There are two primary objectives of this experiment. The first is to determine the relationship between the pressure and volume of a confined gas. The gas we use will be air, and it will be confined in a syringe connected to a gas pressure sensor (see Figure 1). Moving the piston changes the volume of the syringe. We will use the gas pressure sensor and a data logger to record the change that occurs in the pressure exerted by the confined gas as we change its volume. It is assumed that temperature will be constant throughout the experiment. The amount of gas, that is the number of moles of gas, is also assumed to be constant. Pressure and volume data pairs will be collected during this experiment and then analyzed. From the data and graph, you will be able to determine what kind of mathematical relationship exists between the pressure and volume of the confined gas. Historically, this relationship was first established by Robert Boyle in 1662 and has since been known as Boyle’s law.
Gases are made up of molecules that are in constant motion and exert pressure when they collide with the walls of their container. The velocity and the number of collisions of these molecules is affected when the temperature of the gas increases or decreases. The second primary objective In this experiment is to study the relationship between the temperature of a gas sample and the pressure it exerts. Using the apparatus shown in Figure 2, you will place an Erlenmeyer flask containing an air sample in water baths of varying temperature. Pressure will be monitored with a gas pressure sensor and temperature will be monitored using a temperature probe. The volume of the gas sample and the number of molecules it contains will be kept constant. Pressure and temperature data pairs will be collected during the experiment and then analyzed. From the data and graph, you will determine what kind of mathematical relationship exists between the pressure and absolute temperature of a confined gas. You may also do the extension exercise and find a value for absolute zero on the Celsius temperature scale.
Objectives
In this experiment you will…
• use a gas pressure sensor and a gas syringe to measure the pressure of an air sample at several different volumes.
• determine the relationship between pressure and volume of the gas.
• describe the relationship between gas pressure and volume in a mathematical equation.
• use the results to predict the pressure at other volumes.
• study the relationship between the temperature of a gas sample and the pressure it exerts.
• determine from the data and graph, the mathematical relationship between the pressure and absolute temperature of a confined gas.
• find a value for absolute zero on the Celsius temperature scale.
Materials
computer and Logger Pro software
LabQuest Mini
gas pressure sensor
temperature probe
plastic tubing with two connectors
125-mL Erlenmeyer flask
20-mL gas syringe
2 600-mL beakers
non-iodized salt
rubber stopper assembly
ice cubes
hot plate
utility clamp
ring stand
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Procedures
Part I: Boyle’s Law
1. Place the FRCC CHE CD in the CD Drive.
2. Connect the LabQuest Mini to the computer using the USB cable.
3. Connect the gas pressure sensor to the LabQuest Mini in CH 1.
4. Start the Logger Pro software.
5. Open the file “Boyle’s Law – Pressure-Volume Relationship” from the CHE 101 directory on the D:/ drive.
6. Prepare the gas pressure sensor and an air sample for data collection.
a. With the 20-mL syringe disconnected from the gas pressure sensor, move the piston of the syringe until the front edge of the inside black ring (indicated by the arrow in Figure 1) is positioned at the 10.0 mL mark.
b. Attach the 20-mL syringe to the valve of the gas pressure sensor. It is only necessary to use a half twist motion to attach the two pieces together. More than this can damage either the sensor or the syringe.
7. To obtain the best data possible, you will need to correct the volume readings from the syringe. Look at the syringe; its scale reports its own internal volume. However, that volume is not the total volume of trapped air in your system since there is a little bit of space inside the pressure sensor.
To account for the extra volume in the system, you will need to add 0.8 mL to your syringe readings. For example, with a 5.0 mL syringe volume, the total volume would be 5.8 mL. It is this total volume that you will need for the analysis.
8. Click to begin data collection.
9. Collect the pressure vs. volume data. It is best for one person to take care of the gas syringe and for another to operate the computer.
a. Move the piston to position the front edge of the inside black ring (see Figure 2) at the 5.0 mL line on the syringe. Hold the piston firmly in this position until the pressure value stabilizes.
b. When the pressure reading has stabilized, click . (The person holding the syringe can relax after is clicked.) Type in the total gas volume (in this case, 5.8 mL) in the edit box. Remember, you are adding 0.8 mL to the volume of the syringe for the total volume. Press the ENTER key to keep this data pair. Note! You can choose to redo a point by pressing the ESC key (after clicking but BEFORE entering a value).
c. Move the piston to the 7.0 mL line. When the pressure reading has stabilized, click and type in the total volume, 7.8 mL.
d. Continue to repeat this procedure with the piston positioned at syringe markings of 9.0, 11.0, 13.0, 15.0, 17.0, and 19.0 mL.
e. Click when you have finished collecting data.
