"Molecular Weight" of Air


To determine the "molecular weight" of air.


    The physical behavior of gases is described by various laws, which have resulted from hundreds of years of experimentation.   Your text book details many applications of the ideal gas equation,  PV = nRT .  This equation provides a very useful, general description of the physical behavior of “ideal” gases.  Although real gases rarely behave in a completely “ideal” manner, this equation will provide a good basis for the investigation you are performing in the laboratory this week.  When any three of the four variables in this equation (pressure, volume, temperature, and moles of gas) are known, the equation may be rearranged and easily solved to determine the fourth. The gas constant, R, is usually expressed as 0.082057 L atm / K mol, although it can be expressed in different units.

    When describing the physical behavior of a gas, it is important to remember that gases are fluids.  We are used to thinking of liquids as fluids, but gases may also be so defined.  Fluids are substances that flow and can change shape to "fill" their container easily.  Under standard temperature and pressure conditions (STP), O°C and one atmosphere, gases exist as discrete isolated particles in large volumes of empty space.  Because of the low concentration (number/volume) of the molecules present and the almost negligible molecular attraction and repulsion between the particles, increasing the pressure forces them to occupy a significantly smaller volume of space.  For this reason, gases are called compressible fluids, whereas liquids are described as incompressible fluids. 

    Archimedes’ principle describes one property of fluids; they exert a buoyant force on objects wholly or partially immersed in them.  Air, being a fluid, exerts a buoyant force on objects submerged in it.  Balloons that are filled with a gas that is less dense than air will float if the weight of the balloon and the gas inside is less than the weight of the air it displaces.

Gas Cylinder Safety:

       Gas cylinders contain gas that is under high pressure and they should be handled with great care.  This type of container is very sturdy and well adapted for the purpose it serves.  It must remain firmly clamped and strapped into the bracket specifically designed to secure it.  NEVER attempt to unfasten or move the cylinder, and do not attempt to adjust the pressure gauge in any way.  If the cylinder were to become loose, topple over, and the valve broken, it would function as a rocket!   theRE ARE INSTANCES IN WHICH GAS cylinderS HAVE BEEN propelled through cinderblock walls and anything else in THEIR path.  Your TA will show you how to fill your Mylar balloons using the balloon valve on the gas cylinder.   


    Consider the following pairs of substances.  Think about the physical behavior of these substances if each pair were placed together in a separate container.  Explain what would occur for each set and why.  In each case, indicate which substance is more dense.

1.      Salad oil and vinegar

2.      Gasoline and water

3.      Oil and water

4.      Water and ice cubes

5.      Water and liquid mercury

6.      A hydrogen-filled balloon and the atmosphere



Laboratory Part 1:

            As a group, discuss your answers to the prelab question.  Did all the teams come to the same conclusion about the pairs of substances?  What physical properties are responsible for this behavior?

1.     Using the top-loading balance, obtain the combined mass of the empty Mylar balloon and plastic tie. (How much would you expect the balloon to weigh filled with air?  Try it!)

2.      Ask your TA to carefully fill the Mylar balloon with helium.  Care should be taken to avoid overfilling the balloon.  It should be completely expanded to fill the volume but not stretched tight.  When full, twist the neck of balloon closed and fasten it using the plastic tie.  Obtain the mass of the filled balloon assembly on the top-loading balance.

3. Find the difference in mass, Dm, of the empty balloon and the helium-filled balloon.

4.      Repeat parts 2 and 3 two more times.  Find the average difference in mass, Dm, and record your team's value on the board. Think about how you would expect a balloon filled with helium to behave in air (and in helium).  Why would it behave this way?

 As a group:

  Discuss the data that has been collected.  Is there anything unusual about the change in mass? Is it what you expected? What does Dm represent?  Could Dm be related to Archimedes' principle?

Laboratory Part 2: 

The objective of your investigation is to determine the "molecular weight" of air. Read carefully through the material on this page as well as the 'Procedures' page before you begin to plan your investigation.

    Consider the types of data you have collected in the laboratory this semester.  What physical properties have you been able to measure using the Vernier probes and Logger Pro software?

    One type of physical data that you have not yet measured is the pressure of a gas.  A Vernier pressure sensor is available in the lab, which can measure pressure changes in gases, but can also easily measure the current atmospheric pressure.  The procedure for setting up this sensor, analogous to the one you use for the direct-connect temperature probe, is briefly described below. 

 Plug the cord from the Pressure Sensor Box into one of the four channels (CH1, CH2, etc.) on the LabPro box. Go to the Setup pull-down menu and select "Set Up Sensors" followed by "Show All Interfaces." A Dialog Box will open which shows the four channels along with a series of boxes that show which probe is plugged into each port. Right click on the box corresponding to the Channel you're using for the pressure sensor. Select "Choose Sensor" and follow through the menus until you find "Pressure Sensor." A tiny picture of the pressure sensor should appear in the box corresponding to the selected Channel.

             Consider the techniques you have used to determine physical properties.  Recall that volume can be measured using water displacement.  This method was used to determine the volume of pennies in an earlier experiment.  A sizeable overflow tank is available in the lab, should you wish to obtain the volume of larger objects.  The procedure for using this tank is straightforward. 

1.      Place the overflow tank in or over a large sink (if it is not already there) and fill the tank until water just begins to spill out the overflow spout. When water ceases to flow from the spout, you are ready to begin. If a few drops of water are remaining at the mouth of the overflow tube, remove these before you start your measurement.

2.      Place a large, empty container below the spout. 

3.      Place the object whose volume you wish to measure, inside the tank.  Gently push it down until all of it is just submerged.  Try not to measure the volume of your hand along with the object!  (If you are measuring the volume of a filled balloon, be sure that no gas can escape from the neck of the balloon as you are taking the measurement. This would change the volume you wished to measure.)  Hold the object below the surface until water stops overflowing. 

4.      Remove the object from the tank.

5.      Measure the volume of the overflow water, which will be the same as the volume of the submerged object.

 In this part of today’s investigation, you will be using the instruments and equipment available to you in the lab, your knowledge of the ideal gas equation (PV = nRT), and the data for Dm you collected in Laboratory Part 1.  You have the all tools necessary to design and execute an investigation that will provide you with the information needed to calculate the molecular weight of air.

After you have developed a plan, discuss it with your TA.  Now, proceed with your investigation. 

As you write up your conclusion and results, use the accepted value for the average molecular mass of dry air, which your TA will provide to you before you leave, to calculate the % error in your experimentally determined value for the molecular weight of air.   This calculation is performed as shown below:

 ˝(accepted value – experimental value)/accepted value˝x 100 = % error.

 Briefly explain the factors that you think contributed to the error in your experimental value. Also, include a value for the actual mass of the He in the balloon as well as  calculations for the densities of air and helium.



       As a team, you will design and plan the procedure for your investigation. Which instruments and procedures are appropriate to an investigation in which a helium-filled balloon is used to determine the molecular weight of air?  Instrumentation and equipment available to choose from includes:

      Vernier temperature, pressure, and pH probes

      Top-loading balance

      Large overflow tank

      2-L volumetric cylinder

Questions your team may want to discuss as it plans:

      Think about what Dm represents. How is this  useful?

Dm = [n * molar mass of air] [n * molar mass of He]

      What equations describing the physical behavior of gases may prove useful?

     In these equations, what variables can you determine through physical measurements?

     What are the known values of the constant(s) used in these equations?

      What information can you obtain from the Periodic Chart of the elements?