Definitions and descriptions.docx

Definitions and descriptions: Buffer A buffer is a solution that resists pH changes when acids or bases are added to the solution. Most buffer solutions consist of a weak acid and its conjugate base (salt of the weak acid). Examples of Weak Acid Buffer Systems. Acetic acid Sodium acetateBoric acid Sodium borateCitric acid Sodium citratePhosphoric acid Potassium phosphate

Ca (M)buffer pH found with the pH calculator0.14.760.014.7610-34.7910-44.9510-55.4710-66.3110-76.89

Henderson-Hasselbalch Equation

pH = 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 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