Title:
Determination of Empirical Formula Laboratory, Experiment 6:
Name:
Lawrence Baisley,Denise Nguyen
Department of Chemistry
Los Angeles Trade Technical College
Received: March 20, 2014

This experiment demonstrates the technique used to determine the simple mole ratio of the elements in a compound. Magnesium metal will be made to react with oxygen of the air to produce a binary compound, magnesium oxide. (x)Mg(s) + (y/2) O2(g)  MgxOy(s) where x & y are the mole ratios of Mg & O, respectively. The magnesium burns by heating with a very hot flame. Under high temp the magnesium also combines w/ the atmospheric nitrogen to produce magnesium nitride, which is not the desired product for this experiment. 3Mg(s) + N2(g)  Mg3N2(s)
This compound can be converted to the desired product, magnesium oxide, by adding a few drops of water to the burned reaction mixture, then heating the mixture again. The excess water is removed by evaporation during the heating step. Mg3N2(s) + 6H2O(l)  3Mg(OH)2(s) + 2NH3(g) ? Mg(OH)2(s)  ? MgxOy(s) + ? H2O(l)

Equipment
Crucible & lid, tongs, wire gauze, ring stand, iron ring, clay triangle, Bunsen burner, analytical balance

Procedure
Washed crucible & lid. Heated with strong flame for 10 min. Allowed to cool to room temp. Crucible & lid was weighed w/ analytical balance reading 46.3543g. Strip of Mg metal was rolled up and placed in crucible with lid weighing 46.2588g before heating. Heated crucible & metal with lid closed for five mins. w/ gentle heat. High heat w/ 2-cone blue flame was applied using tongs to open and close lid every 2 mins. until reaching full 10 min. duration of high heat. Allowed crucible to cool and added three drops of H2O to react with magnesium nitride. Applied heat gently for 2 mins. Followed by high heat for 10 mins. Allowed to cool and weighed contents of crucible with lid weighing 46.3543g.

Results
Metal
Magnesium
Mass of crucible, lid & metal (before heat)
46.2588g
Mass of crucible, lid & metal oxide (after heat)
46.3543g
Mass of empty crucible & lid
46.0748g
Atomic Mass of metal
24.3050amu
Atomic Mass of Oxygen atom
15.9994amu

Calculations
Mass of metal oxide
0.2795g
Mass of metal
0.1840g
Mass of Oxygen
0.0955g
Moles of metal
0.00757 mol of Mg
Moles of Oxygen
0.00597 mol of O
Mole ratio
1 Mg : 1 O
Empirical formula of compound
MgO
Formula Weight of compound
40.30g
Formula of metal oxide
MgO

Mass of crucible, lid & metal (before heat) - Mass of empty crucible & lid = Mass of metal
46.2588g - 46.0748g = 0.1840g
Mass of crucible, lid & metal oxide (after heat) - Mass of empty crucible & lid = Mass of metal oxide
46.3543g - 46.0748g = 0.2795g
Mass of Mg + Mass of O = Mass of MgxOy
0.184g + 0.0955g = 0.2795g w grams Mg × 1 mol Mg/24.3050g Mg = mol of Mg
0.1840g Mg (1 mol Mg/24.3050g Mg) = 0.007570458 mol of Mg z grams O x 1 mol O/15.9994g O = mol of O
0.0955g O (1 mol O/15.9994g O) = 0.005968973 mol of O
0.00757 moles Mg : 0.00597 moles O
0.00757/0.00597 = 1.2680067 & 0.00597/0.00597 = 1
1.27 Mg : 1 O (with 0.27 in error)
Mole ratio = Mg : O
EF = MgO