Determination of a Ksp Using
Spectrophotometry
Lab 6
Outline
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Purpose
Solubility
Experiment
Reaction and equilibrium expression
Safety Concerns
Waste
Spring Break and Next Lab Reminder
Purpose
The Ksp for the insoluble compound, copper (II) tartrate
will be determined using a spectrophotometric
method.
The experiment will provide the student with more
experience in completing serial dilutions and
equilibrium calculations.
Solubility
 The solubilities of solutes are different in different solvents
because of the “like dissolves like” principle. Other factors
that affect solubility are temperature and the common ion
effect.
 Even insoluble compounds dissolve to a small extent.
 Ksp is a measure of the solubility of solutes.
Solubility
 The larger the Ksp, the greater the solubility; the smaller the Ksp,
the smaller the solubility.
 Ksp of soluble and insoluble compounds can be determined
spectrophotometrically.
 Factors that affect the amount of light absorbed by a compound
are concentration of the absorbing species, path length through
the solution and molar absorptivity of the light absorbing species,
per Beer’s Law.
Experiment
The copper species in CuSO4 is the
same as the copper species in
CuC4H4O6.
The saturated CuC4H4O6 solution
we will be analyzing has an
absorbance value that is already
very low. Dilutions are out of the
question.
We can therefore construct a
calibration curve using dilutions
of CuSO4, since Cu2+ absorbs
light of a 775 nm wavelength
and SO42- is colorless.
Experiment
 Measure the absorbance of Cu2+(aq) at different concentrations to
make up a calibration curve of Abs vs. [Cu2+], M.
 This calibration curve can be used to determine the concentration,
and therefore solubility, of any Cu2+(aq) solution, if the unknown
solution has an absorbance that falls in the same absorbance
range as our calibration curve.
 The absorbance can be measured for copper (II) tartrate to
determine its solubility, based on the equation of the calibration
curve of Cu2+(aq) (with appropriate significant figures) and the
stoichiometric dissociation ratio of copper tartrate.
Reaction and Equilibrium Expression
CuC4H4O6(s)
Cu2+(aq) + C4H4O62-(aq)
Ksp = [Cu2+] [C4H4O62-]
(Remember that solids are not included in the equilibrium
expression!)
Since copper (II) tartrate dissociates in a 1:1 ratio, it’s easy to
determine the Ksp once [Cu2+] is found using a calibration
curve.
Safety Concerns
 Reagents:
 Cupric sulfate
 Copper (II) tartrate
 Eye Contact:
 Irritation, conjunctivitis, ulceration, clouding of cornea
 Skin Contact:
 Irritation and itching
 Inhalation:
 Coughing, sore throat, shortness of breath, ulceration and perforation
of the respiratory tract. Fumes from heating may cause symptoms
similar to a cold.
 Ingestion:
 Burning of the mouth, esophagus, and stomach. Hemorrhagic gastritis,
nausea, vomiting, abdominal pain, metallic taste, and diarrhea. Systemic
copper poisoning with capillary damage, headache, cold sweat, weak
pulse, kidney and liver damage, CNS excitation and depression, jaundice,
convulsions, blood effects, paralysis, coma and death.
Waste
 Copper is toxic and must be disposed of in the
appropriate container in the fume hood.
 Be sure to return your prealiquoted copper tartrate
solutions to the same place you found them. DO NOT
DISCARD THESE!
Next Week: Spring Break
 No labs next week
In 2 weeks: Lab 7 Reminder
 Lab 7 in 2 weeks.