Chapter 16 pH and Titration

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Chapter 16pH and Titration
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I. Concentration Units for Acids and Bases

• A. Chemical Equivalents
• 1. Definition quantities of solutes that
  have equivalent combining capacity
• a. Acid mass of one equivalent is
  numerically equal to the mass of one
  mole of the acid divided by the number of
  protons( H3O ) that one mole of the acid
  can provide
• Example
• HCl 36 g/mol 1 eq 1 H3O 36 g/mol
  H3O
• H2SO4 98g/mol 2 eq 2 H3O 49 g/mol
  H3O

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• b. Base mass of one equivalent is numerically
  equal to the mass of one mole of the base divided
  by the number of protons(OH-) that one mole of
  the base can provide
• Example
• NaOH 40 g/mol 1 eq 1 OH- 40
  g/mol OH-
• Ca(OH)2 74 g/mol 2 eq 2 OH- 37
  g/mol OH-

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B. Normality

• Definition number of equivalents of solute per
  liter of solution
• N eq of solute
• L of solution

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C. Relationship Between Normality and Molarity

• N nM
• N Normality
• n number of equivalents ( of H or OH-)
• M Molarity
• Example
• 1M HCl 1N HCl 1M NaOH 1N
  NaOH
• 1M H2SO4 2N H2SO4 1M Ca(OH)2 2N
  Ca(OH)2

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II. Aqueous Solutions and the Concept of pH

• A. Self-Ionization of Water
• 1. Definition Two water molecules
  interact to produce a hydronium ion and a
  hydroxide ion by proton transfer -
  forms a weak electrolyte
• 2. is symbol used to indicate
  concentration in moles per liter (Molarity)

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• 3. H2O H2O lt---gt H3O OH-
• in pure water H3O OH-
• 4. H3O OH- 1 x 10-14
• 5. If the H3O increases then the
  OH- decreases or
• If the H3O decreases then the
  OH- increases

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• Example
• What is the hydronium and hydroxide concentration
  of a 3 x 10-4 M Ca(OH)2 solution?

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B. The pH scale

• 1. pH -- the negative of the common logarithm of
  the hydronium ion concentration
• pH -logH3O
• 2. Acid pH lt 7
• 3. Base pH gt 7
• 4. Neutral pH 7

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C. Calculations involving pH

• pH -logH3O
• 0.001 M HCl H3O 1 x 10 -3
• pH -log1 x 10-3
• pH 3 (acid)
• Remember that H3O OH- 1 x 10-14 so if
  H3O 1 x 10-3 then OH- 1 x 10-11
• FYI there is also pOH - logOH- and
• pH pOH 14

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• Example
• 1. Determine the hydronium concentration,
  hydroxide concentration of a solution that has a
  pH of 7.52.

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III. Acid-Base Titrations

• A. Indicators
• 1. Definitions
• a. indicators - weak acid or base
  dyes whose colors are sensitive to
  pH, or hydronium, concentration
• b. transition interval - the pH range
  over which an indicator changes color

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• 2. Types of indicators
• a. Change color at about pH 7
• b. Change color below pH 7
• c. Change color above pH 7

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B. The Principle of Titration

• Definitions
• 1. Titration - the controlled addition and
  measurement of the amount of a solution of known
  concentration that is required to react
  completely with a measured amount of a solution
  of unknown concentration
• 2.Standard solution - a solution that
  contains a precisely known concentration of a
  solute

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• 3. Equivalence point - in a neutralization
  reaction, the point at which there are equivalent
  quantities of hydronium and hydroxide ions
• 4. End point - the point in a titration where an
  indicator changes color
• 5. Primary standard - a highly purified compound,
  when used in solution to check the concentration
  of the known solution in a titration

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C. Molarity and Titration

• 1. Determine the moles of acid (or base) from the
  standard solution used during titration
• 2. From a balanced chemical equation, determine
  the ratio of moles of acid (base) to base (acid)
• 3. Determine the moles of solute of the unknown
  solution used during the titration
• 4. Determine the molarity of the unknown solution

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D. Examples

• 1. In a titration of vinegar, you find that it
  requires 11.10 mL of 0.748M NaOH to neutralize a
  10.0 mL sample of vinegar. What is the
  concentration of acetic acid in this sample of
  vinegar?
• NaOH HC2H3O2 ?

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• 2. What is the molarity of a magnesium hydroxide
  solution if 45.45 mL of a 0.50M HCl solution is
  used to titrate 25 mL of magnesium hydroxide?

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D. Normality and Titrations

• Va x Na Vb x Nb
• Va volume of the acid
• Na normality of the acid
• Vb volume of the base
• Nb normality of the base