Question 6

Calculate the pH of a solution prepared by mixing equal volumes of 0.050 M sodium oxalate, Na2C2O4 (aq), and 0.010 M ammonium chloride, NH4Cl (aq).

For ammonium, Ka = 5.70 × 10–10; for oxalic acid, Ka,1 = 5.60 × 10–2 and Ka,2 = 5.42 × 10–5.

Hint #1: Use the charge balance equation as your main equation. Use the mass balance and equilibrium equations to find an expression for each species in terms of [H3O+] or [OH–], then substitute into the charge balance equation.

Hint #2: If no simplifying assumptions are obvious, you can always solve the exact final equation numerically using algebra and Excel. See the pre-exercise for a pair of methods.

I got 7.65 and I'm unsure as to why I'm wrong. I tried to calculate the OH- and H30 concentration for the two salts separately and determine pH from that.