Determining the Empirical Formula of Copper Chloride

Purpose of the Experiment

Determine the empirical formula of a compound containing only

copper and chlorine

The mass in grams of 1 mole of a compound

Molar Mass (Molecular Weight)

H2O 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 gMass of H = 6.048 gMass of O = 15.999 g

Mass of 1 mole of C2H5OH = 24.022 + 6.048 + 15.999 = 46.069

52.144%100%46.069g

24.022g100%

OHHC 1mol of mass

OHHC of mol 1in C of massC ofpercent Mass

52

52=×=×=

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.

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:

Empirical Formula Example

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

Dividing both mole values by the smaller one gives:

O 2.51.4093.523

and P 11.4091.409

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

Empirical Formula Example, contd.

Mg(s-silvery-white) + O2(g)

limiting reagent

MgxOy(s, white)

(0.353 g)

Atmospheric oxygen in excess

heat

Result: Mass of MgxOy = 0.585 g

Another Empirical Formula Example

60.3%100585.0

353.0100

OMg of mass total

g Mg, of mass% Mg,Percent

yx=×=×=

%7.39100g 0.585

g 0.232100

OMg of mass total

O of mass% O,Percent

yx=×=×=

Mass of O = total mass of compound – mass of Mg = 0.232 g

Another Empirical Formula Example, contd.

Mg = 60.3% and O = 39.7%

Formula masses and percent composition of three theoretical compounds of Mg and O

Formula of Oxide MgxOy %Mg %O

MgO 60.3% 39.7%MgO2

43.2% 56.8%Mg2O 75.2% 24.8%

Mg = 60.3% and O = 39.7%

Today’s Experiment

Al(s, silvery white) + CuxCly(aq, blue) AlCl3(aq) + Cu(s, reddish)

known mass

25 mLLimiting reagent

~0.3 g ea. strip(excess) known mass

Zn(s, silvery white) + CuxCly(aq, blue) ZnCl2(aq) + Cu(s, reddish)

Original

Modified

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Copper is a transition metal, with unfilled d orbitals.

Ground state electron configuration: [Ar].3d10.4s1Shell 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.

**

*

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

Other Reactions in the Procedure:

Removal of Excess Reducing Agent

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)

Checkout2 pc Al foil (~0.3 g)1-pr Beaker Tongs

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

Flow Chart for Procedure

waste

Do not overheat to avoid oxidation

Wash with distilled water to remove aluminum chloride

Transfer Cu residue to a pre-heated and pre-weighed

casserole

heat

Determine the mass of Cu

Cu

Flow Chart for Procedure

waste

waste

Procedure NotesRecord 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

Hazards10% HCl-strong acid, corrosive

CuClx solution-heavy metal, irritant

Hot surfaces - hotplates, glassware

WasteLiquid Waste: Al+3 / HCl

Cu solids

Results (calculations)Collected data

Mass of CuClxMass of Cu

Mass percent of CuMass of ClMass percent of ClEmpirical formula

Summary of Data & Calculations

Additional background reading for Antacid Analysis/Titrations:

Atkins, “Chemical Principles”, 3rd ed, pp. F67-F72, F85-F88, 415-425

Next Week’s Experiment:

Antacid Analysis (green book)