Graham’s Law
Diffusion of Gases
Adapted from: Addison Wesley Chemistry Manual
Purpose: To verify Graham’s law of diffusion.
Background:
Have you noticed that, if someone enters a room and sits near you, it can take several
minutes before the smell of his or her cologne reaches you? You become aware of the scent
because molecules in the cologne diffuse through the surrounding air. Diffusion is the
process in which particles in a system move from an area of high concentration to an area of
low concentration. Diffusion continues until a uniform concentration of particles is reached
throughout the system. The rate at which gas molecules diffuse, at constant temperature,
decreases as the molar mass of the gas increases. In fact, the rate of gas diffusion at
constant temperature is inversely proportional to the square root of the molar mass of the
gas. This proportionality is called Graham’s law of diffusion.
rate of difusion 
1
molar mass
In this experiment, you will determine the relative rates of diffusion for two gases with
significantly different molar masses. The gases that you will study are ammonia, NH 3, and
hydrogen chloride, HCl.
Safety First!
In this lab, observe all precautions, especially the ones listed below.
o Wear your safety goggles. (All steps)
o Hydrochloric acid is corrosive and can cause severe burns.
o Concentrated ammonia solution is corrosive. Avoid skin contact. Avoid the
inhalation of ammonia fumes. Use a fume hood.
o Note: Return or dispose of all materials according to the instructions of
your teachers.
Procedure:
This experiment should be conducted in a fume hood.
1. As the experiment is performed, record your observations in the Qualitative Data
Table.
2. Obtain a clean glass tube. Using a q-tip cut in half, make sure the cotton plug fits
snugly into each end of the tube.
3. Go to the fume hood. Each partner will dip their own q-tip into a beaker. One
partner dips their q-tip into the beaker of concentrated (12M) HCl while the other
partner dips their q-tip into the beaker of concentrated ammonia (NH3).
4. Then each will insert the q-tip into the ends of the glass tubing simultaneously.
5. Now carefully carry the glass tube back to your lab table, being careful to keep it
completely level.
6. Observe the tube for the reaction that will form the causing the NH4Cl white
precipitate.
7. Measure the distances (in centimeters) from the inside end of each cotton plug to
the center of the white deposit. Record these measurements in Quantitative Data
table.
8. Properly dispose of the materials.
Calculations:
1. Calculate the ratio of the distances (rate of diffusion) for the NH3 compared to
the HCl.
2. Calculate the theoretical ratio of the rates of diffusion for these gases, using the
following equation [Molar Mass Values].
diffusion rate of NH 3

diffusion rate of HCl
molar mass HCl
molar mass NH 3
3. Calculate the percent error in your experimentally determined value for the ratio of
diffusion rates of NH3 and HCl. Use the theoretical ratio calculated in problem 2 as
the accepted value for the ratio.
percent error 
theoretical ratio  experimental ratio
theoretical ratio
x100%
Questions:
1. Would a change in temperature affect the diffusion rates you calculated?
Explain.
2. Would a change in temperature affect the ratio of diffusion rates? Explain.
3. Suppose an opened bottle of vinegar (CH3COOH) was placed next to an opened
bottle of pentylamine (C5H13N). Which would you smell first? Calculate the
diffusion ratio [show all work]?
4. Are the results of this experiment consistent with Graham’s law of diffusion,
explain?
5. The white substance in the tube is ammonium chloride, NH 4Cl. It is the only product
in the reaction. Write a balanced equation for this reaction. What type of chemical
reaction is this?
Conclusion:
Please write a conclusion where you;
o restate the purpose (1 sentence).
o write a brief procedure (1-2 sentences).
o discuss the theory of the laboratory experiment, the results, things that were
learned, process that were new, etc. (4-6 sentences).
o write two sources of error, the error must be explained and the effect the error
has on the results (diffusion rate of the gases) must be explained.