Chemistry 1011
TOPIC
Acids and Bases
TEXT REFERENCE
Masterton and Hurley Chapter 4.2 (Review), 13,
14.1, 15.1 (page 427), 21.2 (page589)
Chemistry 1011 Slot 5
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13.3 pH and pOH
YOU ARE EXPECTED TO BE ABLE TO:
• Define pH and pOH.
• Calculate the pH and pOH of a solution given
the concentration of hydrogen ions or the
concentration of hydroxide ions, and vice versa.
• Identify strong acids and bases
• Calculate the pH and pOH of solutions of strong
acids and bases of known concentration.
• Identify methods of measuring pH
Chemistry 1011 Slot 5
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Defining pH
• The acidity or basicity of a solution can be
described by the [H+] or [OH-]
• These concentrations can vary from 10-1 to
10-14
• The pH scale was proposed in order to
provide an easier means of describing
acidity
pH = -log10 [H+]
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Logarithms and pH Examples
• If a = bc, then logba = c
• If a = 10c, then log10a = c
• If [H+] = 10-1, then log10 [H+] = -1
or -log10 [H+] = 1
• What is the pH of a solution with a
hydrogen ion concentration of 10-2mol/L
pH = -log10 [H+] = -log10 [10-2] = 2
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pH Examples
• What is the pH of a water sample that has a
hydrogen ion concentration of 4.0 x 10-5?
pH = -log10 [H+] = -log10 [4.0 x 10-5] =
• What is the hydrogen ion concentration of a
solution with a pH of 5.6?
pH = -log10 [H+]
5.6 = -log10 [H+]
[H+] =
Chemistry 1011 Slot 5
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pOH
• pOH is determined in the same way as pH
pOH = -log10 [OH-]
• Since KW = [H+]x[OH-] = 1.0 x 10-14
It follows that pH + pOH = 14
• If a solution has a pH of 5.6, then the pOH
will be 8.4
• It will be basic
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Interpreting the pH Scale
• In an acidic solution,
[H+] > 1.0 x 10-7 [OH-] < 1.0 x 10-7
pH 0 to 7
• In a basic solution,
[H+] < 1.0 x 10-7 [OH-] > 1.0 x 10-7
pH 7 to 14
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Calculating the pH of a Strong
Acid
• Strong acids and bases are fully ionized in
dilute aqueous solution
HCl(aq)
H+(aq) + Cl-(aq)
NaOH(aq)
Na+(aq) + OH-(aq)
• The [H+] or [OH-] can be determined from
the [acid] or [base]
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Example - Calculating the pH of
a Strong Acid
• What is the pH of a 2.5 x 10-2 mol/L
solution of sulfuric acid, H2SO4(aq)?
H2SO4 is a strong acid
H2SO4
2H+(aq) + SO42-(aq)
[H+] = 2 x 2.5 x 10-2 mol/L
pH = -log10 [H+] = -log10 [5.0 x 10-2] =
Chemistry 1011 Slot 5
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Example - Calculating the pH of
a Strong Base
• What is the pH of a 5.0 x 10-2 mol/L solution of
barium hydroxide, Ba(OH)2(aq)?
Ba(OH)2 is a strong base
Ba(OH)2
Ba2+(aq) + 2OH-(aq)
[OH-] = 2 x 5.0 x 10-2 mol/L
pOH = -log10 [OH-] = -log10 [1.0 x 10-1] = 1
pH = 14 - pOH = 13
Chemistry 1011 Slot 5
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pH of Common Materials
•
•
•
•
•
•
•
•
Lemon Juice
Vinegar
Tomato Juice
Cheese
Cow’s Milk
Human Saliva
Human Blood
Seawater
2.2 - 2..4
3.0
4.0
4.8 - 6.4
6.3 - 6.6
6.5 - 7.5
7.3 - 7.5
8.3
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Measuring pH
• pH Meter
• Acid-base indicators
– fruit juices
– litmus
– universal indicator (a mixture of different
indicators that change colour at different pH’s)
– indicator paper
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