Transcript Acids and Bases

The pH Scale
The power or potential of
Hydrogen
Topic 8.3
The Nature Of Science (NOS)
• Occam’s razor
–a scientific and philosophic rule requiring
that the simplest of competing theories be
preferred to the more complex
–the pH scale was proposed to simplify the
comparison of very, very low
concentrations of H+ ions in solution
relative to water
pH scale
• the pH scale is a way of
expressing the strength of
acids and bases
• instead of using very small
numbers for the
concentration of H+, we just
use the negative power of 10
on the Molarity of the H+ ion
• no units
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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
• 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
Properties of Water
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• water will self-ionize to a very small extent
or
• 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
• Kc?
• the ionic product (the [H2O] is considered
a constant so it is left out)
• (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
• acid solutions have a greater
concentration of H+ than OH• basic solutions are opposite and have
greater concentration of OH-
Solving problems involving H+, OH-, pH,
and pOH
pH
[H+]
[OH-]
pOH
Solving for pH when given [H+]
pH = - log [H+]
• 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
3) A 1.7 X 10-3 M solution of nitric acid
(H2NO3)
•
pH = 2.77
Solving for [H+] when given pH
[H+] = 10-pH
• If the pH of Coke is 3.12,
what is the [H+]?
• [H+] = 10-pH
• [H+] = 10-3.12
= 7.6 x 10-4 M
• 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+]
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Solving for pOH when given [OH-]
• since acids and bases are opposites, pH
and pOH are opposites
• pOH looks at the perspective of a base
pOH = - log [OH-]
• since pH and pOH are on opposite ends,
pH + pOH = 14
What is the pH of the 0.0015 M NaOH solution?
[OH-] = 0.0015 M
pOH = - log 0.0015
pOH = 2.82
pH = 14 – 2.82 = 11.18
Solving for [OH-] when given pOH
[OH-] = 10-pOH
• If the pOH of a solution is 11.5, what is the
[OH-]?
• [OH-] = 10-pOH
• [OH-] = 10-11.5
= 3.16 x 10-12 M
– On a side note- remember: [H+] [OH-] = 1.0 x 10-14
» (x) 3.16 x 10-12 = 1.0 x 10-14
» therefore [H+] would be 3.2 x 10-3
Yet another problem
• A solution of fresh milk has a pH of
6.7. Calculate [H+] and [OH-].
• [H+] = 10-6.7
• [H+] = 1.99 x 10-7 mol dm-3
• pOH is 7.3
(14 - 6.7 = 7.3)
• [OH-] = 10-7.3
• [OH-] = 5.01 x 10-8 mol dm-3
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pH testing
• There are several ways to test pH
–Blue litmus paper (turns red = acid)
–Red litmus paper (turns blue = basic)
–pH (Hydrion) paper (multi-colored)
–pH meter
–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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