Qualitative analysis is the branch of chemistry that involves the identification of the substances present in a sample, while quantitative analysis indicates the amounts of each substance present. In this experiment you will perform a quantitative analysis technique that potentially could be used for the determination of the amount of copper in a sample. Using a two-step reaction sequence, copper(II) oxide will be synthesized from a weighed amount of copper(II) sulfate pentahydrate.

Stoichiometry allows us to use the balanced equation to calculate the amounts of product and/or reactant involved in a reaction. Because the starting material will be a pure compound, it will be possible - to check the validity of the technique by comparing the amount of copper(II) oxide predicted theoretically to the amount experimentally obtained.

Copper(II) pentahydrate will be dissolved in water and reacted using a double replacement reaction with sodium hydroxide. The addition of hydroxide ions to a solution containing copper(II) ions results in the precipitation of copper(II) hydroxide.

CuSO4 + 2 NaOH  Cu(OH)2(s) + Na2SO4

Subsequent heating of the copper(II) hydroxide results in its decomposition to copper(II) oxide and water.

Cu(OH)2(s) +   CuO(s) + 2 H2O

The CuO can be quantitatively filtered, dried and weighed. The overall reaction for the sequence is:

CuSO4•5H2O(s) + 2 NaOH(aq)  CuO(s) + Na2SO4(aq) + 6 H2O(l)

You will perform the sequence above with an accurately weighed amount of CuSO4•5H2O. From this amount it is possible to calculate the amount of copper(II) oxide that should be formed (the theoretical yield). By performing the experiment, the experimental (actual) yield is obtained and this value is compared with the theoretical yield. Thus the %yield is

A percent yield close to 100% indicates that the technique is sound and could be used as a quantitative analysis technique for copper.

Procedure

1. Tare a clean, dry 150 mL beaker on the balance. Add between 1.8 and 2.2 grams of copper(II) sulfate pentahydrate (CuSO4•5H2O) into the beaker. Record to the nearest mg.

2. Add 10.0 mL of distilled water to the beaker and dissolve the copper salt by swirling. Then add 10.0 mL of 6.0 M NaOH to the solution with stirring.

3. Prepare for heating using the iron ring and wire gauze as shown in pre lab.

4. Place a watch glass over the beaker and heat the mixture to the boiling point. Try to avoid splattering especially onto the watch glass. If splattering occurs use a wash bottle to wash all the solid back down into the solution.

5. Heat until all of the blue solid has been decomposed to black copper(II) oxide (CuO).

6. Allow the mixture to cool while you prepare for vacuum filtration.

7. Obtain a Büchner funnel, 5.0 cm diameter piece of filter paper, filtration flask, neoprene adapter, and thick rubber hose. Set up for filtration as shown in pre lab. Pre-weigh your filter paper and record its mass before filtering.

8. Place the filter paper in the funnel, turn on the water, and moisten the filter paper using your wash bottle. Secure the vacuum with both hands and press the Büchner funnel gently into the neck of the filtration flask.

9. Test your vacuum system by squirting a few mL of distilled water onto the filter paper. The water should be sucked through the filter paper and into the flask almost immediately.

10. When the vacuum has been established transfer the previously heated mixture to the filter paper. When all the liquid has been transferred to the funnel, use a wash bottle to direct squirts of water at the remaining solid in the beaker and transfer this mixture to the funnel. Continue this