10: Gas Laws

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This lab was adapted from:
Bopegedera, A. M. R. P. (2007). An inquiry-based chemistry laboratory promoting student discovery of gas laws.
Journal of Chemical Education, 84(3), 465-468. https://doi.org/10.1021/ed084p465

Learning Objectives and Skills

Students will...
Students will...

INTRODUCTION

The pressure, volume, temperature, and amount of a gas are interrelated. Some of these relationships are directly proportional and some are indirectly proportional to each other. In this lab we will determine the relationship between the pressure and volume of a confined gas (temperature & moles remain constant) and then the relationship between the pressure and temperature (moles and volume remain constant). After taking measurements, you will record data, create graphs and determine the equation of the best fit lines on the graphs. Also from the data of the second experiment you will estimate absolute zero. If we assume that pressure is zero when all thermal activity ceases, then this is an estimate of absolute zero.

PROCEDURE

PART A. Setup & Data Collection for Pressure vs Volume (Moles and Temperature Constant)

1. Connect Equipment

Turn on LabQuest.
Plug the pressure sensor into any channel (CH1–CH4).

2. Set Up LabQuest on the Meter Screen (has a red banner)

Tap the Mode button → Select Events with Entry.
Enter “Volume” for “Name”, and “mL” for “Units”, then tap OK.

3. Change Pressure Units to atmospheres (atm)

Go to Sensors → Change Units → (your channel) → Gas Pressure Sensor → atm.

4. Prepare Syringe

Set syringe to 5 mL (don’t push or pull after this).
Attach syringe gently by twisting onto the pressure sensor (snug, not tight!).

5. Start Collecting Data

Tap the green Start button.
One partner adjust the syringe to 2 mL, the other tap “Keep” once pressure stabilizes.
Enter volume as 2.7 (for 2.0 (the volume of air in the syringe) + 0.7 (the volume of air in the pressure sensor) = 2.7 mL), then tap OK.

6. Collect More Data

Repeat step 5 for:
- 5 mL → Enter 5.7 mL
- 10 mL → Enter 10.7 mL
- 20 mL → Enter 20.7 mL
Tap the Stop button when done.

7. Graph and Record

Download and save a local copy of the linked Excel spreadsheet
On the pressure vs. volume graph that appears, tap points to view data, then record them in Part A of the provided Excel sheet.

Questions for Part A

What happens to pressure when volume approximately doubles (5.7 → 10.7 mL)? Include values to support your answer.
What happens to pressure when volume approximately halves (20.7 → 10.7 mL)? What happens to pressure when volume approximately doubles (5.7 → 10.7 mL)? Include values to support your answer.
What happens to pressure when volume nearly quadruples (5.7 → 20.7 mL)? Include values to support your answer.
Is the pressure-volume relationship direct or inverse? Explain with your data. It may be useful to look at your graphs.
Predict the pressure at 40 mL. Show your calculation.
Predict the pressure at 1.0 mL. Show your calculation.
Which variables are constant in this experiment?
One way to determine if a relationship is inverse or direct is to find a proportionality constant, k, from the data. If this relationship is direct, k = P/V will all have quite similar answers. If it is inverse, k = P*V will have quite similar answers. On your Excel spreadsheet, create formulas to calculate the columns for P/V and P*V.
Calculate average and standard deviation for k calculated each way in Excel. Which k is more consistent: P/V or P*V?
Write an expression relating P, V, and a constant k. This is Boyle’s law!
The values for k should be relatively constant. Calculate the average and standard deviation for each column using formulas in Excel. Which value (P*V or P/V) is closer to constant?
Plot the values pressure (y) against volume (x) by using the “Insert” menu on Excel and plotting a scatterplot. Add a linear trendline for this plot, making sure to display both the equation and R² value. Is it a good fit?
To confirm that an inverse relationship exists between pressure and volume, a graph of pressure as reciprocal of volume (1/volume) needs to be plotted. To do this in Excel:
1. Highlight the cells B3:B9. Right click and select “Insert”. Choose “Shift cells right”.
2. Add a new label “1/V” in cell B3, and create a formula in each cell for 1/V.
Plot the values pressure (y) against 1/V (x) by using the “Insert” menu on Excel and plotting a scatterplot. Add a linear trendline for this plot, making sure to display both the equation and R² value. Is it a good fit?
What is the relationship between the slope of this line (the “m” value) and the constant (k) you calculated?
Make sure both of these plots are clearly labeled and not overlapping in the spreadsheet. You may find it convenient move the chart to a new sheet. Click so the whole chart is selected, then right clicking to select “Move Chart” and select “New Sheet”. Make sure to give each sheet an appropriate name like “Pressure vs. 1/V”.

