Molecular Mass of a Gas Lab

Mrs. Friedmann began class by checking our procedures/data tables that we prepared over Thanksgiving break for the lab test today. We'll be getting our Unit 5 tests back tomorrow because she wanted to give us plenty of time to complete the lab.

Introduction:

Using whatever lab equipment necessary, we were supposed to determine the molecular mass of our gas (Butane) which was contained in a lighter. Butane's insoluble in water, so, using water displacement we could collect it in a eudiometer and solve using Dalton's Law of Partial Pressures (to account for the water vapor) and the Ideal Gas Law.

Procedure:

1. Gather one gas eudiometer with stand, one large beaker, and one Butane lighter
2. Measure and record the mass of your lighter
3. Fill the large beaker with tap water and take its temperature (record the temp.)
4. Record the barometric pressure for the day
5. Completely fill the eudiometer with room temperature water. Place finger over the mouth and flip it, then insert it into tap water in beaker. ONLY REMOVE YOUR FINGER ONCE IT'S UNDERWATER
(No air bubbles)
6.  Now, carefully bring the lighter underwater - beneath the opening of the eudiometer - and depress the lever on the lighter. This will release the Butane gas into the eudiometer and it should bubble all the way up the tube. Hold the lever down long enough to collect a significant amount of Butane but DO NOT GO PAST 48 mL! You will have to start over if you do.
7. Adjust the height of the eudiometer until the level of water inside the eudiometer is the same as the water
level in the beaker. Now the pressure in the eudiometer should be the same as the atmospheric pressure.
8. Record the volume level of the Butane gas
9. Dry the lighter off and weigh. Record new mass.
10. Clean up!

Calculations:

1. Calculate the mass of gas that went into the eudiometer.

2. The gas in the eudiometer is a mixture of butane and water vapor. Dalton’s Law allows you to calculate just the pressure of the butane.
Ptotal = Patmosphere = Pbutane + Pwater vapor

3. Calculate the moles of gas collected (n) using the ideal gas equation (PV=nRT)

4. Now you can calculate the molecular mass of the gas (g/mol)

Data:

Mass of lighter before: 12.15g
Mass of lighter after: 12.07g
Water temperature: 20.2 C
Barometric pressure: 757 mmHg
Volume of gas: 23.5 mL
Vapor pressure of water: 17.769 mmHg
Pressure of Butane: 739.231 mmHg


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