Oxalic acid is the first in the series of dicarboxylic acids (HOOC-COOH, H2C2O4). It occurs naturally in many plants. Oxalic acid content is high in the leaves of rhubarb (we don't eat the leaves because they are poisonous). Calculate the pH of a 0.100 M oxalic acid solution. Ka1 = 5.6x10-2, Ka2 = 5.4x10-5 Enter to 2 decimal places.

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

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