Summary

In this lab, students will investigate the relationships of the variables related to gases. They will draw particle diagrams and derive equations to express these relationships. They will then combine these relationships to derive the combined gas law and the ideal gas law. Finally, they will use the molar volume of a gas at STP to derive the ideal gas constant, R.

Grade Level

High school

NGSS Alignment

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

• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Developing and Using Models
  • Analyzing and Interpreting Data

Objectives

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

• Understand how to represent direct and inverse relationships using multi-variable equations.
• Understand and model the relationships between
  • Temperature and volume of a gas at constant pressure.
  • Moles and volume of a gas at constant volume.
  • Pressure and volume of a gas at constant temperature.

Chemistry Topics

This lab supports students’ understanding of

• Gases
• Gas Laws
• Properties of Gases (temperature, volume, pressure and moles of particles)

Time

• Teacher Preparation: 20 minutes
• Lesson: one or two class periods, depending on student familiarity with using equations to model direct and inverse relationships, and with drawing particle models

Materials

Station 1:

• Balloons (standard 12 inch latex balloon)
• Trash Can

Station 2:

• Sealed Luer-lok syringe (with plunger and screw-on cap)
• Room thermometer
• 2 Thermometers
• Two 400-mL or 600 mL beakers for ice and warm water baths
• Hot plate
• Ice

Station 3:

• Mini marshmallows
• Sharpie or other marker
• Unsealed Luer-lok Syringe with plunger and cap
• Trash can

Safety

• Always wear safety goggles when handling glassware and chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• Food in the lab should be considered a chemical not for consumption.
• Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.

Teacher Notes

• Many teachers find that gas behavior and the gas laws are some of the easier topics to teach. Students enjoy the many demos and labs available, and the equations they use to “plug and chug” are relatively straight-forward. However, students can memorize the relationships between the variables affecting gases (pressure, volume, temperature, and moles) without knowing what causes them on a particulate level. Likewise, students can successfully solve gas law problems without a conceptual understanding of what the equations represent. This activity was designed to help students connect macroscopic observations of gases with the equations which model the behaviors and with what is happening at the particulate level. First, students learn how to model direct and inverse relationships using multi-variable equations. They then make observations at three lab stations. At each station they come up with an equation modeling the behavior they observe, and draw diagrams depicting what the gas particles are doing. After the lab, students use the equations they generated to derive the Combined Gas Law and the Ideal Gas Law. Finally, using the value of molar volume at STP, then derive the ideal gas constant, R.
• My students have found this activity very helpful in understanding how the equations are connected with the particle behavior. Some of them have even derived the ideal gas constant on tests in extra credit questions!
• This lab is similar to the Three Station Gas Lab, but adds connecting the macroscopic observations with the gas law equations. It can be used for day one of a seven day gas law unit. See the full article in Chemistry Solutions for the other parts of the unit.
• Before the lab, guide students through the Prelab. Show them that direct relationships do not always have multiplication in them, nor are inverse relationships always shown with a division.
• At each station, student observe gas properties, write equations, and draw particle diagrams. If students have not drawn particle diagrams before, spend some time showing them how to model particles, their collisions, and how to represent temperature (with longer/shorter arrows for velocity).
• Example particle diagrams for Station 2:
  • Notice that there are the number of particles (arrows) in both syringes, but the hotter ones have longer arrows to show that they are moving faster.

Ice Bath	Warm Water Bath

• Station 1: Students simply blow up a balloon.
  • It seems simplistic, but it is helpful to start with a straight-forward direct relationship of # of moles to volume. More particles, more collisions.
  • Point out that this is a constant-pressure situation.
  • Watch that they throw the used balloons away so that the used balloons are not re-used.
• Station 2: Students cool, then heat a syringe.
  • Set up an ice water bath in one beaker.
  • Set up a second beaker of water on a hot plate. Turn on the hot plate before class to give it time to warm up (but don’t let it boil).
  • Double-check that the plungers of the syringes can slide easily (you may need to give them a little surreptitious help after cooling or heating).
  • Guide students to understand that pressure and moles are both constant at this station.
• Station 3: Students modify the volume of a sealed syringe.
  • Note that the volume of the marshmallow changes as the volume of the syringe changes.
  • When the volume of the syringe is decreased, the air molecules in the syringe collide with the marshmallow more often, while the air molecules inside the molecules are still hitting at the same rate, increasing the pressure on the outside of marshmallow and making it look “smushed”.
  • When the volume of the syringe is increased, the air molecules in the syringe collide with the marshmallow less often (less pressure), but the air particles inside the marshmallow are still colliding as often as before, so the marshmallow expands.
  • The instructor must point out that there are air molecules trapped inside the marshmallow, and that the # of particles in the syringe is constant (as well as temperature).
  • This is the station where it is most tempting for students to just say “pressure went up (or down)” without give a particle-level explanation.
• Parts II and III may be completed on a second day. Students may need some guidance on how to combine multiple equations into one. They may also require reminders about the values of molar volume and STP.
• Note that Part IV can be challenging for students since they will need to access their prior knowledge about Molar Volume and STP conditions in order to determine the value for “R”.
• Based on the ability level of students, teachers may provide a hint, or the value(s) for Molar Volume and/or STP for use in Part IV.
• If students have difficulty with a particular section of the lab, it is suggested that teachers encourage students to consider the question(s) over a longer period of time rather than rushing to a conclusion too quickly. 
• It is recommended that students do all parts with their lab partners and not as homework.