Question

Ascorbic acid (AscH2) , C₆H₆O₆H₂, MW = 176, better known as Vitamin C is a diprotic acid.

a) Write the reactions that occur when ascorbic acid is dissolved in water and the equilibrium expressions that go with the reactions.

b) Estimate the pH that results from the dissolving of 7.30 g of ascorbic acid in 400 ml water.

c) Determine the concentration of the ascorbate ion, C₆H₆O₆^2-, in the solution

Answer 1

C6H6O6H2 +   2 H2O ---------> C6H6O6 2-   + 2 H3O+

concentration of Ascorbic acid = (7.3 / 176) * (1000 / 400)

   = 0.1037

Ka1 = [H+] [C6H6O6H -] / [C6H6O6H2]

7.9 * 10^-5 = (x) * (x) / (0.1037 - x )

x = 2.822 * 10^-3 = [H+] = [C6H6O6H-]

C6H6O6H- -----------> H+ + C6H6O6 2-

Ka2 = [H+] [C6H6O6 2-] / [C6H6O6H -]

1.6*10^-12 = (x) (x) / (2.822 * 10^-3 - x)

x = 6.719 * 10^-8 = [C6H6O6 2-]

pH = -log (2.822 * 10^-3 + 6.719 *10^-8)

pH = 2.549