Does a Gas Have Mass?

“Of course,” you will say. But what do your students think when you first tell them about Avogadro’s law? In many cases, it is quite difficult for them to perceive that equal volumes of gases, at the same temperature and pressure, contain the same number of molecules. Do these invisible molecules really have appreciable masses that they could measure? What happens if you squeeze additional volumes of gases into a container? Does the mass increase? What happens to the pressure of the gases? Is the increase in mass related to the increase in pressure? Here is a simple experiment that can answer these questions, and help students hypothesize about this very abstract concept.

This experiment uses a Vernier Gas Pressure Sensor and its accessories, Logger Pro 3, an Ohaus Scout Pro balance, LabPro, and a plastic bottle. In the first trial, students measure the mass of a plastic bottle as more and more air is added to the bottle using a syringe. Students add air in 20 mL increments. They use the Events with Entry mode of Logger Pro to record the mass and enter the corresponding volume of air present in the container. In the second trial, students repeat the experiment using a Gas Pressure Sensor. They add air to the bottle using the same techniques used in Trial 1, and then record pressure readings and corresponding volumes of air. Download the complete instructions for this experiment.

Conclusion
By examining the plot of Mass vs. Volume Added, students can see that adding equal volumes of gases to the same container appears to increase the mass by equal increments (due to the linear relationship between mass and volume). For at least one gas composition (air), these data are certainly consistent with the assumption, attributed to Avogardro, that equal volumes of gases, at the same temperature and pressure (before compressing), contain equal numbers of molecules. It should also be noted that these data enable students to easily calculate the density of air from the slope of the linear fit of the plot.

The second run is interesting because it has a similar linear relationship, but this time between Pressure and Volume Added. This behavior is consistent with kinetic molecular theory, since greater numbers of molecules should result in proportionally greater numbers of collisions (and therefore, increased pressure).

Finally, since the two trials were run using precisely the same procedure of adding 20 mL volumes, students might find it interesting to view a plot of Pressure vs. Mass. They can do this by adding a new graph in Logger Pro, and then choosing to plot Pressure on the y-axis, and Mass on the x-axis. These data show a linear relationship between Mass and Pressure.

There are many possible extensions of this experiment. If, instead of using air samples in the syringe, you were to substitute other pure gases, say oxygen or carbon dioxide, students could see that the ratio of slopes is related to the ratio of molecular weights of gases. Note that because the gases are compressed into the same container each time, it is not necessary to take buoyancy of the gases into account (normally a cumbersome step).