one of the ingredients.)
I obtained the same answer and I did it with mols ONLY (which doesn't involve dilution and I like that way) but I also did it with molarity and took into account dilution. Same answer either way.So I think the answer is that you do not need to worry about dilution UNLESS you plug in concns in M (and not mols).