CHEM 403
Exp 10
Experiment 10:
MOLAR MASS OF A GAS*
One of the important applications of the Ideal Gas Law is found in the experimental
determination of the molar masses of gases and vapors. If the gas obeys the Ideal Gas Law,
PV = nRT
or
n=
PV
RT
(1)
If the pressure P is in atmospheres, the volume V in liters, the temperature T in Kelvin,
and the amount n in moles, then the gas constant R is equal to 0.0821 atm∙L/(mol∙K). From the
measured values of P, V, and T for a sample of gas, Equation 1 can be used to find the number of
moles of gas in the sample. The molar mass in grams, MM, is equal to the mass of the gas sample
divided by the number of moles.
MM =
mass of gas sample
n
(2)
This experiment involves measuring the molar mass of butane, C4H10, experimentally. A
simple calculation using a periodic table would give the correct answer for the molar mass;
however, you are going to conduct an experiment using the Ideal Gas Law in the lab to see how
close you can come to the accepted value.
You will measure the mass of the butane released from a pressurized container, and
measure the volume of the gas that you collect. You can measure the temperature of the lab and
atmospheric pressure, and use Equation 1 to find n, the number of moles of butane. Once you
know the mass, and the number of moles, you can use Equation 2 to calculate an experimental
molar mass.
Procedure
SAFETY WARNING: Butane is a flammable gas, and at NO TIME during this lab
should there be any use of an open flame or other heat source!
Determine the initial mass for the butane lighter provided by your instructor to the nearest
0.01 g. Set up a water basin for collecting a gas in an inverted graduated cylinder by water
displacement. Fill a 100-mL graduated cylinder completely with water and carefully turn it upside
down in the water basin so that there are no gas bubbles trapped in the cylinder. Using a ring stand
and clamp, secure the graduated cylinder in place above the water basin.
Submerge the butane lighter in the water basin and hold it directly underneath the graduated
cylinder. Collect a sample of gas with a volume of approximately 70.0-90.0 mL. Take care to
collect every bubble that leaves the lighter. If you miss a bubble, you will have to repeat the entire
procedure beginning with re-weighing the clean and dry lighter.
To perform your calculations correctly, the pressures inside and outside the graduated
cylinder must be equal. Carefully adjust the cylinder up or down so that the water level inside the
*Adapted from University of Florida’s Center for Precollegiate Learning and Training laboratory.
CHEM 403
Exp 10
graduated cylinder is the same as the level outside the cylinder. This will equalize the pressure
inside the graduated cylinder and is a key step in this experiment.
Once the water levels are equal, record the volume of butane that has been collected in the
cylinder. Measure and record the temperature of the water in the basin to 0.1 °C. This temperature
will be used in your Ideal Gas Law calculation and will also be used to determine the vapor pressure
of water in the graduated cylinder.
Dry the lighter, then measure and record its mass to the nearest 0.01 gram.
Refill your graduated cylinder with water and repeat the collection process for a second
trial. Remeasure the temperature of the water after the second trial to account for any change.
CHEM 403 – Exp 10
Name:______________________________ Section: _________
Data & Calculations
Mass
Trial 1
Trial 2
Initial mass of lighter
______________ g
______________ g
Final mass of lighter
______________ g
______________ g
Mass of butane collected
______________ g
______________ g
Volume of gas collected*
*after adjusting to equalize pressure
______________ mL
______________ mL
Volume converted to liters
______________ L
______________ L
Temperature of water in basin
______________ °C
______________ °C
Temperature, in Kelvin, of butane
(assume same temperature as water)
______________ K
______________ K
Barometric pressure in laboratory, Ptotal
___________ mmHg
___________ mmHg
Vapor pressure of water, Pwater
(from Table provided by instructor)
___________ mmHg
___________ mmHg
Pbutane (Ptotal – Pwater)
___________ mmHg
___________ mmHg
Pbutane converted to atm
(1 atm = 760 mmHg)
___________ atm
___________ atm
Volume
Temperature
Pressure
Number of moles
Using Equation 1, calculate the number of moles of butane collected in each trial, using your
experimental values of P (in atm), V (in L), T (in K), and R = 0.0821 atm∙L/(mol∙K).
n (Trial 1) = _______________ mol
n (Trial 2) = _______________ mol
Experimental Molar Mass of Butane
Using Equation 2, and your experimental values, calculate the molar mass of butane:
MMexperimental (Trial 1) = _______________ g/mol
MMexperimental (Trial 2) = _______________ g/mol
MMexperimental (average) = _______________ g/mol
Using a periodic table, calculate the accepted molar mass of butane, C4H10:
MMaccepted = _______________ g/mol
Calculate the percent error in your experimental value using the following equation:
% error =
|MMexperimental (average)–MMaccepted |
MMaccepted
x 100%
% error = _______________ %
CHEM 403 – Exp 10
Name:______________________________ Section: _________
Prelab Assignment
In this experiment, there are several key steps that, if skipped, can have significant impact on the
accuracy of your experimental molar mass of butane. For each of the following situations, explain
which measured values (pressure, volume, temperature, mass of butane) would be affected, and
follow the error through Equations 1 and/or 2 to determine how your experimental molar mass
value would be affected. Would your molar mass value end up too high or too low?
1. During the butane gas collection, some of the butane bubbles were not collected in the graduated
cylinder.
2. The water level in the graduated cylinder was not adjusted prior to recording the volume; the
level of water in the graduated cylinder was above the water level.
3. You did not dry the butane lighter completely before obtaining a final mass.
4. You forgot to subtract the vapor pressure of water from the atmospheric pressure before
performing your calculation of n.