Unit 9: Ideal Gas Law & Molar Mass of a Gas
Name__________________________________________Date_____________________Period_____
Introduction:
When the temperature, pressure, and volume of a gas is measured, the ideal gas law allows the
number of moles of the gas to be calculated. If the percent composition is known, the number of moles
allows the molecular formula to be calculated. The ideal gas equation is the following: PV = nRT
Pre-lab Questions:
1. Write the ideal gas equation; define and specify the unit for each variable.
2. Describe the experimental data which would be needed to determine the molar mass of an unknown
gas.
Purpose:
The student will collect a sample of gas by water displacement, measure the volume, temperature
and pressure of the gas and calculate the molecular mass of the gas using the ideal gas equation.
Equipment & Materials: 100 mL graduated cylinder, Disposable butane lighter, pan,
Thermometer, scales
Procedure:
1. Fill a eureka pan with water. Record the temperature of the water.
2. Place the lighter under water in the can. Remove the lighter, shake off the water, and dry the outside
with a towel. Then, mass the lighter to the nearest 0.01 gram. Record this measurement in the data
table.
3. Place the lighter back in the can. Fill the graduated cylinder completely with water. Cover it with
your hand and carefully invert it into the can of water. Remove your hand, keeping the mouth of the
graduated cylinder under water.
4. Release the gas from the lighter by pressing the small lever near the flint wheel. Release the gas under
water being careful that all of it is collected in the graduated cylinder by water displacement.
Release enough gas to fill the graduated cylinder to within 3 mL of its calibrated capacity. DO NOT
EXCEED THE SCALE AND DO NOT LIFT THE CYLINDER OUT OF THE WATER.
5. Allow the gas to reach room temperature (about 2 minutes). Then adjust the level of the water inside
and outside the graduated cylinder until they are the same by raising and lowering the cylinder in the
can. With the pressure inside and outside the graduated cylinder the same, read the volume of gas
collected using the cylinder’s calibration. Record this volume on the data table.
6. Remove the lighter from the can. Shake off any excess water and dry off with a towel. Measure and
record the new mass of the lighter.
7. Record the barometric pressure (from your beloved & all-knowing teacher).
Table of water vapor pressures at normal lab temperatures
temperature (C) pressure [torrs]
17
14.5
18
15.5
19
16.5
20
17.5
21
18.6
22
19.8
Data:
temperature (C) pressure [torrs]
23
21.0
24
22.4
25
23.7
26
25.2
27
26.7
28
28.3
Data Table
1. Mass of disposable lighter and contents
before experiment
2. Mass of disposable lighter and contents
after experiment
_________g
3. Mass of gas released from lighter
_________g
4. Volume of gas collected from lighter
_________mL
5. Atmospheric pressure
_________atm
6. Room temperature (temp of water and gas)
7. Water vapor pressure at water temp
(see water vapor pressure table)
_________C
8. Partial pressure of dry gas (Pgas = Patm – PH2Ovapor)
_________atm
_________g
_________atm
Observations:
Calculations:
3. Calculate the partial pressure of the dry gas. (Use this pressure in the ideal gas law.)
4. Use the ideal gas equation to determine the number of moles of the gas collected. Remember to
convert the room temperature from Celsius to Kelvin.
5. Use the number of moles collected to calculate the molecular mass of the gas. (moles/gram)
6. Butane (C4H10) is the most common gas found in disposable lighters. Compare your calculated
molecular mass with that of butane (C4H10). Calculate percent error.
Results:
7. Molar mass of butane?
Discussion:
8. How do you explain the fact that the gas is a liquid in the lighter, but a gas when it is collected?
9. Why must you use the water vapor pressures table when finding the pressure of the butane gas?
Conclusion:
Include errors & inaccuracy in results. Can all gases be measured in this manner?
Lab write up: You need to have the following sections in your TYPED,
DOUBLE-SPACED, WELL ORGANIZED, lab report. The lab report is due on
____________________.
Title:
Data: Record all your data in a table or chart.
Observations: Record all observations made during the lab.
Calculations: Show all calculations. If necessary you can also add an explanation.
Results: What did your data tell you (what is the molar mass of the gas?) Is this
the correct molar mass? Percent error?
Discussion: Pre-lab & post lab questions should be addressed here. Along with any
other info you needed to do the lab & find your answers
Conclusions:
Discuss places where errors did occur. Do not blame equipment error or just say
“human error.” Discuss places where errors could occur. Can all gases be
measured in this manner? You’ll need to do research – this is not an opinion
question.