Titrations and Solubility
By Courtney Page and Jake Estes
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.
How it works
• Slowly add base to the unknown acid until the
indicator changes color.
Before
After
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 what’s the difference??
BCA Tables
• Use a BCA table when titrating a strong acid by a
strong base.
ICE Tables
• Use an ICE table when titrating a weak acid and
weak base.
Finding EQ point through calculations
• Strong acid and strong base
No EQ, because it is 100% ionized. pH=7
• 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.
What is Ka and Kb?
• Ka for the example reaction
HA(aq) + H2O(l)  H3O+(aq) + A-(aq)
[H3O+][A-]
K a=
[HA]
• Kb for the example reaction
B(aq) + H2O(l)  HB+(aq) + OH-(aq)
[HB+][OH-]
Kb=
[HB]
How to navigate through titration problems
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?
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?
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•
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A) 0.03 %
B) 0.09 %
C) 3 %
D) 5 %
E) 9 %
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?
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•
•
•
•
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
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.
What’s Ksp?
• Ksp = [products]
[reactants]
• (except when reactants are solids, you do not
include them in your Ksp equation)
The Ksp of Pb(OH)2 (MM=241 g/mol) is
1.2 x 10-15.
• What is the solubility in units of mols/L and g/L?
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)
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•
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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
Factors that affect solubility
• Solubility depends on what else is in
the solution.
• The common-ion effect.
• Acids and bases.
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.
Now some example AP questions…