DETERMINATION OF THE MOLECULAR

MASS OF A VOLATILE LIQUID USING THE

DUMAS METHOD!

Purpose: Determine the molecular mass of your unknown

liquid using the Dumas method.

Materials: 250 mL round bottomed flask, clamp, ring stand,

iron ring, wire gauze, Bunsen burner, aluminum foil,

rubber stopper, glass pipette

Procedure: Design a procedure to determine the molecular mass of your unknown liquid. You must include a detailed rationale in your lab write-up outlining why you performed your specific procedure. Things to remember:

-  The only temperature you can really hold constant is the boiling point of water. However, most volatile liquids boil well below this temperature!

-  If there is liquid left in the bottom of your flask when you remove it from heat, it is almost a guarantee that the liquid is not a gas, and therefore has not filled the container completely!

-  Use about 5-10 mL of your unknown liquid – too little liquid will vaporize too quickly, too much liquid too slowly!

-  Remember – liquids that vaporize at low temperatures (having high vapor pressures, or are volatile) are flammable! Use your aluminum foil shield to protect the liquid from dripping into your flame!

-  Inserting a glass pipette into the rubber stopper may allow some of the excess liquid to boil out of your container!

Data: Construct a data table that includes all appropriate measurements from lab, whether measured or calculated. ANYTHING THAT IS NOT INCLUDED HERE, BUT SHOWS UP IN YOUR CALCULATIONS, WILL CAUSE YOU TO LOSE POINTS!

Calculations/Conclusions:

1.  Calculate your molecular mass using the Dumas version of the ideal gas law. Show all appropriate conversions when necessary!

2.  What was the identity of your substance? You might have to consult a chart or table online or in your book to identify your liquid!

3.  Calculate the percent error from your measured molecular mass and the accepted molecular mass! Use the identity of the substance referenced above to calculate percent error, using it as your accepted molecular mass. You will be graded on this!

4.  Discuss what laboratory errors caused a difference in our measured vs. your accepted molar mass! Discuss at least two sources of error.

5.  What are two practical purposes of this lab - meaning - why are we conducting it?

JJ