Question

Oxalic acid, H2C2O4 has a pKa1 of 1.25 and a pKa2 of 4.14.

When sodium hydrogen oxalate is put into water, will the solution be acidic, basic, or neutral? Justify your answer with some equations and calculations and words.

Answer 1

For diprotic acid like Oxalic acid, H₂C₂O₄

H₂C₂O₄ H⁺ + HC₂O₄^-   pK_a1 = 1.25

HC₂O₄^- H⁺ + C₂O₄^2-    pK_a2 = 4.14

Sodium hydrogen oxalate, NaHC₂O₄ sits between H₂C₂O₄ and Na₂C₂O₄ in a titration curve of oxalic acid and sodium hydroxide.

Hence, the pH of NaHC₂O₄ is the between pH = pK_a1 and pH = pK_a2.

Thus, the pH of sodium hydrogen oxalate = (pK_a1 + pK_a2)/2 = (1.25 + 4.14)/2 = 2.70

Therefore, when sodium hydrogen oxalate is put into water, will the solution be acidic as pH of the solution will be 2.70 which is less than pH 7.