What is the pH of a buffer solution that is 0.15 M propionic acid and 0.20M sodium propionate?

How do I start this problem

4 answers

Use the Henderson-Hasselbalch equation.
pH = pKa + log[(base)/(acid)]
Show your work if you get stuck.
well the only problem im having is finding the pKa
My quant book lists 1.34 x 10^-5 for Ka. And Wikipedia lists a pKa of 4.88 which is almost the same, but not quite. Take your pick.
i think i will choose the 4.88 thank you