Acidity, Basicity, and pH

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Acidity, Basicity, and pH
http//videos.howstuffworks.com/hsw/25353-compound
s-ph-scale-and-indicators-video.htm
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Self-Ionization of Water

• Water acts as both acid and base in same reaction
• Proceeds minimally to the right
• H2O (l) H2O (l) H3O(aq) OH-(aq)
• Pure H2O, 25C, H3O OH-1.0x10-7
• Concentration is small but significant
• Keq H3OOH-/H2O2

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Self-Ionization of Water

• Pure water and dilute solutions H2O same-H2O
  moved to left side
• Keq H2O2H3OOH-
• Kw H3OOH- ion product constant
• Pure water, 25C, Kw 1.0 x 10-14
• Kw (1.0 x 10-7)(1.0 x 10-7)
• Applies for all water solutions at 25C, even
  when H3OOH- are not equal

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Self-Ionization of Water

• Acidic, basic and neutral solutions all contain
  H3O and OH-
• Acidic H3O gt 10-7
• Basic OH- gt 10-7
• If the concentration of H3O in blood is 4.0x10-8
  M, is blood acidic, basic, or neutral? What is
  the concentration of OH- ions in blood?

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Self-Ionization of Water

• Blood is basic, H3O 4.0x10-8 lt 1.0x10-7
• Kw H3OOH-
• 1.0x10-14 (4.0x10-8)OH-
• OH- 2.5x10-7 M

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Example

• What is the concentration of H3O ions in a wheat
  flour and water solution if OH- 1.0x10-8 M?
  Is wheat flour and water acidic, basic, or
  neutral?
• 1.0 x 10-14 H3O(1.0 x 10-8 M)
• H3O 1.0 x 10-6 M
• Acidic

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pH Scale

• Based on logarithms pH -log H3O
• Numbers base 10 logarithm is its exponent when
  10 is the base, log (0.001) 3 10-3 0.001
• 1 unit change in pH is 10-fold change in
  concentration of H3O
• Sig. Figs. number of digits after decimal
  should equal number of sig. figs. In original
  number

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pH

• In one brand of vegetable juice, the
  concentration of H3O ions is 7.3 x 10-5 M What
  is the pH of the juice?
• pH -log H3O
• pH - log (7.3 x 10-5)
• Using log key -4.13667714
• pH -(-4.13667714) 4.14

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Example

• In household bleach, the concentration of OH-
  ions is 5.0 x 10-2 M. What is the pH?
• 1.0 x 10-14 H3O(5.0 x 10-2)
• H3O 2.0 x 10-13
• pH -log (2.0 x 10-13)
• pH 12.70

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Measuring pH

• Use acid-base indicator
• Indicator equation
• HIn (aq) H2O (l) H3O (aq) In- (aq)
• clear red
• Add acid, shift equilibrium to left
• (litmus HIn red)
• Add base, shift equilibrium to right
• (litmus In-blue)

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Measuring pH

• Each indicator has a different equilibrium
  constant
• Different pH range over which color changes
• Select indicator depending on pH range