Lab 9 Gases

1. Objectives

• Represent an ideal gas process in multiple ways.

• Reason about ideal gas processes.

• Experimentally determine the relationship between state variables in a gas.

Pre-lab problem: Complete Problem 9.1 before lab. Write in hand-in sheet.

2. Representing an ideal gas law

Problem 9.1: Pressure, volume, and temperature are fundamental physical quantities used to describe an ideal gas.

1. If we hold the volume of an ideal gas constant by putting it in a container with rigid walls (like a metal can) and increase the temperature of the gas, describe what happens to the third quantity, the pressure of the gas. Give a microscopic explanation (no formulas) using your knowledge of the behavior of an ideal gas. How might the temperature of the gas been increased—give a microscopic explanation using everyday language.

2. How would your explanations from (a) be different if the temperature of the gas increased the same amount but now in a container with a moveable piston that kept the gas at constant pressure but not at constant volume?

Problem 9.2: Complete this problem with your lab partner. Let’s now explore how to represent the physical behavior of an ideal gas. Imagine you perform experiments in which you keep one of the three physical quantities P, V, or T constant and graph the other quantities that change (there is more than one possibility for each case). Draw graphs, as shown below, to describe what you would expect if you performed the following experiments.

1. You have two samples of gas each held at a constant volume V₁ and V₂, respectively, where V₁ > V₂ and you measure pressure vs. temperature.

2. You have two samples of gas each held at a constant pressure P₁ and P₂, respectively, where P₁ > P₂ and you measure volume vs. temperature.

3. You have two samples of gas each held at a constant temperature T₁ and T₂, respectively, where T₁ > T₂ and you measure pressure vs. volume.

III. Testing a model and estimating absolute zero

Problem 9.3: Design and perform an experiment to test how the pressure of an ideal gas depends on the absolute temperature when the volume of gas is held constant. Check with your lab instructor before starting the experiment. Note: In order to reach thermal equilibrium, you should wait two minutes for the temperature to stabilize before recording temperatures and pressures.

1. Use Excel to find a best-fit function for your data.

2. Decide how you can use this graph to determine value of absolute zero temperature in K. Evaluate your error and decide if the value you get is acceptable. Check with your lab instructor.

After completing your readings, remove the gas container from the ice water and leave it exposed to air so that it can reach thermal equilibrium for the next class.

IV. Summary questions

Problem 9.4: Two closed 1-liter samples, A and B, of different monatomic ideal gasses are heated at a constant volume. A graph of the pressure vs. temperature for each sample is shown below.

1. What do you know about the number of the atoms of gas A as compared to gas B?

2. What do you know about the mass of the atoms of gas A as compared to gas B?

Problem 9.5: Identify which of the following statements are correct and correct those that are not. Explain your choice.

1. Both samples are exhibiting a close approximation to ideal gas behavior.

2. Compared with sample 1, sample 2 must contain a larger mass of gas.

3. At any given pressure, sample 2 always has a higher temperature.

4. The behavior of the two samples points the definition of absolute zero temperature.

When finished with the summary questions, show your work to the lab instructor. Be prepared to answer questions about what you did.