What are the concentrations of acetic acid (pKa = 4.76) and acetate in a buffer

Question

What are the concentrations of acetic acid (pKa = 4.76) and acetate in a buffer solution of 0.15 M at pH 4.5? Enter your answer to two significant figures. (For example, 0.0090 and 0.0097 each have two significant figures, whereas 0.009 has only one significant figure.)

What are the concentrations of acetic acid (pKa 4.76) and acetate in a buffer solution of 0.15 M at pH 4.5? Enter your answer to two significant figures. (For example, 0.0090 and 0.0097 each have two significant figures, whereas 0.009 has only one significant figure.) H3C OH acetic acid acetate Number Number M M acetic acid acetate

Explanation / Answer

Apply:

henderson haselbach equation for buffers

pH = pKa + log(Ac-/HAc)

4.15 = 4.76 + log(Ac-/HAc)

4.50 = 4.76 + log(Ac-/HAc)

Ac- / HAc = 10^(4.50-4.76)

Ac- / HAc = 0.5495

and we know

Ac- + HAc = 0.15

so

Ac- = 0.5495*(HAc)

substitute

0.5495*(HAc)- + HAc = 0.15

(1.5495)*HAc = 0.15

0.15/(0.5495) = HAc

HAC = 0.2729 M

so

Ac- = 0.5495*0.2729 = 0.1499

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