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pH scalepH scale
• Logarithmic scale expressing the H+ concentration, [H+].
• If the pH changes by a factor of 1, the [H+] changes by a factor of 10.
• pH = -log[HpH = -log[H++]]
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Water & Self-Ionization
• H2O (l) + H2O (l) H3O+ (aq) + OH-
(aq)
– HH33OO++ is called hydronium ion
– OHOH-- is called hydroxide ion
• In reality, H+ and H3O+ are used interchangeablyinterchangeably.
• HH++ is called a proton or a hydrogen ion.
-
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pH of Water
• Water ionizes to produce both the hydronium ion and the hydroxide ion in equal amounts.
• Ionization happens to a very small extent
• [H+] = [OH-] = 1 X 10[H+] = [OH-] = 1 X 10-7-7 M M • At this concentration:At this concentration:
pH = -log[H+] = 7
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pH to Molarity
• pH = -log[H+]. Solve for [H+].
• -pH = log[H+]
• Antilog(-pH) = [H+]
• Say pH = 5. Then –pH = -5.
• Antilog(-5) = 10-5.
• The –pH becomes the power of 10!The –pH becomes the power of 10!
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•If the pH = 4If the pH = 4
• The [H+] = ?
• The pOH = ?
• The [OH-] = ?
1010
Antilog(-pH) = [HAntilog(-pH) = [H++]]Antilog(-4) = 10Antilog(-4) = 10-4-4
Antilog(-10) = [OHAntilog(-10) = [OH--]]1010-10-10 = [OH = [OH--]]
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Difference in pH = 22, which means
the [H+] differ by 1022 or 100100.
The pH of 5 solution has an [H+] that is 100 X greater than that of the pH 7 solution.
• If the pH changes from 5 to 7, how do the [H+] compare?
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Significance of pH differences
• Difference in pH =5 and pH =7
– Difference is 2 units of pH
• The [H+] differ by 1022 or 100100
• The pH of 5 solution has an [H+] that is 100 X greater than that of the pH 7 solution.
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pH
[H+]1 M
or
1X1000 M
0.1 M
or
1 X 10-1-1
M
0.01 M
or
1 X 10-2-2
M
.001 M
or
1 X 10-3-3
M
pH 00 11 22 33
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Ion Product Constant for Water
• Value of the equilibrium constant expression for the self-ionization of water.
H2O (l) + H2O (l) H3O+ (aq) + OH-
(aq)
Keq= [H+] [OH-]
[H2O]
Kw= [H+] [OH-] = 10-14
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•If the pH = 2
The [H+] = ? 1 x 10-2M
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• If the [H+] = 1 X 10-5 M
• The pH = ? 5
How many time stronger is an acidic solution withA pH of 2 than an acidic solution with a pH of 5?
PROBLEM: Relative acid strength
1000x
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pOH
• By analogy, pOH is defined as –log[OH-]
• Express [OH-] in scientific notation
• If [OH-] = 1.0 X 10-3 M
• Then pOH = -log(10-3) = -(-3) = 3.
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• If the [OH-] = 1 X 10-3
• The pOH = ?
• The pH = ?
• The [H+] = ?
pOH = -log[OHpOH = -log[OH--]]= -log(10= -log(10-3-3) = --3 = 3) = --3 = 3
14 – 3 = 1114 – 3 = 11
Antilog(-11) = 10Antilog(-11) = 10-11-11 M M
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• If the [H+] = 1 X 10-5 M
• The pH = ?
• The pOH = ?
• The [OH-] = ?
55
14 – 5 = 914 – 5 = 9
Antilog(-9) = 10Antilog(-9) = 10-9-9
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pH + pOH = 14pH + pOH = 14
• pH = 3, pOH =
• pH = 7, pOH =
• pH = 10, pOH =
• pH = 9, pOH =
• pH = 1, pOH =
1111
77
44
55
1313
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Acid, Base, or Neutral?
• All H2O contains some H+ and some OH-
• Pure H2O – concentrations are very low.
• [H+] = [OH-] = 1 X 10-7 Molar.
• When [H+] = [OH-], solution is neutral
• Acidic solution: H+ > OH-
• Basic solution: OH- > H+
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[H+] pH [OH-] pOH Acidic or
Basic
3
10-5 M
10-2 M
6
11111010-3-3 1010-11-11 AA
55991010-9-9 BB
22 12121010-12-12 M M AA
881010-8-8 M M 1010-6-6 M M BB
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How to safely test pH
• Instrumental– use a pH meter
• Indicators– use a series of indicators
• Reaction– Acid if reacts with metals
(other than Cu, Ag, or Au)
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Indicator
• A substance that changes color over a narrow pH range.
• Use several indicators to narrow down the pH range of the substance.
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