1. Determining the Empirical Formula Purpose of the Experiment
of Copper Chloride
Purpose of the Experiment
Determine the empirical formula of a
compound containing only copper
and chlorine

2. Molar Mass or Molecular Weight
The molecular weight is the mass, in grams, of one mole of a compound.
H2O MWt = g/mol
AlCl3 MWt = g/mol

3. Percentage Composition
The percentage composition is the percent of a compound’s mass that results from each of its constituent elements.
For ethanol, C2H5OH,
2  g/mol C = g/mol of C2H5OH
6  g/mol H = g/mol of C2H5OH
1  g/mol O = g/mol of C2H5OH
Molecular Weight = the sum = g/mol of C2H5OH
% C = ( g/mol ÷ g/mol) 102 = %
% H = ( g/mol ÷ g/mol) 102 = %
% O = ( g/mol ÷ g/mol) 102 = %
Sum = 100 %

4. Empirical Formula Molecular Formula
The empirical formula is the simplest whole-
number ratio of the atoms in a compound.
Examples: benzene, CH
phosphorus pentoxide, P2O5
Molecular Formula
The molecular formula is the simplest whole-
number ratio of the atoms in a single
molecule of a compound.
Examples: benzene, C6H6
phosphorus pentoxide, P4O10

5. Schematic diagram of the combustion device used
to analyze substances for carbon and hydrogen
CxHy + O2 ----> y/2H2O + xCO2
excess

6. Determination of the Empirical Formula
A white compound has been analyzed and found to contain 43.6 wt. % phosphorous and 56.4 wt. % oxygen. In a separate study it molecular weight has been
found to be 284 g/mol.
What is the empirical formula of this compound?
Assume exactly 100 g of the compound, then one has
43.6 g P ÷ g/mol P = mol of P
56.4 g O ÷ g/mol O = mol of O
then the molar ratios are
1.408 mol ÷ mol = mol P
3.525 mol ÷ mol = mol O
and the empirical formula is P2O5.

7. Determination of the Molecular Formula
A white compound has been analyzed and found to
contain 43.6 wt. % phosphorous and 56.4 wt. % oxygen.
In a separate study it molecular weight has been
found to be 284 g/mol.
What is the molecular formula of this compound?
First determine the empirical weight of P2O5:
2  g/mol P = g/mol P
5  g/mol O = g/mol O
The empirical weight of P2O5 = the sum = g/mol
Molecular weight ÷ Empirical weight = 284 g/mol ÷ g/mol
= 2.001
and the molecular formula is twice the empirical formula and
the molecular formula is P4O10.

8. Determining the Empirical Formula of Magnesium Oxide
The goal is to determine x and y in the unknown magnesium oxide, MgxOy, by burning a known
mass of Mg in an excess of oxygen.
Mg(s, silvery-white) + O2(g)
limiting reagent
MgxOy(s, white)
0.353 g of Mg solid
Atmospheric oxygen in excess
heat
Result: Mass of MgxOy = g

9. Determining the Empirical Formula of Magnesium Oxide
Note: mass of O = total mass – mass Mg
= g – g = g
then
% Mg = (mass Mg ÷ total mass MgxOy) 102
= (0.353 g ÷ g) 102 = 60.3 %
% O = (mass O ÷ total mass MgxOy) 102
= (0.232 g ÷ g) 102 = 39.7 %
Mg = 60.3 % and O = 39.7 %

10. and the unknown oxide is MgO
Mg = 60.3% and O = 39.7%
Stoichiometry and the percentage composition
of three possible magnesium oxides
Possible Oxides
MgxOy Mg O
MgO
60.3 %
39.7 %
MgO2
43.2 %
56.8 %
Mg2O
75.2 %
24.8 %
and the unknown oxide is MgO

11. Today’s Experiment Original experiment Modified experiment
Zn(s, silvery white) + CuxCly(aq, blue)
ZnCl2(aq) + Cu(s, reddish)
Modified experiment
Al(s, silvery white) + CuxCly(aq, blue)
AlCl3(aq) + Cu(s, reddish)
~0.3 g ea. strip
(excess)
25 mL
Limiting reagent
known mass
known mass

12. From: http://wine1.sb.fsu.edu/chm1045/tables/period/PT_large.jpg

13. Copper is a transition metal with partially filled d orbitals.
A transition metal may exhibit multiple oxidation states, such as +1, +2, or +3, oxidation states which are not easily predicted by its position in the periodic table.
Because of the partially filled d orbitals, a transition metal ion in aqueous solution frequently is brightly colored, e.g., copper ions are blue in aqueous solution.
Zinc and aluminum are both stronger reducing agents than copper, see their redox potentials, so either will yield metallic copper from a solution of a copper salt.
Dr. Terry Bone, September 22, 2005

14. These potentials indicate the relative thermodynamic
tendency for the indicated half-reaction to occur.

15. Other Reactions in the Procedure:
Removal of Excess Reducing Agent
Zn(s) HCl(aq) ----> ZnCl2(aq) + H2(g)
2 Al(s) HCl(aq) ----> 2 AlCl3(aq) H2(g)
Cu(s) + n HCl(aq) --x--> no reaction
Note: Cu is below H (0.00 V ref) in table, so
It will not react with acids H+(aq) to form H2(g)
Metals above H in the table will react with acids.

16. Checkout Reagents in Lab 2 piece Al foil, ~0.3 g ea 1 pr Beaker Tongs
CuClx solution in 4 L spigot jugs, use ~25 ml for each analysis
Record data: g CuClx/ml, d = g/ml
10 % HCl in 1 L wash bottles, use ~5 ml
Use solid NaHCO3 on acid spills

17. Flow Chart for Procedure
25 mL copper chloride, weigh and
use exact density to get mass of CuClx
Add Al foil
Stir (takes about 5 min)
Add 5-10 drops of 10 % HCl and stir
(HCl will dissolve excess Al)
Decant the supernatant liquid Cu
waste

18. Flow Chart for ProcedureCu
Wash with distilled water to
remove aluminum chloride
Transfer Cu residue to a
pre-heated and pre-weighed
casserole
waste
CAUTION:
Do not overheat
to avoid oxidation
heat
Determine the mass of Cu
waste

19. Procedure Notes Record all weights to 0.001g
Weigh 25 ml of CuClx solution, use exact density to calculate exact
volume, then calculate the mass of CuClx
Do not use metal forceps or spatulas
Add Al foil until blue color is gone, allow excess foil to dissolve
Allow container to cool before weighing
Speed up cooling by placing flask in front of hood sash raised 4-6”
The second beaker does not have to be 150 mL
A casserole will also work as an evaporating dish

20. Hazards Waste 10 % HCl is a corrosive strong acid
CuClx solution-heavy metal, irritant
Hot surfaces - hotplates, glassware
Waste
Liquid waste: Al+3 solution and HCl
Cu solids

21. Summary of Data & Calculations
Collected data
Results (calculations)
Mass percent of Cu
Mass of Cl
Mass percent of Cl
Empirical formula
Mass of CuClx
Mass of Cu

22. Antacid Analysis (packet)
Next Week’s Experiment:
Antacid Analysis (packet)
Additional background reading for Antacid Analysis/Titrations:
Atkins, “Chemical Principles”, 3rd ed,
pp. F67-F72, F85-F88,