Posted by Nat on .
The pH of a 0.10 M solution of a certain acid is 3.25 at 25°C. What is the pH of a 0.010 M solution of the same acid at the same temperature?

Chemistry 
DrBob222,
pH = log(H^+)
3.25 = log(H^+)
Solve for (H^+). I estimated 5E4 but you need to be more accurate than that.
..........HA ==> H^+ + A^
I.........0.1.....0.....0
C.........x.....x.....x
E......0.1x....x......x
and you know x = 5E4 from above (again that is an estimate)
Ka = (H^+)(A^)/(HA)
Solve for Ka.
Then .....HA ==> H^++ A^
I........0.01.....0....0
C.........x.....x......x
E......0.01x.....x.....x
Substitute the E line into Ka expression along with Ka determined from the first part and solve for x = (H^+), then convert to pH. 
Chemistry 
Nat,
Oh I see what I did wrong! I got the answer 3.75. Thanks for the help!