Materials Included In Kit

Additional Materials Required

Disposal

Save all equipment for future use.

Teacher Tips

• A common kitchen “turkey baster” is a convenient large pipet to remove ice water from the bath and also to add tap water or hot water as needed (steps 11, 17).
• If you are working with air, the Mason jar is already filled with air. To fill the jar with another gas such as oxygen, nitrogen, carbon dioxide, or methane, remove the pressure gauge and open both stopcocks A and B.
• Collect oxygen or carbon dioxide in the jar by the upward displacement of air. Nitrogen and methane can be collected by the downward displacement of air. A cylinder of compressed gas is best for this but you can also use a plastic bag–gas collection and distribution kit. Once the gas is flowing into the Mason jar count to 10 and shut off the gas flow.
• Special care must be taken during the assembly, use and dismantling of the Gas Law Apparatus to be certain that the apparatus is always “gas tight” and that parts of the kit are not damaged. The rubber stopper was manufactured specifically for this kit. The stopper should be dry and clean.
• The following information describes the assembly of the Gas Law Apparatus.

  Clean the stopper and the mouth of the Mason jar with a dry cloth. Insert the rubber stopper into the mouth of the Mason jar. Force the stopper as far as possible into the Mason jar. The lower surface of the rim should be in complete contact with the top edge of the jar. Take note that two of the holes in the stopper are larger than the third hole. Syringe extenders will be inserted through each of the larger holes. Complete the insertion of the first syringe extender before inserting the second syringe extender. Insert the threaded end of a syringe extender into one of the larger holes in the rubber stopper. Push the syringe extender into the hole so that only the top 5 mm of the threaded part of the syringe extender is visible above the rubber stopper. You may not be able to complete this step without some mechanical assistance, as described.

  1. Place the rubber stopper on a solid surface. With a hammer, gently tap the Luer Lock end of the syringe extender until a depth of only 5 mm of the threaded end of the syringe extender is exposed above the rubber stopper.
  2. Invert the Mason jar and the rubber stopper assembly (Luer Lock of the syringe extender facing down) on the hard surface. Grasp the Mason jar with both hands and push down on the hard surface until a depth of only 5 mm of the threaded end of the syringe extender is exposed above the rubber stopper.
• A digital thermometer must be present in the small hole in the large rubber stopper. The experiment cannot be performed with an unplugged hole through the large rubber stopper. Remove the digital thermometer only if the apparatus is not going to be used in the very near future. Insert the thermometer through the hole of a size #1 one-hole rubber stopper. This is not designed to be a tight fit. A clamp will be used later to hold the whole assembly in place after it is placed in a water bath. Moisten your fingers with a drop of water. Moisten the long metal tube by rubbing your moist fingers along the length of metal tube. Hold the plastic housing of the digital thermometer and insert the metal tube through the small hole in the large rubber stopper. Push on the plastic housing until the tip of the long metal tube is approximately 1 cm above the bottom of the Mason jar.
• Push the threaded metal extension of the pressure gauge as far as possible into the hole of a small (#1) rubber stopper. Push the female end of a female/spinner stopcock as far as possible into the other end of the rubber stopper. Ideally, the pressure gauge extension and the female end of the stopcock should meet in the rubber stopper. This will be referred to as the “Pressure Gauge Assembly.” Connect the spinner end of the stopcock (this stopcock will be referred to as stopcock A) to one of the syringe extenders in the large rubber stopper. Adjust (rotate) stopcocks A and B so that their handles point outward (toward the brass ring). When tightening or loosening the spinner on a stopcock, grasp the barrel of the stopcock with the finger and thumb of one hand and grasp the spinner in the finger and thumb of the other hand. When opening or closing the stopcock, grasp the blue handle of the stopcock with the finger and thumb of the other hand. Be careful that stopcock A, attached to the pressure gauge, is always closed (the handle must be perpendicular to the barrel of the syringe) whenever you are preparing to inject gases into the system. Open stopcock A (the handle must be parallel to the barrel of the syringe) only when you are ready to read the pressure gauge. Connect the female/spinner stopcock (this will be referred to as stopcock B) to the other syringe extender.

