Question

How much volume of 0.2 M solution of acetic acid should be added to 100 ml of 0.2 M solution of sodium acetate to prepare a Buffer solution of pH 5. pK_a for acetic acid is 4.74 ?

Answer 1

pH = pK_a + log\(\frac{[Salt]}{[Acid]}\)

log\(\frac{[Salt]}{[Acid]}\) = pH - pK_a

= 5 - 4.74

= 0∙26

log\(\frac{[Salt]}{[Acid]}\) = 0.26

\(\frac{[Salt]}{[Acid]}\) = 1.82

No. of moles of sodium acetate in the solution = \(\frac{100\times0.2}{1000}\) = 0.02 mol

Suppose volume of 0∙2 M acetic acid added

= V ml

No. of moles of acetic acid = \(\frac{V\times0.2}{1000}\)

= \(\frac{0.2}{V\times\frac{0.2}{1000}}\) = 1.82

V = 55 ml.