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Gas Lab Demonstration: Ideal Gas Law Lab
Determining the Molar Mass of a Butane Gas
1. Submerge butane lighter in basin of room temperature water and press lever to release gas for about
45-60 seconds. Remove lighter from water, shake excess water off and dry off outside very well.
2. Take the mass of the dry butane lighter and record in the observation table.
3. Fill a 100 mL graduated cylinder completely with water. Tightly cover the opening with the concave side of a watch
glass or a piece of weighing paper. Carefully invert the cylinder into the water without losing any water. Once in the
water, the watch glass or weighing paper can be removed. If there are air bubbles in the cylinder, repeat this step until
there are none.
4. Lift the graduated cylinder so that it is still completely submerged in the water, and position the butane lighter under
water and below the opening of the graduated cylinder. Press the lever and release butane gas into the graduated
cylinder. The gas will displace the water in the cylinder. Do not allow any of the butane gas to escape through the
outside of the cylinder.
5. Collect about 90 mL of butane gas or until the water level in the cylinder is equal to the water level in the basin.
Remove the butane lighter and stabilize the graduated cylinder, so that it remains steady in the water.
6. Shake and dry the butane lighter as in step 1. Take the mass of butane lighter and record in the observation table. (If
the final mass of the lighter is bigger than its initial mass, repeat steps 1 through 5.)
7. If the water level is not equal in the graduated cylinder and basin, adjust the level of water inside and outside the
cylinder until they are the same, by raising and lowering the graduated cylinder in the basin.
8. Repeat steps 2-8 two more times, for a total of three trials as time permits.
9. Record the temperature of the water and air in the room; they should be the same.
10. Record the atmospheric pressure from Ms. Humora.
Initial Mass of
Butane Lighter
Final Mass of
Butane Lighter
Volume of Butane
Collected
Table 1: Collected Data over 3 Trials of the Butane Lighter
Trial 1: 2B Data
Trial 2
21.142g
20.856g
90 mL
Temperature of Water and Room
Table 2: General Data
20.95°C
Atmospheric Pressure
99.7 kPa
Water Vapor Pressure at Measured Temperature
18.7 mm Hg
Trial 3
DATA ANALYSIS
1. Use the rearranged ideal gas law to determine the molar mass (M) in g/mol of butane for each trial.
Calculate the average molar mass of butane from your best and consistent trials.
2. Calculate the number of moles of butane collected in each trial—using the actual molar mass of butane, C4H10. Take
the average number of moles and use this number to determine the value of R, using PV=nRT.
POST-LAB QUESTIONS
1. Calculate the molar mass percent error using the average experimental value determined for butane.
2. Calculate the R percent error using the accepted value of R.
3. Describe two sources of error and how they affected your experimentally derived R value and molar mass value.
4. Do you think the same experimental technique could be used to determine the molar mass of any gas? Explain your
answer.