Question

calculate the ph of resulting solution when 2.50 ml of the 2.60 M acetic acid is diluted to make a 250.0 ml solution

Answer 1

Let's call acetic acid, CH3COOH, HAc. It saves time in typing.
(HAc) = 2.60M x (2.5/250) = 2.60 x (1/100) = 0.0260 M
Next set up an ICE table.
................HAc ==> H^+ + Ac^-
begin.........0.0260....0......0
change..........-x......+x....+x
final.......0.0260-x.....x......x

Ka = (H^+)(Ac^-)/(HAc)
Substitute from the ICE table into Ka expression and solve for x, then convert to pH.

Answer 2

Oh! wait but don't i have to plug it into ka=x^2/(0.0260-x) , ka of the acetic acid is 1.8 x 10^-5 and then solve for x ?