Welcome
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Saturday School: 10-11ish
Monday: Cold Call Quiz
I have two beakers
Beaker 1 – 1 L of pure
water
 Beaker 2 – 1 L of human
blood
 I pour 5 mL of NaOH in
the pure water and the pH
goes from 7 up to 13.2
 I pour 5 mL of NaOH into
the blood and it goes from
a pH of 7.2 to 7.3
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I have the same two
beakers
Beaker 1 – 1 L of pure
water
Beaker 2 – 1 L of human
blood
I pour 5 mL of HCl in the
pure water and the pH goes
from 7 up to 2.2
I pour 5 mL of NaOH into
the blood and it goes from
a pH of 7.2 to 7.1
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A buffer is any solution that resists changes in pH
Two components of a buffer:
1. A component that neutralizes an acid
2. A component that neutralizes a base
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A weak acid or a weak base are capable of
creating a buffer because they have both of these
components, but a strong acid or base cannot create
a buffer.
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A weak acid or base make an
excellent buffer because they
have a component that can
react with an acid and a
component that
⇌ can react
with a base.
CH3COOH + H2O
Can
with
a H+ Can react with an
-+
CHreact
COO
3
base!
acid!
Therefore, reacting each component
produces a part of the equilibrium and
little change in pH is observed!
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The best buffers have close to equal concentrations
of the conjugate acid/base pair.
We can think about the Ka expression for a weak acid
[H + ][ A - ]
Ka =
[HA]
[HA]
+
[H ] = K a
[ A- ]
Therefore, pH is determined by the ratio of
conjugate acid/base pair and the value of Ka.
As long as the change in ratio of [HA]/[A-] is
small, the change in pH will be small.
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Adding base to a buffer shows
minimal change
Adding base to a neutral
solution causes huge changes
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Could each of the following solutions can be
classified as buffer systems? Why/why not?
1. KH2PO4/H3PO4
2. NaClO4/HClO4
3. C5H5N/C5H5NHCl (C5H5N is a base)
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Students explore using
http://www.mhhe.com/physsci/chemistry/ess
entialchemistry/flash/buffer12.swf
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Calculate the pH of a buffer that is 0.12 M lactic acid
and 0.10 M sodium lactate. The Ka for lactic acid is
1.4 x 10-4
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Calculate the pH of a buffer composed of 0.12 M
benzoic acid and 0.20 M sodium benzoate. The Ka
for the solution is 6.4 x 10-5
The pH at which any buffer works most effectively is
when
pH = pKa
 This is known at the ½ equivalence point
 Buffers usually have a useable range within ±1 pH
unit of the pKa
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