Lab #5 – Molar Mass using PV = nRT
Name
Background
In order to determine an unknown compound’s molar mass, one must be able to determine the number of moles
in the unknown. If the substance is a gas, or an easily vaporized liquid, this can be done with relative ease using
the Ideal Gas Law, PV = nRT, where n is the number of moles. By determining the Pressure of the gas, its
Volume, and its Temperature, we can calculate n = PV/(RT).
In this procedure we will introduce a small sample of a volatile liquid into a specialized piece of glassware
called a Dumas flask (MSRP $20). The liquid will be completely vaporized by heating in a water bath so that
the flask contains only vapor. The temperature of the water bath is taken to be the same as the temperature
inside the flask. Since the flask is an open container, the pressure inside the flask is the same as atmospheric
pressure. Determining the volume inside the flask is problematic and will be discussed in class.
Procedure
Fill a 1000 mL beaker to within 4 finger widths of the top edge. Heat the water to boiling.
While waiting for the water bath to heat, use KimWipes to wipe down a Dumas flask. Use the analytic balance
to find the mass of the clean Dumas flask w/stopper. Record the mass of the empty flask w/stopper. Also find
and record the mass of the empty Dumas flask w/out the stopper.
Use the technique discussed in class to introduce about 5 mL of the unknown liquid into the Dumas flask. Once
the water bath is boiling, use the inverted clay triangle/ring apparatus to submerge the unstoppered Dumas flask
in the water bath. Be careful not to get water in the Dumas’ spout. While the flask is being heated, avoid
creating air currents around the mouth of the Dumas flask, as this can create a pressure difference that might
draw vapor out of the flask. Continue heating until no more liquid is visible in the Dumas flask. Record the
temperature of the water bath, then turn off the burner.
Once the water has stopped boiling, stopper the flask to prevent vapor loss during cooling and remove the flask
from the water bath. As the flask cools, remove the stopper briefly every 30 seconds to allow pressure to
equalize as the vapor condenses. Submerge the flask in a beaker of room temp tap water for several minutes to
facilitate cooling, being careful not to get water in the spout.
Once the flask is cooled to room temp, wipe the outside with KimWipes and weigh the flask w/stopper and
remaining sample on the analytic balance. Record the mass.
Dispose of the leftover volatile liquid in the waste beaker in the fume hood. Use the same logic employed to
draw liquid into the flask to expel liquid from the flask.
Use the technique discussed in class to determine the internal volume of your Dumas flask.
Prelab summary
Procedural Tips: Always wipe down any container before weighing on the analytical balance.
Be sure to record your unknown number.
Start the water bath first.
Do not get water into the Dumas flask.
Avoid air currents around the mouth of the Dumas flask
Safety: All of the unknowns are flammable. Keep away from open flame.
Disposal: Leftover volatile liquid goes in waste beaker in fume hood.
Data and Calculations
Unknown #
Show calculations here:
Mass of empty Dumas w/stopper
g
Mass of Dumas w/stopper and condensed sample
g
Mass of condensed vapor
g
Temperature of water bath
ºC
Atmospheric pressure (see Blackboard)
torr
*Internal volume of Dumas flask (see below)
mL
Moles of condensed vapor
mol
Molar Mass of vapor
g/mol
Determining internal volume:
External volume of flask
ml
External volume of flask
ml
Mass of flask w/out stopper
g
Mass of flask w/out stopper
g
Density of glass
2.40
g/mL Density of glass
2.60
g/mL
Volume of glass
mL
Volume of glass
mL
Internal volume of flask
mL
Internal volume of flask
mL
*Average internal volume
mL
Prelab Questions
1. Suggest a procedure for getting a few mL of a liquid UK into the tiny opening of the Dumas flask.
Hypodermic needles will not be available. [Hint: How can we create “suction” inside the flask?]
2. Suggest a procedure for determining the external volume of the flask. Filling the flask with water is not a
practical option.
3. Suggest a way to determine the volume of glass in the flask
4. A student weighs an empty Dumas flask w/stopper and finds its mass to be 36.7925 g. The flask is heated in
a 98.5ºC water bath until the flask contains only vapor. The atmospheric pressure that day is 752 torr. The
flask has an internal volume of 272.4 mL. The mass of the flask and condensed vapor at the end of heating is
found to be 37.7138 g.
(a) What is the pressure of the vapor in the flask during heating? Explain.
torr
(b) What is the temperature of the vapor during heating? Explain.
ºC
(c) Calculate the moles of vapor in the flask. Show all work.
mol
(d) Calculate the molar mass of the vapor. Show all work.
g/mol
2. Would the calculated molar mass of the vapor be higher, lower, or unchanged if the following procedural
errors were to occur? Explain your choice by tracing the error through the calculation of molar mass.
(a) Some unvaporized liquid remains in the flask when heating is stopped.
(b) Some vapor is drawn out of the flask by air currents near the flask’s mouth.
(c) The thermometer is inaccurate, reading about 5ºC too low.