How can I calculate the dissociation constant?
A 0.031 mol/L solution of an unknown monoprotic acid has a pH of 4.69. What is the acid dissociation constant for this acid?
1. Write the equation for the dissociation of a generic monoprotic acid:
HA + H₂O ⇌ H₃O⁺ + A⁻
2. Write the dissociation constant expression.
3. Determine the equilibrium concentrations.
pH = -log[H₃O⁺] = 4.69
From the equation, [A⁻] = [H₃O⁺] = 2.04 × 10⁻⁵ mol/L
[HA] = (0.031 - 2.04 × 10⁻⁵) mol/L = 0.031 mol/L
4. Substitute these values in the expression and that is it!