The Common Ion Effect

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The Common Ion Effect

• Common Ions?
• Not all solutions contain only one compound
• Common Ions will shift Eq.

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The Common Ion Effect

• HC2H3O2 H2O ? H3O C2H3O2-
• Suppose we add NaC2H3O2, which way will the
  reaction shift?

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pH and Common Ions

• 1. What will happen to the pH of a soln of
  benzoic acid, HC7H5O2, if you add potassium
  benzoate, KC7H5O2?
• 2. What will happen to the pH of a soln of NH3
  if you add NH4NO3?
• 3. What will happen to the pH of a soln of HCl
  if you add NaCl?

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pH and Common Ions

• 4. What is the pH of a solution containing 0.30
  mol of acetic acid and 0.20 mol of sodium acetate
  in 1.00 L of solution? (Ans pH 4.6)
• 5. What is the pH of a soln containing 0.16 M
  HNO2 (Ka 4.5 X 10-4) and 0.10 M KNO2? (Ans
  3.1)

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pH and Common Ions

• 6. What is the fluoride concentration and pH of
  a soln containing 0.10 mole of HCl and 0.20 mol
  of HF in 1.0 L of soln? (Ans 1.4 X 10-3 M, pH
  1.00)
• 7. Calculate the CHO2- ion concentration in a
  solution that is 0.050 M HCHO2 and 0.10 M HNO3.
  (Ans 9.0 X 10-5, pH 1.00)

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Buffers

• 1. A solution that resists a change in pH
• 2. Buffer capacity the amount of acid or base
  needed to overwhelm the buffer.
• 3. Composed of a weak acid and its conjugate
  base
• Ex HC2H3O2/NaC2H3O2
• HC2H3O2 H2O ? H3O C2H3O2-

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Buffers

• 4. Examples
• Blood H2CO3, HCO3-
• Sea Water
• Pool water
• Buffered aspirin

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Buffers

• 1. Adding a base (NaOH)
• HC2H3O2 OH- ? H2O C2H3O2-
• Less HC2H3O2
• More C2H3O2-

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Buffers

• 2. Adding an acid (HCl)
• C2H3O2- HCl ? HC2H3O2 Cl-
• Less C2H3O2-
• More HC2H3O2

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Writing Titration Reactions

• Examples
• HCl NaOH ?
• HC2H3O2 NaOH ?
• H2SO4 KOH ?
• Ca(OH)2 HNO3 ?
• NH3 HCl ?
• CH3NH2 HBr ?

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Writing Titration Reactions

• Student Practice
• HC6H7O6 NaOH ?
• H2SO4 NaOH ?
• HClO4 KOH ?
• C5H5N HCl ?
• H2SO4 NH3 ?
• C6H5NH2 HNO3 ?
• C2H5NH2 HBr ?

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Strong Acid/Strong Base Titrations

• 1. A reaction to determine the molarity of a
  solution
• Uses an indicator or a pH meter to find the eq.
  point
• pH at eq. is always 7 (neutral salt is formed)
• Graph if titrating w/ strong base
• Graph if titrating w/ strong acid

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Strong Acid/Strong Base Titrations

• 1. How many mL of 0.0978 M NaOH are needed to
  titrate 15.00 mL of 0.0831 M HBr? (Ans 12.8
  mL)
• 2. How many mL of 0.0978 M NaOH are needed to
  completely titrate 13.82 mL of 0.104 M H2SO4?
• (Ans 29.4 mL)

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Strong Acid/Strong Base Titrations

• 3. What is the pH of 50.0 mL of 0.100 M HCl when
  the following volumes of 0.100 M NaOH have been
  added
• a) 0 mL
• b) 25.0 mL
• c) 50.0 mL
• d) 51.0 mL

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a) Before addition

• (No NaOH yet)
• HCl H2O ? Cl- H3O
• pH 1

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b) Before Equivalence

• For HCl
• Total volume 75.0 mL
• M1V1 M2V2
• (0.100 M)(50.0 mL) x(75.0 mL)
• x 0.0666 M

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b) Before Equivalence

• HCl NaOH ? NaCl H2O
• I 0.0666 0.0333
• C-0.0333 -0.0333 0.0333
• E 0.0333 0 0.0333
• Ans pH 1.5

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c) At Equivalence

• HCl NaOH ? NaCl H2O
• I 0.05 0.05
• C-0.05 -0.05 0.05
• E 0 0 0.05
• Ans pH 7

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d) After Equivalence

• HCl NaOH ? NaCl H2O
• I 0.0495 0.0505
• C-0.0495 -0.0495 0.0495
• E 0 0.001 0.0495
• Ans pH 11

