Question

1.Suppose it is desired to maintain the pH of a solution at 4.2 using only acetic acid and sodium acetate, describe in detail how this could be done. Specify the concentration of each reagent needed.

2.Can you suggest how a solution of acetic acid- sodium acetate could be maintained at pH = 5.2? Specify the concentration of each reagent needed.

Answer 1

1. Acetic acid (HAc) undergoes dissociation as below:

HAc (aq) <====> Ac^- (aq) + H⁺ (aq)

NaAc is the source of Ac^-; find the ratio of concentrations of Ac^- and HAc using the Henderson-Hasslebach equation. We can find out the ratio of the concentrations of the acid and the conjugate base.

pK_a of HAc is 4.75; use the Henderson equation as below:

pH = pK_a + log [Ac^-]/[HAc]

===> 4.2 = 4.75 + log [Ac^-]/[HAc]

===> -0.55 = log [Ac^-]/[HAc]

===> [Ac^-]/[HAc] = antilog (-0.55) = 0.28

===> [Ac^-] = 0.28*[HAc]

We need to know the total acetate concentration to proceed further.