You can calculate the pH of a buffer solution or the concentration of the acid and base using the Henderson Hasselbalch equation. Here's a look at the Henderson Hasselbalch equation and a worked example that explains how to apply the equation.
Henderson-Hasselbalch EquationpH = pKa + log ([A-]/[HA])
[A-] = molar concentration of a conjugate base
[HA] = molar concentration of a undissociated weak acid (M)
The equation can be rewritten to solve for pOH:
pOH = pKb + log ([HB+]/[ B ])
[HB+] = molar concentration of the conjugate base (M)
[ B ] = molar concentration of a weak base (M)
Example Problem Applying the Henderson-Hasselbalch EquationCalculate the pH of a buffer solution made from 0.20 M HC2H3O2 and 0.50 M C2H3O2- that has an acid dissociation constant for HC2H3O2 of 1.8 x 10-5.
Solve this problem by plugging the values into the Henderson-Hasselbalch equation for a weak acid and its conjugate base.
pH = pKa + log ([A-]/[HA])
pH = pKa + log ([C2H3O2-] / [HC2H3O2])
pH = -log (1.8 x 10-5) + log (0.50 M / 0.20 M)
pH = -log (1.8 x 10-5) + log (2.5)
pH = 4.7 + 0.40
pH = 5.1