Let's make a solution with 0.10 M acetic acid and 0.10 M sodium acetate.

 

HOAc H+ + OAc-
K = 1.8x10-5 =	[H+][OAc-] [HOAc]

If [HOAc] = 0.10 M and [OAc^-] = 0.10 M what is [H⁺]?

  0.10 M     1x10^-7 M     1.8x10^-5 M