Question

Gr. 12 Chemistry (Ontario Curriculum)
Q. Oxalic acid, HOOCCOOH, is a weak diprotic acid that occurs naturally in some foods, including rhubarb. Calculate the pH of a solution of oxalic acid that is prepared by dissolving 2.5 g in 1.0 L of water. What is the concentration of hydrogen oxalate, HOOCCOO^- , in the solution? K_a = 5.6 x 10^-2 [1.69, 0.0204 M]
*The final answer is in Bold, but I need solution steps*

Answer 1

mass of oxalic acid = 2.5 g

moles of oxalic acid = 2.5 / 90.035 = 0.0278

concnetration = 0.0278 / 1 = 0.0278 M

H2C2O4   ---------------> H+   +     HC2O4-

0.0278                             0               0

0.0278 - x                         x                x

Ka = x^2 / 0.0278 - x

5.6 x 10^-2 = x^2 / 0.0278 - x

x = 0.0204

[H+] = [HOOCCOO^-] = 0.0204 M

pH = -log [H+] = -log (0.0204)

pH = 1.69

[HOOCCOO^-] = 0.0204 M