Empirical Formula - Faculty web pages - SUNY Plattsburgh

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$Math 361 Problem Set #5 Due 5 October 2012 - Faculty web pages$

$Math 362 Problem Set #9 â€“ Solution 27 April 2012 - Faculty web ...$

4.) The ratio becomes the subscripts in the empirical formula. Since a chemical formulacan’t have a fraction for a subscript (O 1.5 ), the molar ratio needs to be multiplied by afactor to remove the fraction.Fe: 1 X 2 = 2 (6)O: 1.5 X 2 = 3 (7)5.) Now that the fraction has been eliminated, the empirical formula can be written.<strong>Empirical</strong> <strong>Formula</strong> = Fe 2 O 3In order to determine the actual formula, the molar mass of the compound must be known. Todetermine the actual formula, first determine the mass of one mole of the empirical formula(empirical mass). If the empirical mass is the same as the molar mass, then the empirical formulais the actual formula. If they are not the same, then the actual formula is a multiple of theempirical formula.In the next example, the molecular formula of acetic acid will be determined. Theempirical formula of acetic acid is CH 2 O and the molar mass of acetic acid is 60.0 g/mol.1.) Determine the empirical mass of acetic acid.C: 1 (12.0g) = 12.0 gH: 2(1.01g) = 2.02 gO: 1(16.0g) = 16.0 g (8)CH 2 O = 30.0 g/mol (empirical mass)2.) Divide the molar mass by the empirical mass to determine the factor that thesubscripts need to be multiplied by to write the actual formula.molar massempirical mass60.0 g/mol= = 2(9)30.0 g/molThe actual formula is therefore C 2 H 4 O 2In this experiment you will determine the empirical formula of a compound formed bymagnesium and oxygen. The reaction occurs between magnesium metal and oxygen in the air.The reaction will be run at high temperature (using a Bunsen burner flame) in order to speed upthe rate of reaction. Unfortunately magnesium will also react with nitrogen at high temperaturesforming a magnesium-nitrogen compound. This compound can be converted to the magnesiumoxygencompound by heating in the presence of water.
Procedure 1 :Your instructor will review how to operate a Bunsen burner.Caution a hot crucible looks the same as a cool crucible.When weighing your crucible make sure that it has reached room temperature. If you weigh awarm object, the temperature difference will create air currents around the balance and give youin incorrect mass.1.) Wash rinse and dry your crucible and cover. The crucible may have a dark stain that can’t beremoved, this is ok.2.) Place the crucible in a clay triangle. The bottom of the crucible should be approximately 5cm above the top of the Bunsen burner. Heat the crucible with the hottest part of the flamefor two minutes. This will serve to further clean out the crucible. After two minutes, turn offthe burner and allow the crucible to cool. When it has reached room temperature determinethe mass of the crucible.3.) Add approximately 0.5 g of magnesium turnings or magnesium powder to the crucible.Record the exact mass of the crucible and magnesium.4.) Cover the crucible and heat it gently for several minutes. Heat with the full flame for 15minutes.5.) Let the crucible cool for five minutes, and then adjust the cover so that there is a smallopening.6.) Heat the crucible with the full flame again. During this time periodically check the contentsof the crucible by lifting the cover. If the contents glow brightly or emit white smoke then thereaction is not yet complete. Continue with this heating until all of the magnesium hasreacted.7.) Cool the crucible to room temperature. Add 10 to 15 drops of deionized water. Replace thecover so that there is a small opening and heat the crucible with a low flame for five minutes.8.) Cool the crucible to room temperature. Determine the mass of the crucible and magnesiumoxygencompound.9.) The compound may be scraped into the trash when you are finished with the experiment.10.) Repeat the procedure1 Adapted from Dickson, T.R. Laboratory Experiments to Accompany Introduction to Chemistry, 8 th ed., New York:John Wiley & Sons Inc., 2000.
Page 1: Empirical FormulaThomas M. Moffett

Empirical Formula - Faculty web pages - SUNY Plattsburgh

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