1. Determination of the Molecular Weight of Butane
Virtual Lab

2. Objectives of this lab:
Collect a sample of gas using the water displacement method.
Measure the volume, temperature, and pressure of a sample of gas.
Calculate the molecular mass of a gas using the ideal gas equation.

3. Procedure
In this lab, students collected butane gas from a common lighter using the water displacement method using the equipment shown at the right. Advance to the next slide and then read through the procedure of the lab to familiarize yourself with this method.

4. Procedure
Beaker
Eudiometer (gas collection tube)
Thermometer
Trough

5. Step 1
The procedure asks you to read the initial mass of the lighter. The measurement is shown to the right. Record the value on your data table.

6. Step 2
In this step, you are to set up the eudiometer for gas collection. The picture at the right shows the eudiometer filled completely with water.

7. Step 3
The filling of the eudiometer with butane is shown in the pictures below.

8. Step 4
Read the volume of the butane in the eudiometer as shown in the picture at left and record this value on your data table.

9. Step 4
Read the temperature of the room off the thermometer shown below. The units on the thermometer are oC.

10. Step 5
The final mass of the lighter is shown in the picture on the right. Record this on your data table.

11. Step 6
The barometric pressure was obtained from the national weather service website which is shown below. Record this value on your data table under atmospheric pressure. (The unit is inHg.)

12. Determining the pressure of the butane gas
The eudiometer does not just contain butane. Evaporated water is also inside the tube and the pressure of this vapor must be accounted for. To do this, we must use Dalton’s Law which states that the total pressure in a container is equal to the sum of the partial pressures of the gases in the container. Mathematically,
PTotal = P1 + P2 + P3 + …
(continue on next slide)

13. Determining the pressure of the butane gas
So, the total pressure inside the eudiometer is equal to the pressure of the butane plus the pressure of the water vapor. The total pressure is also equal to the atmospheric pressure. The water vapor pressure can be obtained from the table on your lab sheet. Then use the following equation to solve for the pressure of the butane. Be careful with units!
Patm = Pbutane + Pwater vapor

14. Good luck!!!