Sample Data

Answers to Questions

1. Plot or obtain a graph of pressure on the y-axis versus temperature on the x-axis. Note: Extend the scale of the x-axis from –300 to 100 °C.
   {12579_Answers_Figure_2}
2. Looking at the data, is the pressure of a gas proportional to its temperature over the temperature range studied? Use a computer or calculator to generate the best-fit straight line through the data points.

   A plot of temperature versus pressure gives an excellent straight line through the data points. This means that the pressure of the gas is proportional to temperature over the entire temperature range studied. See the graph for the best-fit straight line.

3. Extend the straight line backward to estimate the x-intercept, the point at which the line crosses the x-axis. The x-intercept corresponds to absolute zero—the minimum temperature that would be needed to reduce the pressure of a gas to zero. What is the estimated value of absolute zero? How close is your value of absolute zero to the accepted value?

   The estimated value of absolute zero is –265 °C. The actual value is –273 °C. The error is due to the large distance from the experimental data points to the extrapolated value. Better accuracy would be possible if one could obtain pressure readings at below-zero temperatures.

4. Guy-Lussac’s law is explained on the basis of the kinetic-molecular theory for ideal gases. Would you expect to see greater deviations from ideal gas behavior at high or low temperatures? At high or low pressures? Explain.

   Deviations from ideal gas behavior become more important at low temperatures and at high pressures. At low temperatures, attractive forces between the gas molecules become more important (remember, in the KMT the presence of attractive forces is ignored). At high temperatures, gas particles have large kinetic energies, which helps them “overcome” the attractive forces between molecules. At high pressures, the gas particles are forced closer together and the attractive forces between them become stronger.

5. Safety warnings on aerosol cans illustrate a real-world application of Guy-Lussac’s law. Most aerosol cans will have a warning similar to the following:

   “Do not place in hot water or near radiators, stoves or other sources of heat. Do not puncture or incinerate container or store at temperatures over 120 °F.”

   Use the results of this experiment to predict what will happen to the gas in an aerosol container at elevated temperatures and to explain why the warning label is needed.
   An aerosol can is a constant volume container and is already “pressurized” (the internal pressure is about 2.5 atm). At high temperatures, the pressure of the gas inside the container will increase to an unacceptably high level and the container essentially becomes a bomb ready to explode. Note to teachers: There is a second reason for the safety warning. Many aerosols contain flammable gas propellants, which will ignite when released suddenly at high temperatures.

References

The Gas Laws Apparatus Kit is manufactured by Irwin Talesnick of S17 Science in Canada. Visit the S17 website at www.s17Science.com for information about additional gas laws equipment, supplies, experiments and instructions.

Introduction

Measure the pressure and temperature of a trapped gas directly to demonstrate Gay-Lussac’s Law for gases. The data can be extrapolated to determine the value of absolute zero.

Concepts

• Pressure vs. temperature of gases
• Gay-Lussac’s law
• Absolute zero
• Kinetic-molecular theory

Materials

Safety Precautions

Never add quantities of gas to the Gas Law Apparatus that will result in gauge pressures greater than 600 mm Hg or 80 kP_a. The brass gasket may come loose from the jar and damage other parts of the assembly. Take special care when assembling the apparatus to be certain that it is airtight. The large rubber stopper or gasket was manufactured specifically for this apparatus; keep it dry and clean. Do not exceed the recommended temperature range for working with the Gas Law Apparatus.

