Write the acid dissociation equation: HA ⇌ H⁺ + A⁻
Set up the Ka expression: Ka = [H⁺][A⁻] / [HA]
Let the initial acid concentration be C
Let the amount that dissociates be x
At equilibrium: [H⁺] = x, [A⁻] = x, [HA] = C – x
Substitute into the Ka expression: Ka = x² / (C – x)
Solve for x
Set x = [H⁺]
Calculate pH using: pH = -log[H⁺]
If x is small compared with C, use the approximation: Ka ≈ x² / C
Then [H⁺] ≈ √(Ka × C)
Then pH = -log(√(Ka × C))