Chemistry 115B

Solubility Product

 

In this experiment you will study the heterogeneous equilibrium of a slightly soluble salt and its ions in aqueous solution. The solubility product constant will be determined for copper(II) iodate for several different concentrations of Cu2+ and IO3-. The equilibrium is:

 

Cu(IO3)2(s) <--> Cu2+ + 2IO3-

and the equilibrium constant expression is:

Ksp -= [Cu2+][IO3-] 

To find the numerical value of Ksp, it is necessary to know the concentrations of both Cu2+ and IO3- at equilibrium. The concentration of Cu2+ may be easily determined spectrophotometrically by comparing the absorbance of the unknown to that of a standard. In this experiment the concentrations of Cu2+ are too low to produce a sufficiently high absorbance; therefore, the color is intensified by adding NH3(aq) to form the deep blue ion, Cu(NH3)42+. The concentrations of IO3- are determined indirectly by knowing that the change from the initial concentration to the equilibrium concentration will be twice that of the Cu2+.

Procedure

Clean and dry with a paper towel four 15-cm test tubes. Clean and drain (but do NOT dry) two Spec 20 cuvets. Clean a 25 mL buret and rinse it with two 2-mL portions of 0.150 M CuSO4 solution. Fill the buret with 0.150 M CuSO4 and deliver the volumes as described below into the appropriate test tubes. Also add 2.00 mL of the CuSO4 solution to a clean and drained 25-mL graduated cylinder. Add enough water to bring the level up to 25.0 mL. Stir this solution to make it homogenous. Clean the buret and rinse it with two 2-mL portions of 0.320 M HIO3 solution. Fill the buret with 0.320 M HIO3 and deliver the volume as indicated below into the appropriate test tube.

Test Tube Number

1

2

3

4

mL of 0.150 M CuSO4

5.40

5.70

6.00

6.30

mL of 0.320 M HIO3

6.60

6.30

6.00

5.70
 

Stir with a stirring rod the solution in test tube 1 until precipitation begins (about 2 minutes). Remove the rod, rinse and dry it, and stir test tube 2 until a precipitate forms. Repeat for test tubes 3 and 4. After precipitation has started, shake the solutions frequently for at least 20 minutes.

Filter the solution in test tube 1 into a labeled clean and dry small beaker. Do not wet the filter paper before titration! (Why?) Rinse and dry the funnel and with new filter paper filter the solution of test tube 2 into another clean and dry beaker. Continue for test tubes 3 and 4. To a fifth clean and dry beaker, add the same volume of standard Cu2+ ion solution from your graduated cylinder as in each of the other four beakers. In the hood, add 15 drops of concentrated ammonia solution to each of the beakers and mix well.

Rinse a Spec 20 cuvet with a small portion of the filtrate in beaker 1. Fill the cuvet about half-way with filtrate 1. Measure the absorbance at 603 nm using deionized water in the other cuvet as a blank. Rinse the sample cuvet with deionized water and then with a small portion of the filtrate in beaker 2. Fill the cuvet to the same level as before and measure its absorbance. Repeat for solutions 3, 4, and the standards.

Assuming that Beer's Law is valid, the unknown concentrations of Cu2+ in the four beakers may be calculated using:

[Cx] = [Cstd] Ax/Astd = [Cu2+]

where Cstd is the calculated concentration of the standard Cu2+ solution.

Results

Solution number

number 1

number 2

number 3

number 4

number 5 (standard)

Measured Absorbance

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Measured % T

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Calculated Absorbance (-log T)

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Concentration of Cu2+ in 12 mL mixture before ppt. (calculated)

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After ppt.

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Decrease in Cu2+ concentration

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Decrease in IO3- concentration

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Concentration of IO3- in 12 mL mixture before ppt. (calculated)

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After ppt.

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Solubility product
[Cu2+][IO3-]2

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DDM/MLK/DSR:clm 12/83

 

Les Brooks | 115B Home page | Lecture/Lab Schedule | Homework Assignments

kh 7/30/99