Purpose of the experiment
The main aim of this experiment was to determine the empirical formula of a sample of hydrous copper sulfate. This was done by first determining the percentage of each component in the hydrous copper sulfate sample.
I. Procedure for determination of percentage of water in hydrous copper sulphate
A clean empty crucible was intensely heated for nearly five minutes and then cooled to room temperature. It was weighed and its weight recorded. Approximately 5.0 g of the hydrous copper sulphate sample was added onto the crucible and the weight recorded. The crucible and its content were then gently heated until all water had evaporated. The crucible and its content were then cooled and weighed. The step was repeated until the weight of the crucible and its content remained constant after heating and cooling. The constant weight obtained was used to calculate the percentage of water in the sample.
A general reaction is: MyXz.nH2O MyXz + n H2O.
II. Procedure for determination of percentage of copper in hydrous copper sulphate
Glassware was cleaned; 25 ml of the stock solution and aqueous ammonia solvent added to 50 ml Erlenmeyer flasks. Four diluted standard solutions were prepared for determining the standard graph of concentration versus concentration (Beer’s Law Graph). The unknown sample solution was then prepared; colorimeter plugged in, computer and Micro Lab turned on. The colorimeter was calibrated at 100% T, data acquired and analyzed.
The reaction of ammonia and hydrous copper is as follows:
n NH3 + Cu(H2O)62+ –> Cu(H2O)6-n(NH3)n2+ + n H2O where n = 4 to 6
III. Procedure for determination of percentage of sulphate in hydrous copper sulphate
Approximately 1.5 g of hydrous copper sulphate was weighed and put into a clean 250 mL beaker. To this beaker were added 150 mL of distilled water and 3 mL of 6 M hydrochloric acid. The solution was then heated to just below its boiling point. Then 10 % BaCl2 was slowly added while stirring until precipitation was over. To ascertain complete precipitation an excess of BaCl2 was used. The mixture was then heated for approximately 30 minutes. The mixture was settled and then filtered. The barium sulfate filtered was dried, sample weighed and heated to a constant weight. The reactions taking place can be expressed as follows:
Cux(SO4)y.n H2O x Cu+a + y SO4-b + n H2O
Ba2+ + SO42- BaSO4
The amount of water of crystallization in the sample was more than expected by the theoretical analysis. This could be attributed to the working conditions in the laboratory. For instance, the sample could have absorbed moisture from the surroundings during the cooling process thereby causing a high figure in the experimental analysis.
The values of the percentages of copper and sulphate in the sample were less than found in the theoretical analysis. For the case of copper, the decrease in the expected mass percentage could be attributed to poor analytical skills in the use of the spectrophotometer.
The errors and the percentage errors were calculated as absolute, that is, the negative sign was ignored in the results of the calculations. Basically, the negative sign indicates decrease in the experimental values as compared to that of the expected theoretical values.
R. Bock, Z. Anorg. Chem., 249:146 (1942)
D. F. Peppard, J. P. Gray, and G. W. Mason, Anal. Chem. 27:296 (1953)