Determining the Empirical Formula of Copper Chloride Purpose

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 or Molecular Weight The molecular weight is the mass, in grams, of one mole of a compound. H 2 O MWt = 18. 0148 g/mol Al. Cl 3 MWt = 133. 341 g/mol

Percentage Composition The percentage composition is the percent of a compound’s mass that results from each of its constituent elements. For ethanol, C 2 H 5 OH, 2 12. 011 g/mol C = 24. 022 g/mol of C 2 H 5 OH 6 1. 008 g/mol H = 6. 048 g/mol of C 2 H 5 OH 1 15. 999 g/mol O = 15. 999 g/mol of C 2 H 5 OH Molecular Weight = the sum = 46. 069 g/mol of C 2 H 5 OH % C = (24. 022 g/mol ÷ 46. 069 g/mol) 102 = 52. 14 % % H = ( 6. 048 g/mol ÷ 46. 069 g/mol) 102 = 13. 13 % % O = (15. 999 g/mol ÷ 46. 069 g/mol) 102 = 34. 73 % Sum = 100 %

Empirical Formula The empirical formula is the simplest wholenumber ratio of the atoms in a compound. Examples: benzene, CH phosphorus pentoxide, P 2 O 5 Molecular Formula The molecular formula is the simplest wholenumber ratio of the atoms in a single molecule of a compound. Examples: benzene, C 6 H 6 phosphorus pentoxide, P 4 O 10

Schematic diagram of the combustion device used to analyze substances for carbon and hydrogen Cx. Hy + O 2 ----> y/2 H 2 O + x. CO 2 excess

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 ÷ 30. 974 g/mol P = 1. 408 mol of P 56. 4 g O ÷ 15. 999 g/mol O = 3. 525 mol of O then the molar ratios are 1. 408 mol ÷ 1. 408 mol = 1. 000 mol P 3. 525 mol ÷ 1. 408 mol = 2. 503 mol O and the empirical formula is P 2 O 5.

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 P 2 O 5: 2 30. 974 g/mol P = 61. 948 g/mol P 5 15. 999 g/mol O = 79. 997 g/mol O The empirical weight of P 2 O 5 = the sum = 141. 945 g/mol Molecular weight ÷ Empirical weight = 284 g/mol ÷ 141. 945 g/mol = 2. 001 and the molecular formula is twice the empirical formula and the molecular formula is P 4 O 10.

Determining the Empirical Formula of Magnesium Oxide The goal is to determine x and y in the unknown magnesium oxide, Mgx. Oy, by burning a known mass of Mg in an excess of oxygen. Atmospheric oxygen in excess Mg(s, silvery-white) + O 2(g) heat Mgx. Oy(s, white) 0. 353 g of Mg solid limiting reagent Result: Mass of Mgx. Oy = 0. 585 g

Determining the Empirical Formula of Magnesium Oxide Note: mass of O = total mass – mass Mg = 0. 585 g – 0. 353 g = 0. 232 g then % Mg = (mass Mg ÷ total mass Mgx. Oy) 102 = (0. 353 g ÷ 0. 585 g) 102 = 60. 3 % % O = (mass O ÷ total mass Mgx. Oy) 102 = (0. 232 g ÷ 0. 585 g) 102 = 39. 7 % Mg = 60. 3 % and O = 39. 7 %

Mg = 60. 3% and O = 39. 7% Stoichiometry and the percentage composition of three possible magnesium oxides Possible Oxides Mgx. Oy Mg. O 2 Mg 2 O Mg 60. 3 % 43. 2 % 75. 2 % O 39. 7 % 56. 8 % 24. 8 % and the unknown oxide is Mg. O

Today’s Experiment Original experiment Zn(s, silvery white) + Cux. Cly(aq, blue) Zn. Cl 2(aq) + Cu(s, reddish) Modified experiment Al(s, silvery white) + Cux. Cly(aq, blue) ~0. 3 g ea. strip (excess) 25 m. L Limiting reagent known mass Al. Cl 3(aq) + Cu(s, reddish) known mass

From: http: //wine 1. sb. fsu. edu/chm 1045/tables/period/PT_large. jpg

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.

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) ----> Zn. Cl 2(aq) + H 2(g) 2 Al(s) + 6 HCl(aq) ----> 2 Al. Cl 3(aq) + 3 H 2(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 H 2(g) Metals above H in the table will react with acids.

Checkout 2 piece Al foil, ~0. 3 g ea 1 pr Beaker Tongs Reagents in Lab Cu. Clx solution in 4 L spigot jugs, use ~25 ml for each analysis Record data: 0. 08067 g Cu. Clx/ml, d = 1. 074 g/ml 10 % HCl in 1 L wash bottles, use ~5 ml Use solid Na. HCO 3 on acid spills

Flow Chart for Procedure Add Al foil 25 m. L copper chloride, weigh and use exact density to get mass of Cu. Clx 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 CAUTION: Do not overheat to avoid oxidation waste heat Determine the mass of Cu waste

Procedure Notes v. Record all weights to 0. 001 g v. Weigh 25 ml of Cu. Clx solution, use exact density to calculate exact volume, then calculate the mass of Cu. Clx v. Do not use metal forceps or spatulas v. Add Al foil until blue color is gone, allow excess foil to dissolve v. Allow container to cool before weighing v. Speed up cooling by placing flask in front of hood sash raised 4 -6” v. The second beaker does not have to be 150 m. L v. A casserole will also work as an evaporating dish

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

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

Next Week’s Experiment: Antacid Analysis (packet) Additional background reading for Antacid Analysis/Titrations: Atkins, “Chemical Principles”, 3 rd ed, pp. F 67 -F 72, F 85 -F 88, 415 -425