Buffers and Henderson-Hasselbach Equation

For more lessons, visit www.chalkbored.com Buffers - summary Solutions with buffers resist changes in pH, when small amounts of acid or base are added Buffers are important in blood, cells, resisting the effects of acid rain on lake ecosystems. For more lessons, visit www.chalkbored.com

A buffer is created when a weak acid is mixed with a salt that contains the identical ion. Two equilibria contribute to the consistent [H+] HA  H+ + A– NaA +  Na+ A–

Buffers Why do buffers work? Le Chatelier’s Principle Very little reaction occurs to change either concentration HA dissociates very little, and adding extra A- to the solution will make the HA dissociate even less. A- does not react with water, and addition of HA makes A- react even less.

pH Calculations for Buffers Henderson-Hasselbalch equation Merely a rearrangement of the Ka equilibrium expression pH = pKa + log ([A-] / [HA]) for acids pH = pKa + log ([B] / [BH+]) for bases

Henderson-Hasselbalch Equation Properties All Equilibria must be satisfied simultaneously in any solution at equilibrium For the pH to change by 1 unit, [A-] / [HA] must change by a factor of 10. If HA increases, the pH must decrease If A- increases, the pH must increase