A 1.41 L buffer solution consists of 0.253 M propanoic acid and 0.110 M sodium propanoate. Calculate the pH of the solution following the addition of 0.061 mol HCl . Assume that any contribution of the HCl to the volume of the solution is negligible. The K a of propanoic acid is 1.34 × 10 − 5 .

pH = 5.01

Explanation:

The reaction between the sodium propanoate and the HCl added is the following:

                 CH₃CH₂COO⁻  +  H₃O⁺   ⇄   CH₃CH₂COOH + H₂O      

initial            0.110M          0.061moles       0.253M

The number of moles of the acid propanoic (a) and sodium propanoate (b) is:

After the adding of HCl, the number of moles of acid propanoic and propanoate is:

Hence, the pH of the solution after the addition of HCl is:

Therefore, the pH of the solution is 5.01.

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