Procedure

1. Set up the gas law apparatus as shown in Figure 1. The stopcocks are labeled to simplify the instructions. Insert the digital thermometer through a #1 one-hole stopper.
   {12579_Procedure_Figure_1}
2. Make sure that all the connections are tight. Go through the following steps to verify that the gas kit is airtight.
   1. Connect a 60-mL syringe full of air to stopcock B.
   2. Open stopcock B—blue handle runs along the stopcock.
   3. Close stopcock A—blue handle is at 90° to the stopcock.
   4. Press the piston into the syringe until the piston reaches the bottom of the barrel.
   5. Close stopcock B and remove the syringe.
   6. Open stopcock A.
   7. Measure the pressure.
   8. Observe the pressure reading. If the pressure does not change during the next 60 sec, the gas kit is airtight.
   9. If the pressure changes, wrap a length of the blue friction cloth included in the kit around the Mason jar ring cover and tighten the ring as securely as you can.
   10. After getting a successful test close stopcock A. Open stopcock B slowly to let the trapped air escape. Open stopcock A to release the pressure on the gauge.
   11. Close stopcock A. You are now ready to start the demonstration.
3. Close both stopcocks A and B.
4. Fill a 60-mL syringe with air and connect the full syringe to stopcock B, which is closed.
5. Open stopcock B attached to the syringe and leave it open to control the flow of gas.
6. Slowly push the piston of the 60-mL syringe into the barrel, forcing the air into the Mason jar. When all of the gas in the syringe has been transferred into the jar, close stopcock B.
7. Open and then close stopcock A. The pressure reading should be 8–12 kP_a (50–70 mm Hg). Note: The apparatus may now be cooled. Adding the extra volume of gas avoids a negative pressure reading which could damage the pressure gauge. It also makes it easier to read the first few pressure measurements.
8. Turn on the digital thermometer and select the °C scale. This step may need to be repeated a number of times during the experiment as the display turns off in order to conserve the battery.
9. Fill a 600-mL beaker about one-third full with ice water to create a water bath for cooling the gas. Also prepare hot water for a heating bath: obtain 200–300 mL of hot water. Use a hot plate to keep the temperature at approximately 85–90 °C; do not let the water boil.
10. Immerse the gas law apparatus in the ice-water mixture in the 600-mL beaker. Use an adjustable clamp and support stand to clamp the rubber stopper on the digital thermometer. Adjust the clamp so that only the glass part of the Mason jar is covered by the ice water.
11. Monitor the temperature on the digital thermometer. Add more ice if necessary. When the air temperature drops below 10 °C use a large pipet to remove ice water from the water bath and replace it with tap water.
12. When the gas temperature rises to 10 °C, open stopcock A and count 1001, 1002, 1003, then close stopcock A. Tap the pressure gauge with your finger. Read and record the gauge pressure to ±1 kP_a or ±5 mm of Hg. Always open and close stopcock A and tap the pressure gauge in this manner to obtain the pressure of the gas in the jar. Record the gas temperature to ±0.1 °C
13. The actual pressure of the gas in the Mason jar is equal to atmospheric pressure plus gauge pressure. Use a barometer to determine atmospheric pressure. Record atmospheric pressure to ±0.5 kP_a or ±0.2 mm of Hg. You can also contact the local weather office to obtain atmospheric pressure at your location. Standard atmospheric pressure is 101 kP_a or 760 mm Hg.

    P_actual = P_atmospheric + P_gauge

14. Place the water bath with the gas law apparatus on the hot plate. Use a large pipet to remove cold water and add hot water to the bath. Adjust the heat setting on the hot plate to control the heating rate.
15. Record the gas temperature (digital thermometer) and the gauge pressure after every 10 °C increase in temperature. Continue making the corresponding temperature and pressure readings until the gas temperature reaches 80 °C. Remember to open and then close stopcock A to measure the pressure. Do not keep it open.
16. Turn off the hot plate once the temperature of the gas reaches 80 °C.
17. Allow the temperature of the water bath to drop and record the gauge pressure again when the gas cools to 80 °C. Continue taking and recording gas temperature and gauge pressure readings after every 10 °C drop in temperature.
18. The rate of temperature decrease can be adjusted by using the large pipet to remove hot water from the beaker and replacing it with ice-cold water.
19. Continue making readings until the temperature of the gas in the Mason jar decreases to about 10 °C.

Student Worksheet PDF

12579_Student1.pdf