Summary

In this lab, students will experimentally determine the molar mass of a gas, specifically butane (C₄H₁₀), by collection over water. This experiment is an inquiry based experiment for 2nd year chemistry or AP chemistry students who have previously collected an insoluble gas. If your students have not previously collected an insoluble gas over water you should take the time to demonstrate one or more procedures that they could use.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
• Scientific and Engineering Practices:
  • Asking Questions and Defining Problems
  • Planning and Carrying Out Investigations
  • Constructing Explanations and Designing Solutions

AP Chemistry Curriculum Framework

This lab supports the following unit, topic and learning objective:

• Unit 3: Properties of Substances and Mixtures
  • Topic 3.4: Ideal Gas Law
    • 3.4.A: Explain the relationship between the macroscopic properties of a sample of gas or mixture of gases using the ideal gas law.

Objectives

By the end of this lab, students should be able to

• Develop, test and adapt an experimental design that can effectively collect a measurable gas over water.
• Use a combination of the Ideal Gas Law and Dalton’s Law of Partial Pressure to experimentally determine the molar mass of a gas.

Chemistry Topics

This lab supports students’ understanding of

• Gases
• Dalton’s Law of Partial Pressures
• Ideal Gas Law
• Closed End Monometers
• Vapor Pressure
• Molar Mass

Time

Teacher Preparation: 10 minutes (30 minutes if using disposable lighters)

Lesson: 45 – 60 minute student planning and 45 - 60 minute lab period

Materials available for this activity

• Cans of butane gas
• 50 or 100mL eudiometers
• 600 or 1000 mL beakers Ring stands
• 500 or 1000 mL graduated cylinders
• Single burette clamp
• Large buckets or plastic storage bins
• Pneumatic troughs
• Water (tap water is fine)
• Analytical balance (2 to 3 decimal places)
• Tygon or rubber tubing that fits around the nozzle on the can of butane. (These can be cut to size depending on the students’ experimental design)
• Classroom barometer
• Classroom thermometer

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Keep butane away from open flames as it is highly flammable.
• When students release the collected butane at the end of the lab, be sure that they are in a well ventilated laboratory space or near open window.

Teacher Notes

• Organize students in lab groups of 2 - 3 students depending on amount of materials you have available.
• Adjust the materials and equipment list to reflect what will be available for your students to use.
• When collecting an unknown gas over water, there will be some water vapor mixed in with the gas in the container. You must subtract the vapor pressure of water from the total pressure in the gas collection container to find the vapor of the unknown gas.

• If you assume that the gas collected in the container is an ideal gas, you can use the Ideal Gas Law to derive the equation for calculating its molar mass (g/mol). You can view a derivation of this formula in the table on the right.

MM = mass ● R ● T
V ● P

• Small butane lighters or cans of butane gas can be used in this lab.
  • If you use the disposable lighters you will have to adjust the gas flow so that it is very high. There are several videos online that you can use to learn how to increase the gas flow. This one has a good explanation and pictures. This task should be done before the students arrive. Warn the students not to light the lighters, as the flames will be high. You may also want to remove the striking mechanism. The lighters should only be used to discharge the gas under water and collected it in a container.
  • If you use the cans of butane gas, they can be purchased on Amazon and other home-type stores. Collecting the gas from the cans of butane typically requires a couple of attempts for students to get used to regulating gas flow. Once students realize how fast the butane comes out of the can, they are usually very successful.
  • Students will struggle initially with controlling amounts of butane used in the experiment. The can of butane releases the gas very quickly. The disposable lighter is slower and more controlled.
  • The experiment itself does not take a lot of time, but the design process and acceptable execution does require time.
• During the design portion students normally will try to use all the equipment available instead of thinking about what they need to measure. If time allows, students should try the experiment and fail and redesign to learn from their flaws in the method and reevaluate the goals of the experiment and the equipment available.
• Students may need a reminder of the data that the will need to successfully complete this lab: room pressure, room/water temperature, mass of gas produced, volume of gas produced, and the vapor pressure of water at the specific temperature for the room/water at the time of the lab.
  • You will need a classroom barometer to get the pressure in your classroom. If one is not available, students can get the data from a website such as weather.com. Students will need to subtract the vapor pressure of water from the room pressure to find the pressure of the gas collected. Note – the barometer usually gives the pressure in mmHg. Students will have to convert to atm for lab calculations.
  • If the water that is being used to collect the gas has been exposed to room temperature for a period of time, students can assume that the temperature is the same as the temperature of the room. Note – students will have to convert the temperature to Kelvin for lab calculations.
  • Students will either use a eudiometer or graduated cylinder to collect the gas. The level of the water inside and outside of the collection vessel must be at the same level to get a good volume reading. Note – students will have to convert the volume to liters for lab calculations.
  • Students will measure the mass of the butane lighter or can before and after collecting the gas. The difference in those two masses is equal to the mass of the gas collected. The gas container must be thoroughly dried before taking the final mass. If students are using lighters, they might consider letting them sit and dry overnight to be sure that all of the water has evaporated.
  • The more gas that students collect, the better their results will be. Since gases have very low density, smaller volumes will have much smaller masses. As an example, at 25⁰C and 1 atm the mass of 50 mL of butane is 0.115 grams. At the same temperature and pressure, the mass of 500 mL of butane is 1.15 grams and the mass of 750 mL is 1.75. The precision of your balances can also effect this measurement the percent error in the calculation of molar mass.
• This experiment is designed for students who have already collected gases in the past. If your students have not previously collected an insoluble gas over water you should take the time to demonstrate one or more procedures that they could use.
• Students should be able to write their own background section including information on butane, write their own procedure including a picture of their set up, determine the data to be collected, and perform the necessary analysis to determine their experimental value. The molar mass of butane can be calculated for use in error analysis.
• Reports for these types of investigations are normally collaborative in my classroom, meaning one report for the group with all group members contributing to it.
• I have included a generic rubric used to grade students in the past and can be adapted to address specific goals of individual teachers. It is available for download.

Examples of lab procedures

• Students can choose a eudiometer or a graduated cylinder as the gas collection containers.
• The chosen container should be filled with room temperature tap water and then carefully inverted in a larger container of water. They then release butane gas from a disposable lighter or a butane gas can to displace the water in the collection container with the gas.
  • If they choose to use a eudiometer, it can be filled to the top with water. The opening can then be covered with a finger and inverted into a bigger beaker, trough, or other container for the gas collection. The opening of the eudiometer is small, so using a butane gas can with a tube attached will work best. The eudiometer can be held in place by one student or held in place with a ring stand and clamp. After putting the open side of the tube into the eudiometer, they should slowly turn on the gas and full the tube with gas, stopping before they get past the volume markings. After turning the butane off and removing the tube, students must adjust the tube in the larger container of water and measure the gas volume when the water volume inside and outside of the tube is equal.
  • If they choose a graduated cylinder, they will follow the same basic procedure, except the larger water container will have to be big enough for them to handle the size of the cylinder when they invert it. They can use a lighter or a butane gas can to release the gas into the graduated cylinder, displacing enough water so that the water level inside and outside of the cylinder can be made equal.
• In addition to recording the volume of gas, students will also have to collect or look up the following data:
  • Room/water temperature (make sure water is at room temperature)
  • Mass of gas container before and after releasing the gas. They need to make sure that the gas container is dry before taking the after measurement. If using lighters they might consider letting them sit overnight to make sure all of the water has evaporated.
  • Room Pressure
  • Vapor pressure of water at your room/water temperature
  • Molar mass of butane