Please help. I have tried this a few different times and have come up with the same answer.

Question

What is the pH of the solution if 0.01 of HCl is added to a buffer containing 0.013 mol of RCOOH and 0.035 mol of RCOONa (source of RCOO-)? Ka = 2.7E-5

so far i have tried the following:
pH = pka + log ([base]/[acid])
pH = -log(2.7E-5) + log(0.035/0.013)
pH = 4.9987, rounded to 5.00

Am I missing something?
Amanda

Answer 1

Yes, you are solving for the pH of the solution BEFORE the 0.01 mol HCl is added. The problem is asking for the pH AFTER the addition of 0.01 mol HCl.

.......RCOO^- + H^+ ==> RCOOH
I.....0.035.....0........0.013
add...........0.01.............
C....-0.01...-0.01........+0.01
E.....0.025.....0........0.01

Now substitute the E line into the Henderson-Hasselbalch equation and solve for pH. Post your work if you get stuck. pKa should be approximately 4.6 or so.