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Gas Density and Relative Molecular Mass (1 Favorite)

LAB in Density, Temperature, Density, Pressure, Measurements, Volume, Error analysis, Error analysis. Last updated October 14, 2019.


Summary

In this lab, students will quantitatively determine the densities of four gases. This is the third part of a larger gas unit featured in Chemistry Solutions. The PowerPoint is the fourth part of the gas unit.

Grade Level

High school

Objectives

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

  • Confirm that gases do have a density.
  • Calculate the density of a gas.

Chemistry Topics

This lesson supports students’ understanding of

  • Gases
  • Density

Time

Teacher Preparation: 1 hour
Lesson
: one class period

Materials

  • Sources of oxygen, natural, carbon dioxide, and helium gas
  • Graduated syringe (60 mL) with secure stopper
  • Nail to secure constant volume in syringe
  • Balance

Safety

  • Always wear safety goggles when working with chemicals in the lab.
  • The instructor or lab assistant should operate gas tanks and valves always.
  • Expel natural gas and oxygen in the fume hood to avoid any potential hazards

Teacher Notes

  • Helium sometimes gives wildly erratic results, so you may want to have students repeat this measurement.
  • To debrief from this lab, review this PowerPoint with students to have them consider why densities differ between gases.

For the Student

Lesson

Background

Solids and liquids have a density. In the case of solids, you can find its mass on a balance, then, by water displacement or geometry, you can find its volume. For liquids, you put a measured volume into a tared container and find the mass of the liquid alone. Then you divide mass by volume. The densities of solids and liquids tend to be between 0.5 and 25 g/cm3.

But experience suggests that densities of gases are different. Helium balloons float in air because He is less dense than air. Carbon dioxide balloons fall to the ground because CO2 is more dense than air. Is it even possible to measure densities of gases—we can’t see the gases?

If we can find the densities of several gases at the same temperature and pressure, and the densities are different, then there are two possibilities. Either the denser gas has more particles of the same size, or the heavier gas has heavier particles. Further experimentation will help determine which is correct. So, let’s measure the masses of equal volumes of gases at the same temperature and pressure.

Purpose

To determine the density of gases at the same temperature (room) and pressure (atmospheric).

Safety

  • Always wear safety goggles when working with chemicals in the lab.
  • The instructor or lab assistant should operate gas tanks and valves always.
  • Expel natural gas and oxygen in the fume hood to avoid any potential hazards.

Procedure (Modified from a Flinn Scientific video)

  1. Prepare a table that will hold all of the data that you will collect. You’ll need to review the procedure to create the data table.
  2. Secure a syringe with a rubber stopper seal and a nail that can keep the syringe full of gas at a constant volume. Your teacher will demonstrate how to find the mass of the syringe with a vacuum inside the volume that will eventually contain your gases. Record the mass of the syringe to 0.01 g.
  3. Fill the assembly with natural gas from the gas jet. You must collect the same volume of gas each time. Stopper it when you have the designated volume. Find the mass of the assembly with the natural gas. Expel the gas in the fume hood. Does natural gas have a mass?
  4. Fill the assembly with oxygen to the same volume you did for natural gas. Stopper it, weigh it, and compare it with your previous results. Expel the gas in the fume hood.
  5. Fill the assembly with carbon dioxide. Again, use the same volume. Stopper it, weigh it, and compare it with your previous results. Expel the gas in the room.

Fill the assembly with helium. Again, use the same volume. Stopper it, weigh it, and compare it with your previous results. Expel the gas in the room.

Oxygen Natural Gas
Carbon dioxide
Helium
Gas + apparatus (g)
Evacuated apparatus (g)
Mass of gas alone (g)
Volume of gas (mL)
Results/Observations
Density of gas (g/mL)
Calculations

Analysis

  1. Were you surprised to find that each of these gases have a volume? Explain. (Be honest!)
  2. Find the ratio of the natural gas mass to the oxygen.
  3. Find the ratio of carbon dioxide gas mass to helium.

Conclusion

Answer the question raised in the objective of the experiment.
Mention sources of error in this experiment and suggest ways you might reduce them.