Calculate the concentration of acetic acid, hac, and acetate ion, ac−, in a 0.25m acetate buffer solution with ph = 5.36. "0.25m acetate" means that the sum of the hac and ac− concentrations, the total concentration of acetate, = 0.25 m. the pka of hac is 4.76.

[HAc] = 0.05M

[Ac⁻] = 0.20M

Explanation:

The Henderson-Hasselbalch formula for the acetic acid buffer is:

pH = pka + log₁₀ [Ac⁻] / [HAc]

Replacing:

5.36 = 4.76 + log₁₀ [Ac⁻] / [HAc]

3.981 = [Ac⁻] / [HAc] (1)

Also, as total concentration of buffer is 0.25M it is possible to write:

0.25M =  [Ac⁻] + [HAc] (2)

Replacing (2) in (1)

3.981 = 0.25M - [HAc] / [HAc]

3.981 [HAc] = 0.25M - [HAc]

4.981 [HAc] = 0.25M

[HAc] = 0.05M

Replacing this value in (2):

0.25M =  [Ac⁻] + 0.05M

[Ac⁻] = 0.20M

I hope it helps!