Question

You need to produce a buffer solution that has a pH of 5.03. You already have a solution that contains 10. mmol (millimoles) of acetic acid. How many millimoles of acetate (the conjugate base of acetic acid) will you need to add to this solution? The pKa of acetic acid is 4.74.

Answer 1

Explanation:

We have given the acid that is, acetic acid. It is known that acetic acid is a weak acid.

Therefore, formula that depicts relationship between pH, $pK_{a}$ and weak acid is as follows.

                  pH = $pK_{a}$ + log$\frac{[A^{-}]}{[HA]}$

where,          [HA] = concentration of weak acid

      $[A^{-}]$ = concentration of conjugate base of given weak acid

Since, we have to calculate the concentration of conjugate base of acetic acid. So, let it be equal to x. Whereas concentration of acetic acid is 10 mmol, $pK_{a}$ is 4.74 and pH is 5.03.

Hence, putting these values in the above formula as follows.

                  pH = $pK_{a}$ + log$\frac{[A^{-}]}{[HA]}$

                  5.03 = 4.74 + log$\frac{x}{10}$

              $\frac{x}{10}$ = antilog (0.29)

                      x = 19.4 mmol

Thus, we can conclude that there is 19.4 mmol acetate (the conjugate base of acetic acid) will be needed to add to this given solution.