Identification of an Unknown Gas
PA State Standards:
3.7.10.B
Apply appropriate instruments and apparatus to examine a variety of objects and processes.
3.4.10.A
Describe concepts about the structure and properties of matter.
3.1.10.E
Describe patterns of change in nature, physical and manmade systems.
2.5.11.A
Select and use appropriate mathematical concepts and techniques from different areas of
mathematics and apply them to solving non-routine and multi-step problems.
1.2.11.A
Read and understand the central content of informational texts and documents in all academic
areas.
Introduction:
Solid and liquid substances are often easily identified by many different physical
properties such as appearance, melting or boiling points, and density. Identification of a
gas is not as easily accomplished unless it has a characteristic color or odor. Since this is
not always the case, it is difficult to identify a gas solely by appearance. This experiment
uses the measurable quantities of gases (pressure, volume, and temperature) and the ideal
gas law to determine the molecular mass of the gas. Since molecular mass is
characteristic of the composition of the gas, the gas can be identified.
Guiding Question:
Please answer the following question before beginning the lab.
Use the following data to calculate the molar mass of an unknown gas. What is its
identity?
Mass of gas
4.8962 g
Volume of gas
1.7 L
Temperature
295.1 K
Pressure
762.8 Torr
Gas Constant (R) 62.4 L Torr K-1 mol-1
Obtain the answer from your instructor, and determine the percent error.
Identification of
an Unknown Gas
Revised 7/15/08
1
Science in Motion
Juniata College
Equipment/Materials:
Top loading balance
Balloon
Barometer (or internet connection)
Gas cylinder
Meter stick
String
Tape
Thermometer
Safety:
 Always wear safety goggles in the laboratory.
Procedure:
1. Mass an empty balloon and record the value in the data section.
2.
Give the balloon to the instructor to be filled with a gas. Record the number of the
gas from your instructor.
3.
Tare a piece of tape on the balance. Stick the balloon on the tape to prevent it from
moving. Record the mass of the filled balloon in the data section.
4.
Determine the temperature in the room, which will equal the temperature of the gas.
Record the value in the data section.
5.
Determine the pressure in the room, which will equal the pressure of the gas.
Record the value in the data section.
6.
Use string and a meter stick to determine the circumference of the balloon. Several
values may be averaged together to increase the accuracy of the results.
Data:
Number of gas (from instructor)
Mass of empty balloon
Mass of filled balloon
Temperature of gas
Pressure of gas
Circumference of balloon
g
g
K
Torr
cm
Calculations:
1. Using the circumference of the balloon and the following equations, determine the
volume of gas in the balloon.
4 3
C=2πr
V=
πr
3
Identification of
an Unknown Gas
Revised 7/15/08
2
Science in Motion
Juniata College
2.
Using the combined gas law, determine the volume of the gas at STP.
3.
Determine the apparent mass of the gas in the balloon.
4.
Determine the mass of air displaced by the filled balloon. (Use 29 g/mol as the
molecular mass of air or 1.29 g/L as the density of air at STP).
5.
Since buoyancy is also a factor, the actual mass of the gas in the balloon is the
apparent mass of gas + mass of air displaced. Calculate the actual mass of gas.
6.
Using the ideal gas law, determine the moles of the gas.
7.
Determine the molecular mass of the gas.
Identification of
an Unknown Gas
Revised 7/15/08
3
Science in Motion
8.
Juniata College
Obtain the answer from your instructor, and determine the percent error.
Questions:
1. Explain how the volume of the gas in the balloon was determined.
2.
Why was 29 g/mol used as the molecular mass of air?
3.
Explain why the buoyancy had an effect on the mass of the gas in the balloon.
4.
If the filled balloons would be tested a week later, would the results remain the
same? Why or why not?
Identification of
an Unknown Gas
Revised 7/15/08
4