To determine the molarities of $HC$ ₂H₃O₂ and $C$ ₂H₃O₂^- in relation to each other:

Substitute the given pH of the buffer into the Henderson-Hasselbach equation:

pH =

pK

_a + log { [base]/[acid] }

4.74 = -log (

1.8 x 10

^-5) + log { [

HC

₂H₃O₂] / [

C

₂H₃O₂^-] }

4.74 = 4.74 + log { [

HC

₂H₃O₂] / [

C

₂H₃O₂^-] }

0 = log { [

HC

₂H₃O₂] / [

C

₂H₃O₂^-] }

1 = [

HC

₂H₃O₂] / [

C

₂H₃O₂^-]

[

HC

₂H₃O₂] = [

C

₂H₃O₂^-]

Therefore, the concentrations of $HC$ ₂H₃O₂ and $C$ ₂H₃O₂^- are equal.