A student is given the task of determining the molar concentration of a CuSO4 solution using two different procedures, precipitation and spectrophotometry.
For the precipitation experiment, the student adds 20.0 mL of 0.200 M Ba(NO3)2 to 50.0 mL of the CuSO4(aq).
The reaction goes to completion, and a white precipitate forms. The student filters the precipitate and dries it overnight. The data are given in the following table.
|
Mass of dry filter paper |
0.764 g |
|
Volume of CuSO4(aq) |
50.0 mL |
|
Volume of 0.200 M Ba(NO3)2 |
20.0 mL |
|
Mass of filter paper and dried precipitate |
1.136 g |
Write a balanced net ionic equation for the precipitation reaction.
Calculate the number of moles of precipitate formed.
Calculate the molarity of the original CuSO4 solution.
For the spectrophotometry experiment, the student first makes a standard curve. The student uses a 0.1000 M solution of CuSO4(aq) to make three more solutions of known concentration (0.0500 M , 0.0300 M , and 0.0100 M ) in 50.00 mL volumetric flasks.
Calculate the volume of 0.1000 M CuSO4(aq) needed to make 50.00 mL of 0.0500 M CuSO4(aq).
Briefly describe the procedure the student should follow to make 50.00 mL of 0.0500 M CuSO4(aq) using 0.1000 M CuSO4(aq),a 50.00 mL volumetric flask, and other standard laboratory equipment. Assume that all appropriate safety precautions will be taken.
The standard curve is given below.
The absorbance of the CuSO4 solution of unknown concentration is 0.219. Determine the molarity of the solution.
A second student performs the same experiment. There are a few drops of water in the cuvette before the second student adds the CuSO4(aq) solution of unknown concentration. Will this result ina CuSO4(aq) concentration for the unknown that is greater than, less than, or equal to the concentration determined in part (f) ? Justify your answer.
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