Chemistry 151
Lab 12: Graham’s Law
Last Updated Dec. 2012
Introduction
Graham’s law states that the rate a gas diffuses is inversely proportional to the square root of its molar
mass (ℳ ).
1
Rate α
√ℳ
For any two gases, the ratio of their rates is related to their molar mass is by the following equation
Rate A
ℳB
=√
Rate B
ℳA
In this lab you will use Graham’s law to experimentally determine the mass of a hydrogen chloride gas
(HCl) by reacting it with ammonia (NH3) to form ammonium chloride (NH4Cl).
NH3(g) + HCl(g)  NH4Cl(s)
The source of the two gases will be concentrated solutions of ammonium hydroxide and hydrochloric acid,
which will be placed at either end of a glass tube. As each gas leeches from its respective solution it will
travel down the tube, meeting somewhere in the middle to form NH 4Cl as a white solid. The rate of each
gas (in cm/s) can be found by dividing the distance traveled to from the solid by the time required for the
solid to form.
Procedure
1.
2.
3.
4.
5.
6.
7.
8.
9.
Clamp a glass tube horizontally to a ring stand.
Obtain 2 rubber stoppers and a cotton ball. Place a small amount of cotton into each stopper, through
the smaller end (the end you will insert into the glass tube). Try not to have a lot of cotton sticking out
of the stopper hole.
Take one of the stoppers to a fume hood and, using a disposable pipette, soak the cotton with
concentrated ammonium hydroxide.
Repeat step 3 with the other stopper, only this time soak it with concentrated hydrochloric acid. If you
want, while you’re in the hood briefly bring the two stoppers together in the hood—but without them
touching—just to give you an idea of the reaction you’re about to do. When you get back to your work
area, though, keep the two stoppers away from each other.
Simultaneously insert the stoppers into the ends of the glass tube and the record the time (if you use a
stopwatch simply note the time as “0:00”).
When you first see the appearance of ammonium chloride, note the time. Use a wax pencil to mark its
location on the tube (if the solid forms as a wide band, place the mark in the middle of the band).
Remove the stoppers and measure the distance from each end of the tube to the pencil mark.
After recording your measurements, clean and dry the glass tube as follows
a. Rinse the tube with deionized water to wash away the solid.
b. Do a second rinse with either acetone or propanol to remove the water.
c. Attach a rubber stopper with a glass connector to a rubber hose. Insert the stopper into one
end of your glass tube and the other end of the hose to the vacuum line. Open the vacuum
line and draw air through the tube for a couple of minutes.
Repeat steps 3-8 four more times for a total of five determinations. You don’t need to use a new piece
of cotton each time (just resoak the cotton from step 2).
Waste Disposal
Used cotton should be placed in the waste hood. Do not throw them in the trash.
Name: _____________________________
Section: ________
Data
Trial
1
2
3
4
5
Time stoppers inserted
________
________
________
________
________
Time NH4Cl observed
________
________
________
________
________
Time elapsed, s
________
________
________
________
________
Distance traveled by NH3, cm
________
________
________
________
________
Distance traveled by HCl, cm
________
________
________
________
________
Rate of diffusion of NH3, cm/s
________
________
________
________
________
Rate of diffusion of HCl, cm/s
________
________
________
________
________
17.03
17.03
17.03
17.03
17.03
________
________
________
________
________
Accepted molar mass of NH3, g/mol
Experimental molar mass of HCl, g/mol
Average molar mass of HCl, g/mol
Actual molar mass of HCl, g/mol
Percent error, %
% error =
experimental − actual
x 100
actual
Show your work for each of the following calculations from Trial 1.
1) Rate of diffusion of NH 3
2) Rate of diffusion of HCl
3) Experimental molar mass of HCl
________
36.46
________
Name: _____________________________
Section: ________
Post-lab Questions
1. After completing a trial, if you couldn’t remember which end contained the HCl and which was NH 3,
how could you make an education guess (besides smelling the cotton)?
2. How would each of the following affect your calculated molar mass of HCl (incorrectly high, low or no
effect)? Explain your answers.
a) A student placed the HCl-soaked cotton in first, and then placed the NH3-soaked cotton on the other end.
b) The temperature of the classroom increased during the lab period and the temperature during each trial
was significantly higher than the one before it.
3. Use a lab thermometer to estimate the temperature of the lab then calculate the average rms-speeds (urms)
of NH3 and HCl at that temperature. Use the actual molar mass of HCl.
Given these speeds, why didn’t you see the ammonium chloride form immediately after inserting the
stoppers?
Name: _____________________________
Section: ________
Pre-lab Questions
1. An unknown gas reacts with silane gas (SiH4) to produce a solid product. Using an experiment similar
to the one performed in this lab, a sample of silane was placed at one end of a glass tube and the unknown
at the other end. After 3 minutes, 33 seconds the solid formed in tube, 29.4 cm from the silane end of the
tube and 20.6cm from the end with the unknown.
a) Calculate the rate of diffusion of silane (in cm/s).
b) Calculate the rate of diffusion of the unknown (in cm/s)
c) Calculate the molar mass of the unknown gas.
2. Once you soak each piece of cotton (see steps 3 and 4 of the procedure), why is important not to set the
two stoppers next to each other.
3. When running this experiment, would you expect the formation of ammonium chloride to occur closer
to the ammonia end of the glass tube or the end with hydrochloric acid? Explain.