A buffer solution contains 0.492 M acetic acid and 0.241 M potassium acetate.

Question

A buffer solution contains 0.492 M acetic acid and 0.241 M potassium acetate.  

If 0.0302 moles of sodium hydroxide are added to 225 mL of this buffer, what is the pH of the resulting solution?  

(Assume that the volume does not change upon adding sodium hydroxide).

pH = 

Answer 1

Let HAc = acetic acid

Let Ac^- = acetate anion

HAc + OH- ===> Ac- + H2O

0.492.....0.0302.........0.241.........Initial

-0.0302...-0.0302......+0.0302.....Change

0.4618......0...............0.2712......Equilibrium

pH = pKa + log [Ac-]/[HAc]

pH = 4.74 + log (0.2712//0.4618)

pH = 4.74 + (-0.23)

pH = 4.51

Answer 2

Let HAc = acetic acid

Let Ac = acetate

HAc + OH^- ===> Ac^- + H2O

0.1107... 0.0302.....0.0543...........Initial

-0.0302.....-0.0302....+0.0302.......Change

0.0805........0............0.0845..........Equilibrium

pH = pKa + log [A-]/[HA]

pH = 4.74 + log (0.0845/0.0805)

pH = 4.76