Chemistry

Problem

A laboratory buffer system consists of formic acid (HCOOH) and sodium formate (HCOONa). At 25 °C the

pK_a

of formic acid is 3.75. The buffer is prepared so that the analytical (molar) concentrations are

[\text{HCOOH}]=0.0400\,\text{M}

and

[\text{HCOO}^-]=0.0500\,\text{M}

. Because the total ionic strength of the solution is 0.10 M, the mean activity coefficients for the species have been measured:

\gamma_{\text{H^+}}=0.83

\gamma_{\text{HCOOH}}=0.92

, and

\gamma_{\text{HCOO}^-}=0.76

. (a) Using activities rather than concentrations, calculate the pH of the buffer. (b) Calculate the pH that would be predicted if activity coefficients were assumed to be unity (i.e., if concentrations were used directly). (c) Determine the difference in pH between parts (a) and (b). Explain why including activity corrections produces a more accurate value, citing the role of ionic strength in your reasoning.

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