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) H 2 OMW = g AlCl 3 MW= g

Percent Composition Gives the percentages of a compound’s mass that are due to each of the component elements C 2 H 5 OH Mass of C = g Mass of H = g Mass of O = g Mass of 1 mole of C 2 H 5 OH = =

Empirical Formula Represents the simplest whole-number ratio of the various types of atoms in a compound Examples: CH 5 N, C 2 H 10 N 2, C 3 H 15 N 3

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 g of compound we have: Empirical Formula Example 43.64g P x (1 mol P / 30.97g P) = mol P g O x (1 mol O / 15.99g O) = mol O

Dividing both mole values by the smaller one gives: O and P  This yields the formula PO 2.5 Since compounds must contain whole numbers of atoms, the empirical formula should contain only whole numbers. Empirical Formula = P 2 O 5 Empirical Formula Example, contd.

Mg(s-silvery-white) + O 2 (g) limiting reagent Mg x O y (s, white) (0.353 g) Atmospheric oxygen in excess heat Result: Mass of Mg x O y = g Another Empirical Formula Example

Mass of O = total mass of compound – mass of Mg = g Another Empirical Formula Example, contd. Mg = 60.3%andO = 39.7%

Formula masses and percent composition of three theoretical compounds of Mg and O Formula of Oxide Mg x O y %Mg %O MgO 60.3%39.7% MgO % 56.8% Mg 2 O75.2%24.8% Mg = 60.3%andO = 39.7%

Today’s Experiment Al(s, silvery white) + Cu x Cl y (aq, blue)AlCl 3 (aq) + Cu(s, reddish ) known mass 25 mL Limiting reagent ~0.3 g ea. strip (excess ) known mass Zn(s, silvery white) + Cu x Cl y (aq, blue)ZnCl 2 (aq) + Cu(s, reddish ) Original Modified

Copper is a transition metal, with unfilled d orbitals. Ground state electron configuration: [Ar].3d10.4s1 Shell structure: 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) ----> ZnCl 2 (aq) + H 2 (g) 2 Al(s) + 6 HCl(aq) ----> 2 AlCl 3 (aq) + 3 H 2 (g) Cu(s) + n HCl(aq) --x--> No Reaction Other Reactions in the Procedure: Removal of Excess Reducing Agent

Reagents in Lab CuCl x solution in 4L spigot jugs - take ~25 ml for each run Record data: ( g CuCl x / ml, d=1.074 g/ml) 10% HCl in 1L wash bottles (take ~5 ml) (N.B. solid NaHCO 3 is to be used for acid spills) Checkout 2 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

Procedure Notes  Record all weights to 0.001g  Weigh 25 ml of CuCl x solution, use exact density to calculate exact volume, then calculate the mass of CuCl x  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  CuCl x solution-heavy metal, irritant  Hot surfaces - hotplates, glassware Waste  Liquid Waste: Al +3 / HCl  Cu solids

Results ( calculations ) Collected data Mass of CuCl x Mass of Cu Mass percent of Cu Mass of Cl Mass percent of Cl Empirical formula Summary of Data & Calculations

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