# How can I calculate the dissociation constant?

##### 1 Answer

You need some information to calculate a dissociation constant. Here's how to calculate the dissociation constant of a weak acid from the molarity and the pH.

**Example**

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?

**Solution**

**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

[H₃O⁺] =

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!