Titrations and Solubility

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Titrations and Solubility

• By Courtney Page and Jake Estes

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The Basics of Titrations

• A titration is when a solution of accurately
  known concentration is gradually added to another
  solution of unknown concentration until the
  chemical reaction between the two is complete.
• Equivalence point is the point when the reaction
  is complete. (In an acid-base titration the EQ
  point is when the moles of H from the acid
  equals the moles OH- from the base.)
• Indicator is the substance that changes color at
  the equivalence point.

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How it works

• Slowly add base to the unknown acid until the
  indicator changes color.
• Before?
  ?After

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When do we use titrations?

• We use titrations to find different aspects of
  the experiment. You could be asked to find the
  concentration of a solution before the EQ point,
  at EQ point, or after EQ point. You could also
  be asked to find the pH at a given point.
• To do this, we use a BCA or ICE table.
• But whats the difference??

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BCA Tables

• Use a BCA table when titrating a strong acid by a
  strong base.

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ICE Tables

• Use an ICE table when titrating a weak acid and
  weak base.

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Finding EQ point through calculations

• Strong acid and strong base
• No EQ, because it is 100 ionized. pH7
• Strong acid and weak base
• The base is neutralized, need Ka for conjugate
  acid EQ
• Weak acid and strong base
• The acid is neutralized, need Kb for conjugate
  base EQ
• Weak acid and weak base
• Depends on the strength of both. Could use any of
  the above 3 ways to find EQ.

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What is Ka and Kb?

• Ka for the example reaction
• HA(aq) H2O(l) ?? H3O(aq) A-(aq)
• H3OA-
• Ka HA
• Kb for the example reaction
• B(aq) H2O(l) ?? HB(aq) OH-(aq)
• HBOH-
• Kb HB

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How to navigate through titration problems
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A 25.0 mL sample of 0.20 M HCl is titrated with
0.10 M NaOH

• What is the pH when 0 mL of titrant has been
  added?
• What is the pH when 60 mL of titrant has been
  added?

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Sample Multiple Choice 1

• Ka the acid dissociation constant, for an acid is
  9 x 10-4 at room temperature.
• At this temperature, what is the approximate
  percent dissociation of the acid in a 1.0 M
  solution?

• A) 0.03
• B) 0.09
• C) 3
• D) 5
• E) 9

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Sample Multiple Choice 2

• What is the ionization constant, Ka, for a weak
  monoprotic acid if a 0.30 molar solution has a pH
  of 4.0?

• A) 9.7 x 10-10
• B) 4.7 x 10-2
• C) 1.7 x 10-6
• D) 3.0 x 10-4
• E) 3.3 x 10-8

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The Basics of Solubility

• Solubility is the amount of stuff that can
  dissolve in a given amount of solvent at a given
  temperature. (Units are usually mols/L or g/L)
• Solubility product is the equilibrium constant
  (Ksp) for a given dissolution at a given
  temperature.
• The common-ion effect is the shift in equilibrium
  caused by an addition of a compound having an ion
  in common with the dissolved substance.

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Whats Ksp?

• Ksp products
• reactants
• (except when reactants are solids, you do not
  include them in your Ksp equation)

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The Ksp of Pb(OH)2 (MM241 g/mol) is 1.2 x 10-15.

• What is the solubility in units of mols/L and g/L?

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Sample Multiple Choice 3

• Determine the OH- (aq) concentration in 1.0 M
  aniline (C6H5NH2) solution.
• (Kb for aniline is 4.0 x 10-10)

• A) 2.0 x 10-5 M
• B) 4.0 x 10-10 M
• C) 3.0 x 10-6 M
• D) 5.0 x 10-7 M
• E) 1.0 x 100 M

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Factors that affect solubility

• Solubility depends on what else is in the
  solution.
• The common-ion effect.
• Acids and bases.

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Review of Titrations and Solubility

• Titrations are a laboratory technique to
  determine the concentration of an acid or base
  solution.
• An indicator is used in a titration to indicate
  the equivalence point, or end point.
• The solubility product constant is Ksp.
• Many factors can effect solubility of solutions.

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Now some example AP questions