Discussion: When atoms of one element combine with those of another, the combining ratio is typically an integer or a simple fraction: 1:2,1:1,2:1,2:3 are ratios that one might encounter. The simplest formula of a compound expresses that atom ratio. Some substances with the ratios listed include CaCl₂, KBr, Ag₂O and Fe₂S_3. When more than two elements are in a compound, the formula STILL indicates the atom ratio. Thus Mg₃P₂O₈ indicates that magnesium, phosphorous and oxygen combine in a 3:2:8 ratio. Proteins and high molecular weight compounds typically have ratios considerable higher but the same concept applies.

To find the simplest ratio of a compound, the actual number of moles of each element must be determined for a sample. Then these mole amounts can be analyzed as a mol to mol ratio. After finding the ratio, the simplest formula can be calculated.

In this experiment, you will use this method to find the formula of a compound, Cu_xCl_y^. Z H₂O, where x, y and z are integers, which when determined will establish the formula of the compound.

The compound studies will be copper chloride hydrate. It is a green hydrate and stable. It is readily dehydrated. In addition, since copper is less active than aluminum, aluminum will replace it in a compound. Hence, the waters of hydration may be determined for a sample. Also, the mass of copper replaced in the compound can also be experimentally found. The mass of chlorine will be calculated from assumptions concerning the law of conservation of mass.

When the hydrate becomes the anhydrous salt, the color of the compound changes from the brilliant blue-green to a tan-brown color. When aluminum is placed into the solution of the copper chloride, the solution finally fades in color as the copper is removed from solution. The reddish brown copper can then be recovered.

Procedure:

Find the mass of a clean, dry crucible . Record.

Add one gram to the hydrate to the crucible. Record the mass. (Use the analytical balance.)

Gently heat the crucible via ring stand. DO NOT OVERHEAT THE SAMPLE! Before heating, you may break up any lumps. Heat GENTLY until the green hydrate turns brown at the edges. When all of the hydrate appears now to be anhydrous (brown NOT black), gently heat an additional two minutes.

Remove the crucible. Cover it and allow to cool for at least 8 minutes. Roll the crucible around, if any green crystals are noted, re-heat. Find the mass of the COOL crucible, uncovered. Record.

Transfer the crystals to a CLEAN 50-mL beaker. Rinse the crucible with two 5-7 mL portions of DISTILLED water. The solution in the beaker should now be blue-green. (The anhydrous salt rehydrates!)

Obtain a 20 cm piece of Al wire (~ 0.25 g). Coil it lightly and place into the beaker with the copper chloride solution. When reaction appears to have ceased, add 5 drops of 6M HCl to dissolve any insoluble aluminum salts.

Remove the Cu from the Al wire with the aid of a glass rod and wash bottle. If needed, you may drop drops of HCl solution onto the wire to add in the removal of the Cu. Set the wire aside.

Prepare a Buchner funnel and filter the Cu onto a filter paper in the funnel. You may use the wash bottle to rinse all particles of Cu from the beaker. Transfer must be done quantitatively.

Break up an lumps of Cu. Rinse twice more with distilled water. Turn off the suction.

Rinse with 10 mL of ethanol. Turn on the suction; draw air through the Cu for 5 minutes.

Find the mass of a marked watch glass.

Transfer the Cu to the watch glass. Find its mass. Record.

Dispose to the liquid waste down the sink. Turn the dry Cu in to your instructor.

 

Suggestion:

Design your data table to include masses and moles.

Questions: