Henderson-Hasselbalch Equation and Example

You can calculate the pH of a buffer solution or the concentration of the acid and base using the Henderson-Hasselbalch equation. Heres a look at the Henderson-Hasselbalch equation and a worked example that explains how to apply the equation. Henderson-Hasselbalch Equation The Henderson-Hasselbalch equation relates pH, pKa, and molar concentration (concentration in units of moles per liter): apH pK log ([A-]/[HA]) [A-] molar concentration of a conjugate base [HA] molar concentration of an 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 Equation Calculate 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