1. The problem statement, all variables and given/known data What is the pH of a buffer solution created by combining 100 mL of 0.2 M acetic acid and 400 mL of 0.10 M sodium acetate? Ka= 1.8 x 10^(-5) 2. Relevant equations 1.8 x 10-5=([C2H3O2-][H+])/([HC2H3O2]) 3. The attempt at a solution I know what I'm supposed to do to solve this problem, but I'm not sure how to set up the concentrations in the expression. I think it should go like this: 1.8 x 10-5= (0.08+x)(x)/(0.04-x) The explanation in my book has basically the same thing with just one tiny exception. In the equilibrium expression, it used only 0.08 as a concentration for C2H3O2-, without the x. I thought the x would be necessary b/c 0.08 is only the initial concentration of the acetate ion and the acetic acid will dissociate more before reaching the equilibrium. Any help would be nice. Thanks in advanced.