What is the pH?

Question

A buffer containing 1.0564 M of acid, HA, and 0.1416 M of its conjugate base, A-, has a pH of 3.30. What is the pH after 0.0017 mol NaOH is added to 0.5000 L of this solution?

Answer 1

pH = 3.30 means [H+] = 10^-3.30 M. Also, we may figure out Ka with the data given before adding NaOH into the solution:
HA <==> H+ + A-,
Ka = [H+]*[A-]/[HA] = (10^-3.30)*0.1416/1.0564.

After 0.0017 mol NaOH is added to 0.5000 L of this solution, we have:
[HA] = 1.0564M - 0.0034M = 1.053M
[A-] = 0.1416M + 0.0034M = 0.145M
Therefore, [H+] = Ka*[HA]/[A-]
= (10^-3.30)*0.1416*1.053/(1.0564*0.145)
= 4.879x10-4(M)
Hence the final pH = -log(4.879x10-4) = 3.31