AP Chemistry
posted by Bill .
Consider 67.0 mL of a solution of weak acid HA (Ka = 1.00 106), which has a pH of 4.500. What volume of water must be added to make the pH = 5.000?

HA ==> H^+ + A^
Solve this equation to determine the (HA) at pH 4.50.
As a separate problem, plug in pH 5.000 and determine the (HA) for that value.
Then use the dilution formula using 67.0 mL of the first to determine how much to dilute it.
c1v1 = c2v2
Post your work if you get stuck. 
Thank you very much.