PART B. Investigating Pressure /Temperature Relationship at Constant Volume and Moles

Data Collection

(If doing Part A then Part B, delete the current data by tapping on File/New/Discard.) Connect the pressure sensor and add a temperature probe by connecting it into another CH port. You should now see two sensor readings on the screen (pressure and temp).
Go to the Meter screen with the top left meter icon, tap Mode in the upper right corner and change to Events with Entry by tapping on the down arrow and making the selection. Tap on OK.
Change the pressure units to atmospheres by tapping on the box for pressure and selecting atm. Tap on the temp box and choose Kelvin for the units for temperature.
Set the temperature probe on the bench top and connect the pressure sensor to the small 50mL Erlenmeyer flask with the connector tubing and let it sit on the bench top as well. A stopper that comes loose results in the need to restart the entire experiment; so make sure that the stopper is tightly seated in the flask.
Start the experiment by tapping the green start button in the lower left corner. You will not tap the stop button until you have done all four temperature tests. Do not handle the tubing and flask any more than absolutely necessary since heat from your hands can change the flask’s temperature. When the pressure reading stabilizes, (do not tap the stop button) tap the keep button and enter 1.
Attach a utility clamp to the flask as demonstrated. Take the LabQuest with the pressure sensor connected to the Erhlenmeyer flask and the attached temperature probe to one of the water baths. One is an ice water bath, another is slightly above room temperature and the third is a very warm water bath. Submerge the flask and as much tubing as possible without getting the pressure sensor wet. Place the temperature probe into the bath as well. Gently rock the flask until the pressure reading on the LabQuest stabilizes. (This will take about 5 minutes.) The more patient you are the better your results will be.
After 5 minutes when the pressure reading stabilizes, tap the keep button and enter 2.
Repeat step 6 and 7 until you have repeated the experiment in all three water baths entering 3 and 4 for each of these experiments after tapping the keep button. After the last data set has been “keep”ed, tap the stop button.
Tap the Graph tab and choose Graph Options… to open a window for setting up a proper display of the data. We need to determine if the pressure and temperature values are directly or inversely proportional. To start let’s look at temperature on the x-axis and pressure on the y-axis. Choose temperature in the pull down menu for the X-Axis Column and then check the Pressure under Run 1 and if necessary, uncheck Temperature under Run 1. Also, at the top of the window select “Autoscale from 0” and tap OK.
Tap Graph again, then Show Graph, and select Graph 1. To examine the data pairs on the displayed graph, tap the table icon to view your data. If you are completing part B first, download and save a local copy of the linked Excel spreadsheet. Under Part B, enter the pressure and temperature data you just took.

Questions, Part B:

17. If the temperature is increased, what does your data show happens to the pressure?

18. If the temperature is decreased, what does your data show happens to the pressure?

19. From your answers to the questions above and the shape of the curve in the plot of pressure vs temperature, do you think the relationship between the pressure and temperature of a confined gas is direct or inverse?

20. What experimental factors are assumed to be constant in this experiment? Question 4 on report sheet.

21. One way to determine if a relationship is inverse or direct is to find a proportionality constant, k, from the data. If this relationship is direct, k = P/T will all have quite similar answers. If it is inverse, k = P*T will have quite similar answers. On your Excel spreadsheet, create formulas to calculate the columns for P/T and P*T.

22. The values for k should be relatively constant. Calculate the average and standard deviation for each column using formulas in Excel. Which value (P*T or P/T) is closer to constant?

23. Write an expression relating P, T, and a constant k. This is Charles’ law!

24. Using the average of your four estimates for a constant in the above table. Use this average to estimate the pressure of the gas in the flask if the temperature was 15°C. Show your work

25. Plot the values pressure (y) against temperature (x) by using the “Insert” menu on Excel and plotting a scatterplot. Add a linear trendline for this plot, making sure to display both the equation and R² value. Is it a good fit?

26. Using the equation from your graph, estimate absolute zero (in K and °C). It is the temperature where the pressure of your gas would be 0.00 atm. You will have to enter this value as your “y” and solve for “x”. Show your work or do it in excel in a clearly labeled cell.

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