A laboratory buffer system consists of formic acid,

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

\mathrm{HCOOH}

, and sodium formate,

\mathrm{HCOONa}

. At

25\ ^\circ\mathrm{C}

, the

pK_a

of formic acid is

3.75

. The buffer is prepared so that the analytical molar concentrations are

[\mathrm{HCOOH}]=0.0400\ \mathrm{M}

and

[\mathrm{HCOO^-}]=0.0500\ \mathrm{M}

. Because the total ionic strength of the solution is

0.10\ \mathrm{M}

, the measured activity coefficients are

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

\gamma_{\mathrm{HCOOH}}=0.92

, and

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

. (a) Using activities rather than concentrations, calculate the pH of the buffer. (b) Calculate the pH predicted if activity coefficients are assumed to be unity, that is, if concentrations are 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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