Transcript Function of the Conjugate Base

Function of the Conjugate
Base
• The function of the acetate ion
C2H3O2 is to neutralize added H3O+.
• The acetic acid produced by the
neutralization contributes to the
available weak acid.
C2H3O2(aq) + H3O+(aq)
HC2H3O2(aq) + H2O(l)
acetate ion
acid
acid
water
acetic
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Summary of Buffer Action
Buffer action occurs because
the weak acid in a buffer neutralizes •
base
the conjugate base in the buffer •
neutralizes acid
the pH of the solution is maintained •
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pH of a Buffer
The [H3O+] in the Ka expression is used to determine the pH of a buffer.
Weak acid + H2O
H3O+ + conjugate base
Ka =
[H3O+][conjugate base]
[weak acid]
[H3O+] = Ka x [weak acid]
[conjugate base]
pH = log [H3O+]
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Guide to Calculating pH of a
Buffer
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Example of Calculating
Buffer pH
The weak acid H2PO4 in a blood buffer H2PO4/HPO42
has a Ka = 6.2 x 108. What is the pH of the buffer if
[H2PO4] = 0.20 M and [HPO42] = 0.20 M?
STEP 1 Write the Ka expression for:
H2PO4(aq) + H2O(l)
HPO42(aq) + H3O+(aq)
Ka = [HPO42][H3O+]
[H2PO4]
STEP 2 Rearrange the Ka for [H3O+]:
[H3O+] = Ka x
[H2PO4]
[HPO42]
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Example of Calculating
Buffer pH (continued)
STEP 3 Substitute [HA] and [A]:
[H3O+] = 6.2 x 108 x [0.20 M] =
6.2 x 108
[0.20 M]
STEP 4 Use [H3O+] to calculate pH:
pH =  log [6.2 x 10 8] = 7.21
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Learning Check
What is the pH of a H2CO3 buffer that is
0.20 M H2CO3 and 0.10 M HCO3?
Ka(H2CO3) = 4.3 x 107
1) 6.67
2) 6.37
3) 6.07
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Solution
What is the pH of a H2CO3 buffer that is 0.20 M H2CO3
and 0.10 M HCO3?
Ka(H2CO3) = 4.3 x 107
STEP 1 Write the Ka expression for:
H2CO3(aq) + H2O(l)
HCO3(aq) + H3O+(aq)
Ka = [HCO3][H3O+]
[H2CO3]
STEP 2 Rearrange the Ka for [H3O+]:
[H3O+] = Ka x
[H2CO3]
[HCO3]
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Solution (continued)
STEP 3 Substitute [HA] and [A]:
[H3O+] = 4.3 x 107 x [0.20 M] = 8.6
x 107 M
[0.10 M]
STEP 4 Use [H3O+] to calculate pH:
pH =  log [8.6 x 107] = 6.07
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