Transcript Electrolytes

Chapter 9 Acids, Bases, & Salts
9.13
Buffers
1
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.
2
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.
3
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.
4
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
5
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−
acetic acid
base
CH3COO− + H2O
acetate ion
water
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
6
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
acetic acid
acid
water
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
7
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.
8
Learning Check
Which combination(s) make a buffer solution?
A.
B.
C.
D.
HCl and KCl
H2CO3 and NaHCO3
H3PO4 and NaCl
CH3COOH and CH3COOK
9
Solution
B. H2CO3 + NaHCO3
A weak acid and its salt
D. CH3COOH + CH3COOK
A weak acid and its salt.
10
The pH of buffers

The pH of buffers can be calculated using
[H3O+][A-]
Ka =
[HA]
or
[OH-][HB+]
Kb =
[B]
Where the equation are rearranged to solve for
[H3O+]=[H+] or [OH-]
[H+]
[HA]
= Ka [A ]
or
[B]
[OH-] = Kb [HB+]
Using properties of logarithm (log) concept
[HA]
pH = pKa + log
or
[A-]
[B]
pOH = pKb + log
[HB+]
Henderson-Hasselbach equation
11
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+
12
Solution
pH = pKa + log [CH3COOH]
[CH3COONa]
acid
conjugate base
pH = 4.74 + log [0.10 mol/L]
[0.10 mol/L]
pH = 4.74 + log(1) = 4.74
Note that pH = pKa when the concentration of the acid
and conjugate base are equal
13
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?
14
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
[H3O+][A-]
Ka =
[HA]
rearrange equation to solve for the salt [A-]
-2 M]_
[HA]_
[1.0x10
[A-] = Ka
= 1.0x10-6 M
+
[H3O ]
[1.0x10-3 M]
[A-] = 1.0x10-5 M
15