Stoichiometric Determination:
Empirical Formula of Copper Chloride

• Background
• Procedure
• Calculations

Background

name	Empirical Formula	Molecular Formula
water	H2O	H2O
aluminum chloride	AlCl3	AlCl3
ethene	CH2	C2H4
glucose	CH2O	C6H12O6

Empirical Formulas and mol: The empirical formula is the simplest whole-number ratio of numbers of mols of atoms in one mol of a compound. The mole (mol) is that quantity of matter possessing a mass equal to the formula weight expressed grams. For example:

Cu (a monatomic element)	H2O	Al2O3
Atomic Wt.: 63.546	Formula Wt.: 18.016	Formula Wt.: 102.
Mass of 1 mol = 63.546 g	Mass of 1 mol = 18.016 g	Mass of 1 mol = 102. g
63.546 g Cu/mol Cu	18.016 g H2O/mol H2O	102. g Al2O3/mol Al2O3

If the compound is made of ...	then ... Cl- ions are needed	and the formula would be ...
Cu1+ and Cl1-	1	CuCl
Cu2+ and Cl1-	2	CuCl2
Cu3+ and Cl1-	3	CuCl3

Procedure

Producing solid Cu from a Cu(II) solution by reaction with Zn.

CuxCly (aq) + Zn (s) --->	ZnCl2 (aq) + Cu (s)
(blue)	(colorless)

• Obtain about 25 mL of copper chloride solution in a graduated cylinder. (Be sure to measure the volume to +/- 0.2 mL.) Transfer to a reaction beaker; rinse the graduated cylinder and add the rinse to the beaker.
• Obtain a piece of Zn and obtain its mass to +/- 0.01 g.
• Handling the Zn with tongs, add it to the reaction vessel. Carefully tilt beaker so solution covers much of the Zn.
• Use the rubber policeman attached to your stirring rod to periodically scrape solid Cu from the Zn piece into the solution.
• After the reaction has stopped, add 5-10 drops of HCl solution and stir.
• Using tongs, remove the Zn from the beaker, making sure to leave behind adhering Cu. Dry the Zn with a paper towel and obtain its mass, then dispose of the Zn in a container under the fume hood.
• Carefully decant (pour off) the supernatant liquid from the solid Cu into a waste beaker.
• Wash the solid Cu by adding about 10 mL distilled water to it, stirring vigorously, then decanting the rinse water into your waste beaker. This will remove dissolved ZnCl₂.

Removing water from the solid Cu

• Add about 10 mL isopropyl alcohol (isopropanol) to the Cu, stir thoroughly, and decant the alcohol into the waste beaker. Repeat this step twice more.
• Set up a boiling water bath on a hot plate using a 250-mL beaker almost half full of water. Add 1 or 2 boiling stones to the water. You will use this water bath later to dry the Cu.
• Wipe clean an evaporating dish with your cloth towel. Obtain the mass of the dish to +/- 0.01 g.
• Use the rubber policeman to transfer the Cu from the reaction beaker to the evaporating dish. Make sure to transfer every trace of solid Cu. You may use a bit of isopropanol.
• Place the evaporating dish on the water bath and heat the water to boiling.
• When the Cu appears almost dry, stir the Cu and continue heating. When it appears completely dry, use tongs to place the evaporating dish on a wire gauze on the lab bench. Allow the dish and contents to cool to room temperature.
• Discard the contents of your waste beaker into the waste container labeled Discarded Zn(II) Solutions.
• Determine the mass of the dish and its contents to +/- 0.01 g.
• Using tongs, place the evaporating dish back on the water bath and heat for an additional 5 minutes.
• As before, remove the dish, let it cool, and weigh it. Repeat the heating, cooling, and weighing until successive weighings are within +/- 0.05 g.
• Discard the Cu(s) in the waste container labeled Discarded Cu Residue.

Calculations

• Calculate numbers of mols in original copper chloride sample

  	1 mol Cu
  Let a = mols Cu = g Cu x	------------
  	63.55 g Cu

  g Cl = g copper chloride - g Cu

  	1 mol Cl
  Let b = mols Cl = g Cl x	------------
  	35.45 g Cl

• Write a tentative formula: CuCl_b/a where a = mols Cu and b = mols Cl. The ratio b/a is the number of moles of Cl needed for every mole of Cu in the compound.
• If necessary, convert these ratios to whole numbers. For example, if you get ...
  • ... CuCl_0.96, you have approximately CuCl₁, that is, CuCl.
  • ... CuCl_0.48, you have Cu₂Cl_0.96 (because 0.48 moles Cl for every mole of Cu is the same ratio as 0.96 moles of Cl for every 2 moles of Cu), which is approximately Cu₂Cl.
  • ... CuCl_1.5, you have Cu₂Cl₃.