Making the ICE table for the dissociation of oxalic acid:
H₂C₂O₄ --> H⁺ + HC₂O₄⁻
0.0321 0 0
-x + x +x
0.0321-x +x +x
Kₐ = x²/ 0.0321 –x
pKₐ = - logKₐ
Kₐ = e⁻pKₐ = e⁻1.23 = 0.3
0.3 = x²/ 0.0321 – x
0.3 (0.0321 – x) = x²
-x² -0.3 x + 9.63 x 10⁻³
Using the quadratic formula:
a = -1, b = - 0.3, c = 9.63 x 10⁻³
x = - 0.33, 0.03
Using Henderson-Hasselbalch equation
pH = pKₐ + log [conjugate base]/ [acid]
pH = pKₐ₁ + log [HC₂O₄⁻]/ [H₂C₂O₄]
pH = 1.23 + log (0.03/0.0321) = 1.23 - 0.03 = 1.20
Thus the pH of a 0.0321 M solution of oxalic acid is 1.20
