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
SAFETY PRECAUTIONS
PROCEDURE
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|
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.