DETERMINING MOLAR MASS USING THE IDEAL GAS EQUATION

DISCUSSION & OBJECTIVE
Gases are one of the major products and/or reactants in many chemical reactions.  Of all the states of matter, gases are the most affected by changes in temperature and pressure.  The method of collecting the gas also affects the pressure of the gas.  The relationship between the density of a gas and the pressure and temperature at which it is collected can be used to determine the molecular weight of the gas.

This lab activity will use the Ideal Gas Equation to experimentally determine the molar mass of a common gas - butane.  Since real gases do not behave ideally at room temperature, the results will be expected to vary from the calculated molar mass.  The idea of a dry gas versus one collected over water will also be involved.

MATERIALS
-   butane lighter, large container, large graduated cylinder, thermometer, balance

SAFETY PRECAUTIONS
-      basic safety precautions apply;  Do not try to ignite the gas after collecting it!

PROCEDURE
1.
  Immerse the lighter completely in water.  Then use a paper towel to dry the lighter as best as possible.  Then
     weigh the butane lighter to the nearest hundredth of a gram.  Record this value in the data table.

2.  Fill the container about two-thirds to three-fourths full with water.

3.  Place the graduated cylinder in the container and fill it with water also.  Invert the cylinder and keep the
    opening of the cylinder under the surface of the water to prevent any water from leaving the cylinder.  (You
    should not have any air bubbles at the top of the graduated cylinder.)

4.  Check the lighter to be certain it is open as much as possible to allow gas to escape rapidly.

5.  Place the top of the lighter up into the opening of the cylinder and depress the striker to allow the gas to
     bubble up into the cylinder.  (Butane is not very flammable under water!)

6.  Allow the gas to escape until about 250 mL of gas are collected.  Quickly read the volume of the gas because
     butane is more soluble in water than most hydrocarbons.  Record this volume.

7.  Dry the lighter completely and weigh it again.  Record this value.

8.  Read the temperature of the water to the nearest tenth of a degree.  Record this temperature.

9.  Your instructor will provide you with the barometric pressure reading.

* Click here to go to the water vapor pressure table online.  Or, make sure that you note this measurement for the temperature of your lab before you leave class.




DATA TABLE

Mass of the lighter before collecting gas

_______________ g

Mass of the lighter after collecting gas

_______________ g

Mass of gas collected

_______________ g

Volume of gas collected

_______________ mL

Volume measurement in Liters

_______________ L

Temperature of the water (and gas)

_______________ oC

Temperature measurement in Kelvins

_______________ K

Barometric pressure

_______________ mm Hg

Vapor pressure of water at certain temperature *   

_______________ mm Hg

Pressure of the "dry" gas

_______________ mm Hg

Dry gas pressure measurement in atm

_______________ atm

CALCULATIONS
The ideal gas equation is    P V = n R T
where P = pressure, V = volume, n = moles, R = ideal gas constant, and T = temperature.                        
You need to use what you know about moles (including the fact that the unit for molar mass is g/mole) to manipulate the data to solve and determine the experimental molar mass.

Make sure that your calculations are clearly shown!  (Please note that clearly implies not only legibility, but also a logical progression of calculations.  Numbers written haphazardly all over your paper is not a logical progression.)

Experimental molar mass =     ________________g/mole

2. The formula for butane is C4H10.  Calculate the theoretical molar mass based on this formula.

Theoretical molar mass =         ________________  g/mole

Calculate the % error:    | theoretical value – experimental value|     x 100
                                                    theoretical value
                   

LAB QUESTIONS
1.)
  Identify at least three (3) possible sources of experimental error.  (The errors you include should stem from
      either an assumption that was made about the gas or lab conditions or something that you did or did not do
      during the lab.  Remember, I know that you’re intelligent young adults.  “We read the  insert name of
      measuring device here  wrong.“ is not an acceptable source of experimental error.  Think about your
      answers!)

2.)  Read Step 6 in the PROCEDURE.  What effect would leaving the gas in contact with the water for an
      excessive amount of time have on the experimental molar mass?  (Would the experimental molar
      mass be higher or lower if you did this?  Why?  Think about your calculations.)

3.)  Show in detail how you determined the experimental molar mass of butane, the theoretical molar mass of
      butane, and your percent error.

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