Experiment 8 Molecular Weight of a Volatile Compound.

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Presentation transcript:

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 The number of moles is expressed as w/MW Final equation

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

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, 10cm 2 aluminum foil, and 15cm Al wire. Weigh and record to.001 g

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

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

Calculations Weight of the condensed liquid (same as weight of vapor Molecular wt. of unknown liquid using equation 1 waterofdensity waterof weight flask of Volumevapor of Volume   RT MW w PV 

mole g )g.( L.*atm. K.*) K*mole atm*L.( w PV RT MW = = = Sample calculation

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

Empirical vs. Molecular Compare the E. formula weight to your approximate Mol. Wt. & identify the Mol. Formula Molecular formulaC 6 H 6 Empirical formulaCH

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