Massing Gases

Introduction

Make Avogadro proud by using his law to determine the molar mass of several gases. In this demonstration, the mass of oxygen and various other gases will be determined, and their mass ratios will be used to calculate the molar mass of each gas.

Concepts

• Avogadro’s law
• Ideal gas law
• Molar mass

Materials

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. Carefully release the samples of air and oxygen into the atmosphere. Burner gas or poisonous gases should be released under an efficiently operating fume hood.

Prelab Preparation

Gas Delivery Apparatus

1. Assemble the gas delivery apparatus by first pushing the 1-quart plastic bag through the hole in the #10 stopper, leaving approximately 1 inch of the bag out of the opening (see Figure 1).
   {13842_Preparation_Figure_1}
2. Place the #2 one-hole stopper firmly into the freezer bag opening. The freezer bag should be held tightly between the walls of the two stoppers (see Figure 2).
   {13842_Preparation_Figure_2_Top view}
3. Carefully insert the tapered end of the medicine dropper through the hole of the #2 stopper. Place a drop or two of glycerin in the hole and slowly work the dropper back and forth until the tip is inside the bag (see Figure 3).
   {13842_Preparation_Figure_3_Assembly with dropper}
4. Attach the short piece of latex tubing over the wide end of the medicine dropper.
5. Place the pinch clamp on the latex tubing. The apparatus is now complete.
6. Assemble a new gas delivery apparatus for each additional gas to be measured.

Filling with a Gas

1. Evacuate the bag. Remove the pinch clamp and attach the latex tubing to either a vacuum pump or aspirator. When the bag has been evacuated, replace the pinch clamp on the tubing.
2. Attach the end of the tubing to the stem on the valve of the gas source.
3. Remove the pinch clamp.
4. As slowly as possible, fill the bag assembly with the gas. The bag should be taut when filled, but not ready to burst.
5. Turn off the gas source and replace the pinch clamp.
6. Remove the tubing from the stem. The bag now contains a slightly pressurized sample of gas.
7. Repeat steps 1 through 6 for each gas to be massed.

Procedure

1. Push the plunger of the 140-mL syringe to the bottom of the syringe and attach the syringe tip cap to the tip of the syringe.
2. Pull the plunger to the 140-mL mark and place the nail in the prepared hole in the plunger so that the syringe plunger is at about the 140-mL mark. This step requires two people—one person pulls the plunger out past the 140-mL mark and the other person then inserts the nail in the prepared hole.
3. Find the mass of the complete syringe assembly on a balance capable of reading to the nearest 0.001 g. Record the mass in the data table.
4. Open the syringe tip cap to allow air to enter the syringe. Replace the syringe tip cap and find the mass of the complete syringe assembly with air. Record the mass in the data table.
5. Remove the nail and the syringe tip cap. Depress the plunger to expel the air from the syringe. Go to the gas delivery bag in the fume hood and attach the syringe to the latex tubing. (This can be done by angling the tip of the syringe at 45 degrees to the end of the tubing, then working the tubing over the tip of the syringe.)
6. Release the pinch clamp and draw the gas into the syringe until the plunger is slightly past the 140-mL mark.
7. Insert the nail into the hole in the plunger, then push the plunger forward so the nail rests on the syringe barrel.
8. Reattach the pinch clamp to the latex tubing.
9. Hold the plunger in while releasing the syringe from the latex tubing. Immediately attach the syringe tip cap.
10. Mass the syringe assembly containing gas to the nearest 0.001 g. Record the mass in the data table.
11. Release the cap and expel the gas (use a fume hood when releasing gases that are unknown or poisonous).
12. Repeat steps 5–11 with any additional gases.

Student Worksheet PDF

13842_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration an unlimited number of times: 140-mL plastic syringe, #10 finishing nail, friction cap, 2 lengths of latex tubing, 2 pinch clamps, 2 size #10 rubber stoppers, size #2 rubber stopper, 2 glass medicine droppers and 7 quart-sized plastic freezer bags.
• There are various sources for gases to test. Propane and butane are available from hardware supply stores. Burner gas is a source for methane. Lecture bottles of gases, ranging from low molecular weight gases, hydrogen and helium, to the high molecular weight gas, sulfur hexafluoride, are available from Flinn Scientific.
• The data sheet can be made into a transparency with the data displayed on an overhead projector.
• If a milligram balance is not available, a centigram balance (0.01-g precision) may also be used. There will be some loss in accuracy using a centigram balance.
• Step 2 requires a good amount of force to pull the syringe plunger out. Keep fingers away from the plunger shaft to avoid pinching fingers if the plunger should slip.
• An alternative method can be used to determine the true mass of each gas. Each gas is massed in the syringe to give its apparent mass. The true mass is calculated by adding the mass of the same volume of air to this apparent mass. Use the density of air tables in the CRC Handbook of Chemistry and Physics to determine the mass of air.

Discussion

Air, like water, exerts a positive or upward buoyant force on all objects. This force is compensated for in balances when “massing” liquids and solids. When massing gases, however, this force is not negligible. The apparent mass of gas will be less than the actual mass of the gas.

True mass of gas = apparent mass of gas + mass of air displaced.

By evacuating the syringe volume of all gas (steps 1–3), the last term, mass of air, is eliminated and the true mass of the gas can be directly determined.

The molar mass of any gas can be estimated using the Ideal Gas Law. However, this determination requires accurate values for temperature (T), pressure (P) and syringe volume (V).

The principles of Avogadro’s law can be used to eliminate the need for these values. Avogadro’s law states that the number of moles of a gas is directly proportional to its volume, when pressure and temperature are held constant.

k = P/RT

If two gases are at the same temperature, pressure and volume, it follows that they have the same number of moles. The moles of any gas are equal to the mass of the gas divided by its molar mass. Substituting into Equation 5 If the molar mass of one gas is known, the molar mass of the other gas can be determined from Equation 6.

In this demonstration, pure oxygen or air can be used for the known gas. The molar mass of oxygen is 32.0 g/mol. Air is a homogeneous mixture of gases and has an apparent molar mass of 28.9 g/mol. A sample calculation using oxygen as the reference gas and air as the “unknown” gas is summarized.

If the mass of air is determined to be 0.168 g and the mass of oxygen is 0.176 g, the experimental molar mass of air is calculated as follows:

References

Special thanks to DeWayne Lieneman, retired chemistry teacher, Glenbard South High School, Glenn Ellyn, IL, for providing the idea and procedure for this demonstration.

Shakhashiri, B. Z., Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison; 1985; Vol. 2, pp 44–47.