Hydrogen Ions and
Acidity
The Ionization of Water and pH
Objectives
• When you complete this presentation, you will be able
to:
• describe the autoionization of water
• determine the hydrogen ion concentration in water
when given the hydroxide ion concentration in water
• determine the hydroxide ion concentration in water
when given the hydrogen ion concentration in water
• determine pH or pOH of a solution when given the
hydrogen ion or hydroxide ion concentration
• determine the hydrogen ion or hydroxide ion
concentration when given the pH or pOH
Hydrogen Ions in Water
• We are used to thinking of water as a pure liquid
that contains only H2O molecules.
• However, as a pure liquid, water ionizes into
hydrogen ions and hydroxide ions:
• H2O(l) → H+(aq) + OH-(aq)
• H2O(l) + H2O(l) → H3O+(aq) + OH-(aq)
Hydrogen Ions in Water
• The symbol for the concentration of hydrogen
ions is [H+] and for hydroxide ions is [OH-].
• We can measure the concentration of the ions in
pure water.
• [H+] = 1.0×10-7 M
• [OH-] = 1.0×10-7 M
• By stoichiometry, H2O(l) → H+(aq) + OH-(aq), we
see that [H+] = [OH-] in pure water.
Hydrogen Ions in Water
• We also know that the product of the
concentrations is a constant, Kw.
• Kw = [H+][OH-] = 1.0×10-14
• This means that as we change [H+], we change
[OH-].
• If [H+] goes up, [OH-] goes down.
• If [H+] goes down, [OH-] goes up.
Hydrogen Ions in Water
• The higher the value of [H+] and lower the value
of [OH-], the more acidic the solution.
• The lower the value of [H+] and higher the value
of [OH-], the more basic the solution.
• For example: [H+] = 1.0×10-6 is acidic.
• For example: [H+] = 1.0×10-3 is more acidic.
• For example: [H+] = 1.0×10-8 is basic.
• For example: [H+] = 1.0×10-11 is more basic.
Hydrogen Ions in Water
Example 1:
Colas are slightly acidic. If a cola solution has
[H+] = 1.0×10-5 M, what is the [OH-]?
[H+] = 1.0×10-5 M
Kw = [H+][OH-] = 1.0×10-14
Kw = [H+][OH-]
Kw
[OH ] =
=
+
[H ]
[OH-] = 1.0×10-9 M
1.0×10-14
1.0×10-5
The pH Concept
• pH is another way to measure [H+] and/or [OH-].
• pH = -log[H+]
• pOH = -log[OH-]
• If [H+] = 1.0×10-7
• then pH = -log(1.0×10-7) = 7.00
• If [OH-] = 1.0×10-7
• then pOH = -log(1.0×10-7) = 7.00
The pH Concept
• To find pH on the TI-83 or TI-84 Plus …
• press the “(-)” button
• press the “LOG” button
• enter the value for [H+]
• press the “)” button
• press “ENTER”
The pH Concept
• To find pH on the TI-Nspire …
• press the “(-)” button
• press the “ctrl” button
• press the “10x” button
• enter the value for [H+]
• press the “)” button
• press “ENTER”
• NOTE: The position of these buttons on your
calculator may change with faceplate.
The pH Concept
• If [H+] = 1.0×10-5
• then pH = -log(1.0×10-5) = 5.00
• If [OH-] = 1.0×10-8
• then pOH = -log(1.0×10-8) = 8.00
• If [H+] = 3.1×10-2
• then pH = -log(3.1×10-2) = 1.51
• If [OH-] = 4.5×10-11
• then pOH = -log(4.5×10-11) = 10.35
The pH Concept
Find the pH of a solution that has
1. [H+] = 1.0×10-4 pH = 4.00
2. [H+] = 1.0×10-9
pH = 9.00
3. [H+] = 2.4×10-5
pH = 4.62
4. [H+] = 6.6×10-8
pH = 7.18
5. [H+] = 4.8×10-3
pH = 2.32
The pH Concept
Find the pOH of a solution that has
1. [OH-] = 1.0×10-10pOH = 10.00
2. [OH-] = 1.0×10-3 pOH = 3.00
3. [OH-] = 3.8×10-5 pOH = 4.42
4. [OH-] = 8.1×10-8 pOH = 7.09
5. [OH-] = 2.8×10-3 pOH = 2.55
The pH Concept
• We can also convert from pH and pOH to [H+]
and [OH-] respectively.
• [H+] = 10-pH
• [OH-] = 10-pOH
• If pH = 7.00
• then [H+] = 10-7.00 = 1.0×10-7
• If pOH = 3.25
• then [OH-] = 10-3.25 = 5.6×10-4
The pH Concept
• To find [H+] on the TI-83 or TI-84 Plus …
• press the “2ND” button
• press the “LOG” button
• press the “(-)” button
• enter the value for pH
• press the “)” button
• press “ENTER”
The pH Concept
• To find pH on the TI-Nspire …
• press the “10x” button
• press the “(-)” button
• enter the value for pH
• press the “)” button
• press “ENTER”
• NOTE: The position of these buttons on your
calculator may change with faceplate.
The pH Concept
• If pH = 2.556
• then [H+] = 10-2.556 = 2.78×10-3
• If pOH = 9.27
• then [OH-] = 10-9.27 = 5.4×10-10
• If pH = 12.12
• then [H+] = 10-12.12 = 7.6×10-13
• If pOH = 6.678
• then [OH-] = 10-6.678 = 2.10×10-7
The pH Concept
Find the [H+] of a solution with a
1. pH = 6.00
[H+] = 1.0×10-6
2. pH = 13.00
[H+] = 1.0×10-13
3. pH = 5.25
[H+] = 5.6×10-6
4. pH = 9.42
[H+] = 3.8×10-10
5. pH = 2.32
[H+] = 4.8×10-3
The pH Concept
Find the [OH-] of a solution with a
1. pOH = 4.00
[OH-] = 1.0×10-4
2. pOH = 11.00
[OH-] = 1.0×10-11
3. pH = 3.79
[OH-] = 1.6×10-4
4. pH = 10.46
[OH-] = 3.5×10-11
5. pH = 3.77
[OH-] = 1.7×10-4
The pH Concept
• pH + pOH = 14
• if pH = 2.0,
• then pOH = 14.0 - 2.0 = 12.0
• if pH = 3.250,
• then pOH = 14.000 - 3.250 = 10.750
• if pOH = 3.0,
• then pH = 14.0 - 3.0 = 11.0
• if pOH = 7.451,
• then pH = 14.000 - 7.451 = 6.549
The pH Concept
• Solutions with pH = 7 (pOH = 7) are neutral.
• Litmus paper remains colorless.
• Solutions with pH < 7 (pOH > 7) are acidic.
• Litmus paper turns red.
• Solutions with pH > 7 (pOH < 7) are basic.
• Litmus paper turns blue.
Measuring pH
• Indicators may be added to a solution to tell us
the range of pH of the solution.
Measuring pH
Indicators are most often used where pH may
change, such as in a titration.
But they are also used to tell us where we are in a
particular range of pH.
For example, if we wanted to
tell if a solution has a pH
greater than or less than 7 ...
we might use Bromthymol
Blue (which changes color at
about pH = 7).
Measuring pH
Indicators are most often used where pH may
change, such as in a titration.
But they are also used to tell us where we are in a
particular range of pH.
For example, if we wanted to
tell if a solution has a pH
greater than or less than 7 ...
we would not want to use
Methyl Red (which changes
color near pH = 5).
Measuring pH
• Some meters provide quick and accurate
readings of pH.
• They vary from the simple to the very complex.