Gases have no definite volume. They spread out, or diffuse, and occupy all the space available to them. This spreading of gases is
called diffusion. A gas will diffuse even if another gas is present in the same space. The molecules of gases are far enough apart to
allow other gas molecules to fit in between.
Gases diffuse at different rates. Graham's law states that, under equal conditions of temperature and pressure, gases diffuse at rates
inversely proportional to the square roots of their molecular masses. Mathematically this may be stated as
The term rate implies that something happens in a given period of time. The rate of diffusion of a gas is the distance its molecules travel
per unit time. In an equal period of time, the distances D1 and D2 traveled by molecules of two different gases are related by:
D1 square M2
------ = -----------------
D2 square M1
This inverse relationship indicates that the distance traveled by the heavier gas (gas with greater molecular mass) will be less than that
traveled by the lighter gas in the same period of time.
In this experiment, two gases (HCl and NH3) will be introduced simultaneously into opposite ends of a glass tube. At the point where the
two gases meet inside the tube, a chemical reaction will occur that produces a white powder. The equation for the reaction is:
HCl(g) + NH3 (g) Þ NH4Cl (s)
Using the point where the powder forms as a reference point, the distance traveled by each gas can be measured. By comparing the
ratio of these distances with the ratio of the square roots of the known molecular masses of the two gases, Graham's law can be verified.
Verify Graham's law by measuring the distances traveled during the same period of time by two different gases of known molecular
glass tubing, 10 mm x 60 cm metric ruler
dropper pipets (2) safety goggles
cotton swabs lab apron or coat
wax marking pencil
HCl(con.) NH3 (ammonia)
acetone (for drying test tubes)
Handle both the concentrated HCl and the concentrated NH3 solutions with great care. Avoid getting any of either of these chemicals on
1. Obtain a 50-cm length of glass tubing (10 mm internal diameter). Make sure it is completely dry. Lay the tubing on your bench.
2. Place one cotton swab in each end of the tubing. Using a marking pencil, mark the glass to indicate the position of the end of
each swab as illustrated in Figure 1.
3. Remove the cotton swabs from the tubing. Mark the stem of one of the swabs with the marker for purposes of identification.
Using dropper pipettes, place about five drops of concentrated HCl on the unmarked cotton swab and five drops of
concentrated NH3 • H20 solution on the marked cotton swab.
CAUTION: Handle these chemicals with care. They can cause painful bums if they come in contact with the skin.
4. Immediately and simultaneously insert the moistened ends of the cotton swabs into opposite ends of the tube, to the lines
5. After several minutes, a white ring will form where the gases HCl and NH3 meet inside the tube to form the white compound
NH4Cl (ammonium chloride). Mark the point on the tube where the white ring is formed.
6. Remove the cotton swabs, rinse them with water, and dispose of them as instructed by your teacher.
7. Measure the distance traveled by each gas.
8. Rinse the tubing with water. It may be dried by rinsing it withacetone. CAUTION: Acetone is highly flammable.
9. The procedure can be repeated if time permits.
OBSERVATIONS AND DATA
Trial 1 Trial 2 Average of 2 trials
Distance traveled by NH3 __________ __________ __________
Distance traveled by HCl __________ __________ __________
Molecular masses are: NH3 = 17 HCl = 36,5
Practical Work: Observing diffusion of a gas.
Aim: What is the aim of this experiment?
Apparatus: ______________ , _________________, ______________
1. Having the above apparatus, how would you design this experiment as to observe diffusion in a gas?
2. Why do we use red lithmus paper and ammonia?
3. Do you think ammonia is volatile?
4. How would you set up the apparatus as to see ammonia gas moving from one place to another?
5. From which direction to what other direction does ammonia move: from high concentration to low concentration or from low to high?
1. How did you see ammonia travelling?
2. What was the colour change of red lithmus paper?
1. Why was it important to wet the lithmus paper before starting the experiment?
2. What is diffusion?
3. Did diffusion occur in this experiment?