hydrogen ions and acidity the ionization of water and ph
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Hydrogen Ions and Acidity
The Ionization of Water and pH
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 MKw = [H+][OH-] = 1.0×10-14
Kw = [H+][OH-] [OH-] =Kw
[H+]=1.0×10-14
1.0×10-5
[OH-] = 1.0×10-9
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 ConceptTo 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”
•press the “ctrl” button
The pH ConceptTo find pH on the TI-Nspire ...
•press the “10x” button
•enter the value for [H+]
•press the “)” button
•press “ENTER”
•press the “(-)” button
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• 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 ConceptTo find [H+] on the TI-83 or TI-84 Plus ...
•press the “2ND” button
•press the “LOG” button
•press the “(-)” button
•enter the pH value
•press the “)” button
•press “ENTER”
The pH ConceptTo find [H+] on the TI-Npire ...
•press the “10x” button
•press the “(-)” button
•enter the pH value
•press the “)” button
•press “ENTER”
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• 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
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.
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