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6b-02 - Hydrogen Ions and Acidity

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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 M
Kw = [H+][OH-] = 1.0×10-14

                                  Kw     1.0×10-14
Kw = [H+][OH-]          [OH-] =    +] = 1.0×10-5
                                 [H

[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 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 “10 ” button
               x


• enter the value for [H ]
                        +


• press the “)” button
• press “ENTER”
          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 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 pH value
• press the “)” button
• press “ENTER”
           The pH Concept
To find [H+] on the TI-Npire ...



• press the “10 ” button
                x


• 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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