Determining the Empirical Formula
of Copper Chloride
Purpose of the Experiment
Determine the empirical formula
of a compound containing only
copper and chlorine
Molar Mass (Molecular Weight)
The mass in grams of 1 mole of a compound
H 2O
MW = 18.0148 g
AlCl3
MW= 133.341 g
Percent Composition
Gives the percentages of a compound’s mass
that are due to each of the component elements
C2H5OH
Mass of C = 24.022 g
Mass of H = 6.048 g
Mass of O = 15.999 g
Mass of 1 mole of C2H5OH = 24.022 + 6.048 + 15.999 = 46.069
Mass percent of C 
mass of C in 1 mol of C2H5OH
24.022g
100% 
100%  52.144%
mass of 1mol C2H5OH
46.069g
Empirical Formula
Represents the simplest whole-number ratio
of the various types of atoms in a compound
Examples: CH5N , C2H10N2, C3H15N3
Schematic diagram of the combustion device used
to analyze substances for carbon and hydrogen.
Empirical Formula Example
A white compound is analyzed and found to contain
43.64% phosphorous and 56.36% oxygen by mass.
What is the empirical formula?
In terms of moles, in 100.00 g of compound we have:
43.64g P x (1 mol P / 30.97g P) = 1.409 mol P
56.36 g O x (1 mol O / 15.99g O) = 3.523 mol O
Empirical Formula Example, contd.
Dividing both mole values by the smaller one gives:
1.409
 1 P and
1.409
3.523
 2.5 O
1.409
This yields the formula PO2.5
Since compounds must contain whole numbers of atoms,
the empirical formula should contain only whole numbers.
Empirical Formula = P2O5
Another Empirical Formula Example
Atmospheric oxygen in excess
Mg(s-silvery-white) + O2(g)
heat
MgxOy(s, white)
(0.353 g)
limiting reagent
Result: Mass of MgxOy = 0.585 g
Another Empirical Formula Example, contd.
mass of Mg, g
0.353
Percent Mg, % 
100 
100  60.3%
total mass of MgxOy
0.585
Mass of O = total mass of compound – mass of Mg = 0.232 g
Percent O, % 
mass of O
0.232 g
100 
100  39.7%
total mass of MgxOy
0.585 g
Mg = 60.3% and
O = 39.7%
Mg = 60.3% and
O = 39.7%
Formula masses and percent composition
of three theoretical compounds of Mg and O
Formula of Oxide
MgxOy
MgO
MgO2
Mg2O
%Mg
60.3%
43.2%
75.2%
%O
39.7%
56.8%
24.8%
Today’s Experiment
Original
Zn(s, silvery white) + CuxCly(aq, blue)
ZnCl2(aq) + Cu(s, reddish)
Modified
Al(s, silvery white) + CuxCly(aq, blue)
~0.3 g ea. strip
(excess)
25 mL
Limiting reagent
known mass
AlCl3(aq) + Cu(s, reddish)
known mass
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Copper is a transition metal, with unfilled d orbitals.
Ground state electron configuration: [Ar].3d10.4s1
Shell structure: 2.8.18.1
Transition metals may exhibit multiple oxidation states
(+1, +2, +3, etc…).
These are not easily predicted by position in the periodic
table.
Transition metals ions in aqueous solutions frequently are
brightly colored, also due to d orbitals (e.g. Cu ions are blue).
Zn and Al are both stronger reducing agents than copper.
(Note: the redox potentials on next slide)
Because of this either one would work to produce metallic
copper from a solution of a copper salt.
*
*
*
These potentials indicate the relative thermodynamic
tendency for the indicated half-reaction to occur.
Other Reactions in the Procedure:
Removal of Excess Reducing Agent
Zn(s) + 2 HCl(aq) ----> ZnCl2(aq) + H2(g)
2 Al(s) + 6 HCl(aq) ----> 2 AlCl3(aq) + 3 H2(g)
Cu(s) + n HCl(aq) --x--> No Reaction
Checkout
2 pc Al foil (~0.3 g)
1-pr Beaker Tongs
Reagents in Lab
CuClx solution in 4L spigot jugs
- take ~25 ml for each run
Record data: (0.08067 g CuClx / ml, d=1.074 g/ml)
10% HCl in 1L wash bottles (take ~5 ml)
(N.B. solid NaHCO3 is to be used for acid spills)
Flow Chart for Procedure
Add Al foil
25 mL copper chloride, weigh and
use exact density to get mass of CuClx
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
Flow Chart for Procedure
Cu
Wash with distilled water to
remove aluminum chloride
Transfer Cu residue to a
pre-heated and pre-weighed
casserole
waste
Do not overheat
heat
to avoid oxidation
Determine the mass of Cu
waste
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 also
Allow container to cool before weighing
Speed up cooling by placing 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
Hazards
10% HCl-strong acid, corrosive
CuClx solution-heavy metal, irritant
Hot surfaces - hotplates, glassware
Waste
Liquid Waste: Al+3 / HCl
Cu solids
Summary of Data & Calculations
Collected data
Mass of CuClx
Mass of Cu
Results (calculations)
Mass percent of Cu
Mass of Cl
Mass percent of Cl
Empirical formula
Next Week’s Experiment:
Antacid Analysis (green book)
Additional background reading for Antacid
Analysis/Titrations:
Atkins, “Chemical Principles”, 3rd ed,
pp. F67-F72, F85-F88, 415-425