The pH Scale
The Power or Potential of
Hydrogen
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2
pH scale
• the pH scale is a way of
expressing the strength of
acids and bases
• instead of using very small
numbers, we just use the
NEGATIVE power of 10 on the
Molarity of the H+ ion
• under 7 = acid
7 = neutral
over 7 = base
pH
[H+] ions in
scientific
notation
[H+] ions traditionally
0
1x100
1.0
1
1x10-1
0.1
2
1x10-2
0.01
3
1x10-3
0.001
4
1x10-4
0.0001
5
1x10-5
0.00001
6
1x10-6
0.000001
7
1x10-7
0.0000001
8
1x10-8
0.00000001
9
1x10-9
0.000000001
10
1x10-10
0.0000000001
11
1x10-11
0.00000000001
12
1x10-12
0.000000000001
13
1x10-13
0.0000000000001
14
1x10-14
0.00000000000001
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• pH of 1 is 100x
more acidic than
pH of 3
• pH of 4 is 1,000x
more basic than
pH of 1
• pH of 6 is 10,000
less concentrated
[H+] than pH of 2
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Properties of Water
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• water will self-ionize to a very small extent
• in pure water at 25°C, the [concentration] of H+
(or H3O+) and OH- ions are equal
– [H+] = [OH-]
– both have a concentration of 1.0 x 10-7 M
• the ionic product (the [H2O] is considered a
constant so left out) constant (Kw) for water at
298 K is:
Kw = [H+] [OH-]
= 1.0 x 10-7 x 1.0 x 10-7
= 1.0 x 10-14
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• acid solutions have a greater
concentration of H+ ions (or H3O+) than
OH- ions
• basic solutions are opposite and have
more OH-, referred to as alkaline
solutions when in water
pH calculations – Solving for pH
pH = - log [H+]
[H+] = 10-pH
• Example: If [H+] = 1 X 10-10, what is the pH?
– pH = - log 1 X 10-10
– pH = - (- 10)
– pH = 10
• Example: If [H+] = 1.8 X 10-5
– pH = - log 1.8 X 10-5
– pH = - (- 4.74)
– pH = 4.74
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Try These!
Find the pH of these:
1) A 0.15 M solution of hydrochloric
(HCl) acid
– pH = .82
2) A 3.00 X 10-7 M solution of nitric acid
(H2NO3)
– pH = 6.5
pH calculations – Solving for H+
• If the pH of Coke is 3.12,
what is the [H+]?
• 10-pH = [H+]
• [H+] = 10-3.12
•
= 7.6 x 10-4 M
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• A solution has a pH of 8.5. What is the
Molarity of hydrogen ions in the
solution?
pH = - log [H+]
10-8.5 = [H+]
3.16 X 10-9 = [H+]
solving for pOH
• since acids and bases are opposites, pH
and pOH are opposites
• pOH does not really exist, but it is
useful for changing bases to pH
• pOH looks at the perspective of a base
pOH = - log [OH-]
Since pH and pOH are on opposite ends,
pH + pOH = 14
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What is the pH of the 0.0010 M NaOH solution?
[OH-] = 0.0010 M
pOH = - log 0.0010
pOH = 3
pH = 14 – 3 = 11
13
pH
[H+]
[OH-]
pOH
14
pH testing
• There are several ways to test pH
–Blue litmus paper (turns red = acid)
–Red litmus paper (turns blue = basic)
–pH paper (multi-colored)
–pH meter (7 is neutral, <7 acid, >7
base)
–Universal indicator (multi-colored)
–Indicators like phenolphthalein
–Natural indicators like red cabbage,
radishes
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pH meter
• tests the voltage of the
electrolyte
• converts the voltage to
pH
• must be calibrated with
a buffer solution and
stored in a solution
pH indicators
• indicators are dyes that can be
added that will change color in
the presence of an acid or base.
• some indicators only work in a
specific range of pH
• once the drops are added, the
sample is ruined
• some dyes are natural, like radish
skin or red cabbage
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