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Strong Acid/Strong Base Titrations

• 4. Calculate the pH when the following volumes of
  0.100 M HNO3 have been added to 25.00 mL of 0.100
  M KOH.
• 0 ml (pH 13)
• 24.90 mL (pH 10.30)
• 25.0 mL (pH 7)
• 25.10 mL (pH 3.70)

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Weak Acid/Weak Base Titrations

• 1. pH at eq. Is NOT 7 if a weak acid or weak
  base is involved.
• 2. pH depends on what is leftover after the
  rxn occurs
• 3. Graph

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Weak Acid/Weak Base Titrations

• HC2H3O2 is titrated with KOH. Is the pH at
  equivalence acidic, basic or neutral?
• NH3 is titrated with HCl. Is the pH at
  equivalence acidic, basic or neutral?

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Weak Acid/Weak Base Titrations

• 4. Two step problems
• a) Stoichiometric step same as strong
  acid/strong base
• b) Equilibrium Step uses the leftovers

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Weak Acid/Weak Base Titrations

• 1. Calculate the pH when the following volumes
  of 0.0500 M KOH are added to 50.0 mL of 0.0250 M
  Benzoic acid
• a) 20.0 mL
• b) 25.0 mL
• c) 30.0 mL

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a) Before equivalence

• Stoichiometric step
• KOH HC7H5O2 ? H2O KC7H5O2
• I 0.0143 0.0179
• C 0.0143 0.0143 0.0143
• E 0 0.0036 0.0143

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a) Before equivalence

• Equilibrium Step
• HC7H5O2 H2O ? H3O C7H5O2-
• I 0.0036 0.0143
• C -x x x
• E
• Ka 6.3 X 10-5 x(0.0143)/0.0036
• pH 4.80

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b) At Equivalence

• Stoichiometric step
• KOH HC7H5O2 ? H2O KC7H5O2
• I 0.0167 0.0167
• C 0.0167 0.0167 0.0167
• E 0 0 0.0167

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b) At Equivalence

• Equilibrium Step
• C7H5O2- H2O ? OH- HC7H5O2
• I 0.0167
• C -x x x
• E
• Kb 1.54 X 10-10 x2/0.0167
• pH 8.2

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c) After Equivalence

• Stoichiometric step
• KOH HC7H5O2 ? H2O KC7H5O2
• I 0.0188 0.0156
• C0.0156 0.0156 0.0156
• E 0.0032 0 0.0156
• pH 11.5

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Weak Acid/Weak Base Titrations

• 2. Calculate the pH when the following volumes
  of 0.0300 M HCl are added to 30.0 mL of 0.0200 M
  NH3
• a) 15.0 mL (pH 8.8)
• b) 20.0 mL (pH 5.6)
• c) 21.0 mL (pH 3.2)

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a) Before equivalence

• Stoichiometric Step
• NH3 HCl ? NH4 Cl-
• I 0.0133 0.0100
• C 0.0100 0.0100 0.0100 0.01
• E 0.0033 0 0.0100 0.01

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a) Before equivalence

• Equilibrium Step
• NH3 H2O ? NH4 OH-
• I 0.0033 0.0100
• C -x x x
• E
• Kb 1.8 X 10-5 x(0.01)/0.0033
• pH 8.8

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b) At Equivalence

• Stoichiometric Step
• NH3 HCl ? NH4 Cl-
• I 0.0120 0.0120
• C 0.0120 0.0120 0.0120 0.012
• E 0 0 0.0120 0.012

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b) At Equivalence

• Equilibrium Step
• NH4 H2O ? NH3 H3O
• I 0.0120
• C -x x x
• E
• Ka 5.56 X 10-10 x2/0.0120
• pH 5.6

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c) After Equivalence

• Stoichiometric Step
• NH3 HCl ? NH4 Cl-
• I 0.0118 0.0124
• C 0.0118 0.0118 0.0118 0.0118
• E 0 0.0006 0.0118 0.0118
• pH 3.22

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Weak Acid/Weak Base Titrations

• 3. What is the pH at eq. If you titrate 0.100 M
  HC2H3O2 with 0.0800 M NaOH? (pH 8.7)
• 4. What is the pH at eq. if you titrate 0.0934 M
  CH3NH2 with 0.103 M HCl? (pH 6.1)

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Polyprotic Titrations

• Titration of Na2CO3 with HCl

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• a) 3.62 b) 5.01 c) 3.07
• 18. a) 4.1 b) 1.9
• 22. a) 10.33 b) 10.19

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• Estimate the pH at equivalence if you titrate
  HC3H5O2 with NaOH?
• Estimate the pH at equivalence if you titrate
  CH3NH2 with HNO3.