1. Molar Mass of an Unknown Gas

2. Concept
Gas Laws are a set of variables that predict the properties of a gas.
Application left to the student.

3. Purpose
To experimentally determine the molar mass of an unknown gas.

4. Safety
Substance H F R Special
Unknown gas Flammable

5. Procedure.1
Obtain and mass a clean dry 250 ml erlenmeyer flask with a stopper. 5

6. Procedure.2 Fill the flask with the unknown gas.
Inverted for gases less dense than air 6

7. Procedure.3
Stopper the flask and mass the flask and stopper with its gaseous contents. Record the mass.
Record the atmospheric pressure and the room temperature. 7

8. Procedure.4
Repeat procedure steps 2 & 3 two additional times. 8

9. Procedure.5
Fill the flask to the top (Completely filled) and place the stopper in the flask. (water should overflow)
Measure the volume of the flask to the nearest milliliter by using a 100 ml graduated cylinder.
Record the volume of the flask and stopper system. 9

10. Procedure.6
Clean up and replace the equipment. 10

11. Calculations.1 Determine the mass of the flask only
Determine the mass of the air inside the ‘empty’ flask. (use the table “Density of Air”)
Determine the mass of the ‘truly’ empty flask.
Determine the mass of the unknown gas in the flask. 11

12. Calculations.2
Determine the number of moles unknown gas in the flask (assume the gas is ideal)
Determine the molar mass of the gas. 12

13. Analysis Why was the mass of the gas measured three times?
The unknown gas has an empirical formula of C2H5.
What is the true molecular formula of the compound? 13

14. Conclusion Summary of the procedure Result Sources of error
Error propagation sequence 14