Question

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.

Answer 1

[HAc] = 0.05M

[Ac⁻] = 0.20M

Step-by-step 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!