Answer:
pH = 1.28
Explanation:
The Oxalic acid (HOOC-COOH) that is in solution with water follows the equilibrium:
HOOC-COOH(aq) + H₂O(l) ⇄ HOOC-COO⁻(aq) + H₃O⁺(aq)
Where Ka₁ is defined as:
Ka₁ = 5.6x10⁻² = [HOOC-COO⁻] [H₃O⁺] / [HOOC-COOH] (1)
In equilibrium, the concentration of each specie must be:
[HOOC-COOH] = 0.100M - X
[HOOC-COO⁻] = X
[H₃O⁺] = X
Replacing in (1):
5.6x10⁻² = [X] [X] / [0.100 - X]
5.6x10⁻³ - 5.6x10⁻²X = X²
0 = X² + 5.6x10⁻²X - 5.6x10⁻³
Solving for X:
X = -0.1 → False answer, there is no negative concentrations.
X = 0.0519 → Right answer
That means [H₃O⁺] = 0.0519
As pH = -log [H₃O⁺]
pH = 1.28
