Determination of a K sp Using Spectrophotometry Lab 6

 Purpose  Solubility  Experiment  Reaction and equilibrium expression  Safety Concerns  Waste  Spring Break and Next Lab Reminder Outline

The K sp 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. Purpose

 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.  K sp is a measure of the solubility of solutes. Solubility

 The larger the K sp, the greater the solubility; the smaller the K sp, the smaller the solubility.  K sp 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. Solubility

Experiment The copper species in CuSO 4 is the same as the copper species in CuC 4 H 4 O 6. We can therefore construct a calibration curve using dilutions of CuSO 4, since Cu 2+ absorbs light of a 775 nm wavelength and SO 4 2- is colorless. The saturated CuC 4 H 4 O 6 solution we will be analyzing has an absorbance value that is already very low. Dilutions are out of the question.

 Measure the absorbance of Cu 2+ (aq) at different concentrations to make up a calibration curve of Abs vs. [Cu 2+ ], M.  This calibration curve can be used to determine the concentration, and therefore solubility, of any Cu 2+ (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 Cu 2+ (aq) (with appropriate significant figures) and the stoichiometric dissociation ratio of copper tartrate. Experiment

CuC 4 H 4 O 6(s) Cu 2+ (aq) + C 4 H 4 O 6 2- (aq) K sp = [Cu 2+ ] [C 4 H 4 O 6 2- ] (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 K sp once [Cu 2+ ] is found using a calibration curve. Reaction and Equilibrium Expression

 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. Safety Concerns

 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! Waste

 No labs next week Next Week: Spring Break

 Lab 7 in 2 weeks. In 2 weeks: Lab 7 Reminder