Chemistry Honors
Chapter 7
Stoichiometric Determination: Empirical Formula of Copper Chloride
Background
Empirical Formula: the simplest whole-number ratio in which different kinds of atoms combine to form a
compound (atoms combining as single, distinct units).
For example:
name
Empirical Formula Molecular Formula
water
H2O
H2O
aluminum chloride
AlCl3
AlCl3
ethene
CH2
C2H4
glucose
CH2O
C6H12O6
For ionic compounds: Compound formula is the same as the empirical formula. The compound formula
defines the formula unit, the simplest whole-number ratio of positive and negative ions giving an electrically
neutral unit.
Empirical Formulas and mol: The empirical formula is the simplest whole-number ratio of numbers of moles
of atoms in one mole of a compound. The mole (mol) is that quantity of matter possessing a mass equal to the
formula weight expressedin grams. For example:
Cu (a monatomic element) H2O
Al2O3
Atomic Wt.: 63.546
Formula Wt.: 102.
Formula Wt.: 18.016
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
Empirical formula of Copper Chloride: CuxCly
If the compound is made of ... then ... Cl- ions are needed and the formula would be ...
Cu1+ and Cl11
CuCl
2+
1Cu and Cl
2
CuCl2
3+
1Cu and Cl
3
CuCl3
In this lab you will be producing solid Cu from a Cu(II) solution by reaction with Zn.
CuxCly (aq) + Zn (s) ---> ZnCl2 (aq) + Cu (s)
(blue)
(colorless)
Procedure:
Chemistry Honors
Chapter 7
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Obtain about 25 mL of .1 M copper chloride solution in a 50 mL graduated cylinder. (Be sure to
measure the volume to +/- 0.2 mL.) Transfer to a 100 mL 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. Let it react for 10 minutes.
After the reaction has stopped, add 5-10 drops of 1.0 M HCl solution and stir. This is to make all the Zn
reacts with the Chloride ions.
Using tongs, remove the Zn from the beaker, use the policeman to scrape the Zn,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.
Slowly and 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. After the copper
settles, decant the rinse water into your waste beaker. This will remove dissolved ZnCl2.
Removing water from the solid Cu
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Add about 10 mL isopropyl alcohol (isopropanol) to the Cu, stir thoroughly, and decant (gently tip over
so that only the liquid goes out leaving the solid behind) the alcohol into the waste beaker.
Set up a boiling water bath on a hot plate using a 250-mL beaker almost half full of tap 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 to quickly rinse the beaker.
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. Check the cooling of the dish by bringing your hand close to the dish without
actually touching the dish.
Carefully lift the wire gauze and carry the evaporating dish to the balance. 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.
Discard the contents of your waste beaker into the waste container.
Chemistry Honors
Chapter 7
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Calculations
1. How many moles of Zn reacted?
2. How many atoms of Zn reacted?
3. How many moles of Copper Chloride were used? (0.1 M means there is 0.1 mole of copper
chloride in every liter of solution)
4. How many grams of copper chloride were used?
5. How many grams of copper were produced?
6. How many grams of chloride ions were used?
7. How many moles of chloride ions?
8. How many moles of copper were produced?
9. Two moles of chloride ions reacted for every mole of zinc ions. How many moles of chloride
ions reacted?
10. What is the empirical formula of copper chloride?
Write a tentative formula: CuClb/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.
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If necessary, convert these ratios to whole numbers. For example, if you get ...
o ... CuCl0.96, you have approximately CuCl1, that is, CuCl.
o ... CuCl0.48, you have Cu2Cl0.96 (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.
o ... CuCl1.5, you have Cu2Cl3.
Prelab :
1. Drawing a labeled lab set up including the precision of the equipment.
2. A Data table to record your qualitative and quantitative data.
Lab Write up will include:
1. Data Processing
2. Error Analysis.
Found at website: http://web.lemoyne.edu/~giunta/chm151L/stoichiometry.html