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CHEMISTRY LEVEL 3


1. GAS LAWS
2. THE MOLE: Formulae and Chemical Equations
3. ORGANIC CHEMISTRY 1
4. NITROGEN AND ITS COMPOUNDS
5. SULPHUR AND ITS COMPOUNDS
6. CHLORINE AND ITS COMPOUNDS
7. A guide to chemical tests based on this module
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Gas Laws: Diffusion and Graham's Law

1.0 Gas Laws


1.5 Diffusion and Graham's Law

A perfume sprayed at one corner finally fills the room. We can detect sweet smell from flowers, and aromatic smell from food. These are possible because matter consists of small particles (molecules), and because of diffusion. Some molecules move faster and therefore diffuse faster than others.


diffusion demonstration, chemistry demonstrations, gas laws, high school chemistry

Figure 1.5: Set-up to demonstrate "diffusion" of marble balls


(courtesy Youtube-Diffuion in gases by ormalearn)



The steel balls (glass balls or beads) represent molecules. They all have the same mass except the grey ball, which is lighter.


Observe the video demonstration of movements of steel balls when shaken in a petri dish, to represent diffusion.


Questions 1.5(a): Previous and current experiences

  1. What is diffusion? (Hint: Primary Science.)
  2. The balls move fairly fast. But each of them takes long to reach the farthest end. Why?
  3. Which balls move faster: the lighter or heavier ones?
  4. From your answer to Question 3, which objects move faster: the denser or less dense objects?
  5. How does density affect diffusion rate?
  6. How would an increase in temperature affect diffusion rate? (Hint: Brownian motion, Chemistry Level 2.)

Answers to Questions 1.5(a)


Gas molecules move fairly fast, at typically 600 m/s (over 2 000 km/hr). But they take long to diffuse (or advance) in a given direction because of multiple collisions with other molecules. Collisions aside, rate of diffusion of a gas decreases as its density increases. It is an inverse relationship.


Graham found the relationship to be

Graham's equation, chemistry demonstrations, gas laws, high school chemistry

Plate 1.5(a) Graham's equation


That is, the rate of diffusion of a gas is inversely proportional to the square root of its density. This is called Graham's law.


Let us play around with Graham's equation to see other possible forms it can take, before we use it to solve problems. Always remember, Density = Mass/Volume. That is, rho = M/V.


different forms of Graham's law, chemistry demonstrations, gas laws, high school chemistry

Plate 1.5(b) Different forms of Graham's equation


Questions 1.5(b)

  1. Rewrite the Graham,s equation with an equal (=) sign introduced.
  2. Rewrite the equation in terms of mass, M, instead of density.
  3. Rewrite the equation in Question 2 for a gas labelled A.
  4. Suppose the rate of diffusion of gas A is RA, and of gas B is RB. Write the ratio RA/RB in terms of the densities rho A and rho B of gases A and B respectively.

  5. RA/RB = -----


    NB: Equal sign comes automatically when we are dealing with ratios. That is, if A is proportional to B, and C is proportional to D, then A/B = C/D.


  6. Rewrite the equation for RA/RB in terms of Relative Molecular Mass of A (RMMA) and Relative Molecular Mass of B (RMMB).

  7. RA/RB = -----


  8. Rewrite the equation for RA/RB in terms of volume of A (VA) and of B (VB) diffused.

  9. RA/RB = -----


  10. Write an equation to relate Relative Molecular Mass of A (RMMA) and Relative Molecular Mass of B (RMMB) with volume of A (VA) and of B (VB) diffused.
  11. Rewrite the equation for RA/RB in terms of the time taken by A (tA) and B (tB) to diffuse under the same conditions. (Hint: If the rate is high, time taken is short. So if rate is a numerator (up), time is a denominator (down).)

  12. RA/RB = -----



Answers to Questions 1.5(b)



Questions 1.5(c)

200ml of gas X with a relative molecular mass (RMM) 64 diffuses in 5 minutes. It takes 8 minutes for the same volume of gas Y to diffuse under the same conditions of temperature and pressure.

  1. Gas P is 4 times less dense than X. How many times does P diffuse faster than X?
  2. Determine the relative molecular mass of Y.
  3. How long would it take 200 ml of gas Z, with a relative molecular mass of 2, to diffuse under the same conditions?
  4. Determine the rate of diffusion of gas Q with a relative molecular mass of 71.0 under the same conditions.

Answers to Questions 1.5(c)



1.6 Project 1

In this topic, we have learnt how volume varies with pressure at constant temperature, and the relationship between volume and absolute temperature of a gas at constant pressure.

  1. Design an experiment to investigate the relationship between pressure, P, and absolute temperature, T, of air at constant volume. In the design, explain clearly the procedure you would follow, the materials and apparatus you require and how you would use them, the measurements you would take and how you would use them to determine the relationship.
  2. Carry out the project and prepare a report explaining the method and materials used, measurements and observations made, findings, and any difficulties experienced.

NB: Project work should preferably be done in a group.