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Experiment 8

Experiment 8. Molecular Weight of a Volatile Compound. Purpose and Goals. To determine the molecular weight of an unknown volatile compound using the ideal gas law Use a method developed by J.R. Dumas to determine the vapor density of the unknown. Ideal Gas Law. PV = nRT. Molecular Weight.

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Experiment 8

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  1. Experiment 8 Molecular Weight of a Volatile Compound

  2. Purpose and Goals • To determine the molecular weight of an unknown volatile compound using the ideal gas law • Use a method developed by J.R. Dumas to determine the vapor density of the unknown

  3. Ideal Gas Law PV = nRT

  4. Molecular Weight • The number of moles is expressed as w/MW • Final equation

  5. !!!CAUTION!!! • All unknown compounds are flammable • Run experiment under a hood because the vapor may be toxic

  6. Procedure • This experiment should be done individually • An unknown compound and its elemental percentage analysis will be provided • Weigh and record a clean, dry Erlenmeyer flask to 0.1 g • Collect the flask, 10cm2 aluminum foil, and 15cm Al wire. Weigh and record to .001 g

  7. Procedure • Heat the water bath until boiling • Boil until NO vapor is coming from the pinhole, then turn off burner • Allow to cool 5oC or more, remove from the bath and place on a clean towel

  8. Procedure • Dry and weigh (.001g) the flask assembly after it is allowed to reach room temperature • Record the weight once it stays constant • Record the Barometric pressure • Unroll the Al foil and inspect for water droplets • If droplets are present the experiment must be done again

  9. Calculations • Weight of the condensed liquid (same as weight of vapor • Molecular wt. of unknown liquid using equation 1 · = Volume of vapor Volume of flask weight of water = density of water w = PV RT MW

  10. Sample calculation RT = MW w PV L * atm ( 0 . 0821 ) * 372 . .78 K mole * K = ( 1 . 012 g ) . 989 atm * . 2610 L g = 120 mole

  11. Calculations cont. • Empirical formula from the elemental percentage analysis • From Empirical formula find the empirical formula weight

  12. Empirical vs. Molecular • Compare the E. formula weight to your approximate Mol. Wt. & identify the Mol. Formula • Molecular formula C6H6 • Empirical formula CH

  13. Calculations cont. • From the Molecular Formula calculate the Molecular Weight • Percent error from the true Molecular wt. and your calculated molecular weight

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