Question

A buffer solution is prepared by mixing 26.6 mL of 0.0746 M ascorbic acid with 77.5 mL of 0.736 M sodium ascorbate.

1. Calculate the pH (to two decimal places) of this solution.

Assume the 5% approximation is valid and that the volumes are additive.

2. Calculate the pH (to two decimal places) of the buffer solution after the addition of 0.0647 g of sodium ascorbate (NaC₅H₇O₄COO) to the buffer solution above.

Assume 5% approximation is valid and that the volume of solution does not change.

Answer 1

1)

millimoles of ascorbic acid = 26.6 x 0.0746 = 1.984

millimoles of sodium ascorbate = 77.5 x 0.736 = 57.04

ascorbic acid pKa = 4.2

pH = pKa + log [salt / acid]

     = 4.2 + log [57.04 / 1.984]

     = 5.66

pH = 5.66

2)

millimoles of sodium ascorbate (C) = 0.0647 / 198.1 = 3.266 x 10^-4

                                                 = 0.3266 millimoles

pH = pKa + log [salt + C / acid - C ]

      = 4.20 + log [57.04 + 0.3266 / 1.984 - 0.3266]

      = 2.66

pH = 2.66