10. Record the pressure volume data in Table 1 (see PostLab).
11. Examine the graph of pressure vs. volume. Based on this graph, decide what kind of mathematical relationship you think exists between these two variables, direct or inverse. To see if you made the right choice:
a. Click the Curve Fit button, .
b. Choose Variable Power (y = Ax^n) from the list at the lower left. Enter the power value, n, in the Power edit box that represents the relationship shown in the graph (e.g., type “1” if direct, “–1” if inverse). Click
c. A best-fit curve will be displayed on the graph. If you made the correct choice, the curve should match up well with the points. If the curve does not match up well, try a different exponent and click again. When the curve has a good fit with the data points, then click .
12. To confirm the type of relationship that exists between pressure and volume, plot a graph of pressure versus the reciprocal of volume (1/volume or volume–1).
a. Use the “1/Volume” column of data that the logger automatically calculated from your original volume data.
b. Click on the horizontal-axis label “Volume”, from the popup menu select “1/Volume” to be displayed on the horizontal axis.
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13. Decide if the new relationship is direct or inverse and change the formula in the Fit menu accordingly.
a. Click the Linear Fit button, .
b. A best-fit curve will be displayed on the graph. In the Linear Fit display box, observe the “Correlation” value which should be very close to 1.000 and the b(y-intercept) value which should be close to 0 mmHg. If either of these values is not what is expected, try eliminating the highest pressure data point from your curve fit. This can be accomplished by moving the right (high pressure) bracket “]” left to eliminate one or more points from the linear fit. When the curve has a good fit with the data points, then click .
14. If the relationship between P and V is an inverse relationship, the plot of P vs. 1/V should be direct; that is, the curve should be linear and pass through (or near) your data points. Examine your graph to see if this is true for your data.
15. On a separate piece of paper sketch the graph of the pressure vs. 1/volume. Be sure to label the axis and record the equation for line. Include this with your lab report.
Part II: Guy-Lussac’s Law
16. Place the FRCC CHE CD in the CD Drive.
17. Connect the LabQuest Mini to the computer using the USB cable.
18. Connect the gas pressure sensor to the LabQuest Mini in CH 1 and the temperature probe in CH 2.
19. Start the Logger Pro software.
20. Open the file “Guy-Lussac’s Law – Pressure-Temperature Relationship” from the CHE 101 directory on the D:/ drive.
21. Obtain and wear safety glasses!
22. Prepare a hot-water bath. Put about 400 mL of hot tap water into a 600-mL beaker and place it on a hot plate. Turn the hot plate to a high setting.
23. Prepare a salt ice-water bath. Put about 50 mL of hot tap water into a second 600 mL beaker and add about ten
grams of salt. Stir to dissolve. Add ice.
24. Prepare the temperature probe and gas pressure sensor for data collection.
a. Obtain a rubber-stopper assembly with a piece of heavy-wall plastic tubing connected to one of its two valves. Attach the connector at the free end of the plastic tubing to the open stem of the gas pressure sensor with a gentle clockwise turn. Leave its two-way valve on the rubber stopper open (lined up with the valve stem as shown in Figure 2) until Step 25c.
b. Insert the rubber-stopper assembly into a 125-mL Erlenmeyer flask. Important! Gently twist the stopper into the neck of the flask to ensure a tight fit.
c. Close the 2-way valve above the rubber stopper – do this by turning the valve handle so it is perpendicular with the valve stem itself (as shown in Figure 3). The air sample to be studied is now confined in the flask. Do not open the valve after this point in the experiment.
25. Click to begin data collection.
Figure 3
Figure 2
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26. Collect pressure vs. temperature data for your gas sample:
a. Place the flask into the ice-water bath. Use a ring stand and utility clamp to hold the flask into the water bath. Make sure the entire flask is covered (see Figure 4). Stir with a glass rod.
b. Place the temperature probe into the ice-water bath.
c. When the pressure and temperature readings displayed in the meter stabilize, click . You have now saved the first pressure-temperature data pair for the first, cold water bath.
27. Use a ring stand and utility clamp to place the flask into the boiling-water bath. To keep from burning your hand, hold the tubing of the flask using a glove or a cloth. Once it is secure, hold the temperature probe in the boiling-water bath. After the temperature probe has been in the boiling water for a few seconds, repeat the Step 27 procedure for this second, hot water bath. Remove the flask and the temperature probe after you have clicked . CAUTION! Do not burn yourself OR the probe wires with the hot plate.
28. Pour about 100 mL of water out of your hot water bath (into the sink) leaving about 300 mL. Add cold water from your salt ice water bath to the hot water bath until the total volume is about 400 mL. Repeat the Step 27 procedure using this third, slightly cooler water bath.
