Chapter 9 Acids, Bases, & Salts 9.13 Buffers

Buffers When an acid or base is added to water, the pH changes drastically. to a buffer solution, the pH is maintained; pH does not change.

Buffers Buffers resist changes in pH from the addition of acid or base. in the body, absorb H3O+ or OH- from foods and cellular processes to maintain pH. are important in the proper functioning of cells and blood. in blood maintain a pH close to 7.4. A change in the pH of the blood affects the uptake of oxygen and cellular processes.

Components of a Buffer A buffer solution contains a combination of acid-base conjugate pairs. may contain a weak acid and a salt of its conjugate base. typically has equal concentrations of a weak acid and its salt. may also contain a weak base and a salt of the conjugate acid.

Buffer Action In the acetic acid/acetate buffer with acetic acid (CH3COOH) and sodium acetate (CH3COONa) The salt produces acetate ions and sodium ions. CH3COONa(aq) CH3COO-(aq) + Na+ (aq) The salt is added to provide a higher concentration of the conjugate base CH3COO- than the weak acid alone. CH3COOH(aq) + H2O(l) CH3COO-(aq) + H3O+(aq) Large amount Large amount

Function of the Weak Acid in a Buffer The function of the weak acid in a buffer is to neutralize a base. The acetate ion produced adds to the available acetate. CH3COOH + OH− CH3COO− + H2O acetic acid base acetate ion water Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

Function of the Conjugate Base The function of the acetate ion CH3COO− is to neutralize H3O+ from acids. The acetic acid produced contributes to the available weak acid. CH3COO− + H3O+ CH3COOH + H2O acetate ion acid acetic acid water Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

Summary of Buffer Action Buffer action occurs as the weak acid in a buffer neutralizes base. the conjugate base in the buffer neutralizes acid. the pH of the solution is maintained.

Learning Check Which combination(s) make a buffer solution? A. HCl and KCl B. H2CO3 and NaHCO3 C. H3PO4 and NaCl D. CH3COOH and CH3COOK

Solution B. H2CO3 + NaHCO3 A weak acid and its salt D. CH3COOH + CH3COOK A weak acid and its salt.

The pH of buffers The pH of buffers can be calculated using Ka = or Kb = Where the equation are rearranged to solve for [H3O+]=[H+] or [OH-] [H+] = Ka or [OH-] = Kb Using properties of logarithm (log) concept pH = pKa + log or pOH = pKb + log [H3O+][A-] [OH-][HB+] [HA] [B] [B] [HA] [HB+] [A-] [HA] [B] [A-] [HB+] Henderson-Hasselbach equation

Learning Check Calculate the pH of a buffer solution that contains 0.10 mol of acetic acid (CH3COOH) and 0.10 mol of sodium acetate (CH3COONa) per liter. (pKa for acetic acid is 4.74 given in Table 9.9, pg 294 of your book) CH3COOH + H2O  CH3COONa + H3O+

Solution pH = pKa + log pH = 4.74 + log pH = 4.74 + log(1) = 4.74 Note that pH = pKa when the concentration of the acid and conjugate base are equal [CH3COOH] acid [CH3COONa] conjugate base [0.10 mol/L] [0.10 mol/L]

Learning Check If the Ka of an acid is 1.0x10-6, pH = 3.00, and the concentration of the unionized acid is 1.0x10-2 M, what is the concentration of the salt?

Solution Given: pH = 3.00, Ka = 1.0x10-6 M, and [HA] = 1.0x10-2 M solve for [H3O+] = 10-pH = 10-3 = 1.0x10-3 M Ka = rearrange equation to solve for the salt [A-] [A-] = = 1.0x10-6 M [A-] = 1.0x10-5 M [H3O+][A-] [HA] [HA]_ [1.0x10-2 M]_ Ka [H3O+] [1.0x10-3 M]