CHEMISTRY

EXPERIMENT 17: "Flick Your Bic!" The Determination of the Molar Mass of Butane

The ideal gas equation, PV = nRT, can serve a a way to determine the molar mass of a gas. Identify each variable in the equation, give the values of R, and describe how the equation can be used to calculate the molar mass of a gas.

Procedure:

Remove the striking mechanism (flint, wheel, and spring) from a new disposable pocket lighter, then measure the mass of the lighter on the analytical balance.

Fill a 250 mL or 500 mL graduated cylinder completely full of water, invert it and place it inside a pneumatic trough filled at least half-way with water. Be sure that the cylinder contains no air bubbles.

Hold the lighter under the water and below the mouth of the inverted cylinder and press the release "button" on the lighter. Make sure all the bubbles of the gas are going up into the cylinder. Collect 200-250 mL of gas in the 250 mL graduate, and 300-400 mL gas in the 500 mL graduate.

After recording the volume of gas in the graduate, measure the height difference, if any, between the water in the trough and that remaining in the graduate. If possible, adjust the position of the cylinder in the water so that water levels inside and outside the cylinder are the same.

Shake excess water from the lighter and then use the cool setting on the hair dryer to blow off any remaining water. Lastly, remass the lighter.

Be sure you measure the temperature of the water (it will be assumed that the gas will be at the same temperature) and read the pressure from the barometer.

Results and discussion:

Calculate the molar mass of the gas using the ideal gas equation.

The percentage composition of the gas in the lighter is as follows: carbon, 82.63% and hydrogen, 17.37%. Calculate the empirical formula, and then using your molar mass, see if you can get a whole-number multiple of the empirical formula for the molecular formula of the gas.

Answer the following questions as part of your discussion.