29. Pour about 200 mL of water out of your hot water bath (into the sink) leaving about 200 mL. Add cold water from your salt ice water beaker to the hot water bath until the total volume is about 400 mL. Repeat the Step-27 procedure using this fourth, even cooler water bath.
30. Click when you have finished collecting data. Turn off the hot plate. Record the pressure and temperature values in Table 2 (see PostLab). Note that the temperature is recorded in °C and in K (absolute temperature).
31. Examine your graph of pressure vs. temperature (°C). Notice that the relationship between P and T is direct; that is pressure increases with temperature.
a. Click the Linear Fit button, . A best-fit linear regression curve will be shown for the four data points. The equation for the regression line will be displayed in a box on the graph, in the form y = mx + b. “b” is the y-axis intercept and “m” the slope of the line.
b. Remove the curve fit box on the graph by clicking on its upper-left corner.
c. Click on the vertical-axis label and select “Temperature” to plot the Celsius temperature. In the same way, select “Pressure” to be displayed on the horizontal axis.
d. Rescale the temperature axis from a minimum of –300°C to a maximum of +200°C. This may be done by clicking on the minimum or maximum value displayed on the graph axis and editing them, or by pulling the axes using the squiggly bar (ask your instructor if you are not sure). The pressure axis should be scaled from 0 mmHg to +800 mmHg.
e. Click the Linear Fit button, . The numerical value for b is the y-intercept and now represents the Celsius value for absolute zero. At what temperature does the line intersect zero on the Pressure axis? Record this temperature on your PostLab Part II, Question 3.
f. On a separate piece of paper sketch the graph of the Temperature vs. Pressure. Be sure to label the axis and record the equation for line. Include this with your lab report.
Waste Disposal
All salt/ice water solutions may be flushed down the drain.
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PRE-LAB: Complete Quiz on D2L and record your information
1. Based on your experience with gases in balloons, what would you expect to happen to the volume of a balloon if you took it into the vacuum of space? If you took it outside on a cold winter day?
2. Perform the following conversions:
a. 6.45 atmospheres to torr
b. 760 torr to mmHg
c. 123.0°C to K
d. 30.0 psi to atmospheres
3. Write an equation expressing the Combined Gas Law and describe how the variables are related.
4. Consider a sample of gas that has a volume of 10.0 L at 0.875 atmospheres and 25°C. What will its volume be if the pressure on the gas is increased to 1.25 atmospheres without changing the temperature?
5. Calculate the final pressure exerted by a sample of gas confined in a rigid cylinder at 37°C and 650 mmHg when you heat the cylinder to 73°C.
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Post Lab – Part I: Boyle’s Law – Be sure to attach your sketched graphs
1. Answer each of the following using actual data from the experiment.
a. What does your data show happens to the pressure when the volume is doubled from 5.0 mL to 10.0mL? Show the pressure values from your data in your answer.
b. What does your data show happens to the pressure if the volume is halved from 20.0 mL to 10.0 mL? Show the pressure values from your data in your answer.
c. What does your data show happens to the pressure if the volume is tripled from 5.0 mL to 15.0 mL? Show the pressure values from your data in your answer.
d. From your answers to Parts a, b, and c as well as the shape of the curve in the plot of pressure vs. volume, do you think the relationship between the pressure and volume of a confined gas is direct or inverse? Explain your answer.
2. What experimental factors are assumed to be constant in this experiment?
3. Using the seven ordered pairs in your data table calculate the value of k in the Boyle’s Law equation, k = P·V. Show your answers in the third column of the table at right.
4. How constant were the values for k you obtained in Question 3? Calculate the average and the range for k from your data.
Average __________
Range __________ to __________
5. Why might eliminating the high pressure point in your data set
improve the linear fit?
Volume (mL)
Pressure (mmHg)
Constant, k (P·V)
Table 1: Boyle’s Law Data
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Post Lab – Part II: Guy-Lussac’s Law- Be sure to attach your sketched graphs
1. Refer to your data table and complete the table below. Calculate the constant k using the values for temperature in °C and again using absolute temperature (K).
Pressure (mmHg)
Temperature (°C)
Temperature (K)
Constant, kC P / T (°C)
Constant, kK P / T (K)
2. For which temperature scale is the value of k constant? Explain why this is so.
3. What is the equation relating temperature in °C and absolute temperature (in K)?
Record the value for the zero pressure intercept of your line. _______________ °C
How does this value compare to the equation from above?
4. Write an equation to express the relationship between pressure and temperature (K). Use the symbols P, T, and k. What temperature units are used?
5. According to this experiment, what should happen to the pressure of a gas if the Kelvin temperature is doubled? Check this assumption by finding the pressure at –73°C (200 K) and at 127°C (400 K) on your graph of pressure versus temperature. How do these two pressure values compare?
Table 2: Guy-Lussac’s Law Data
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