Graham's Law: Diffusion of ammonia gas and HCl(g)

Two Erlenmyer flasks, one containing concentrated ammonia solution and the other containing concentrated hydrochloric acid, are placed side by side. The stoppers are removed and an aquarium is immediately inverted over them. A white mushroom cloud of ammonium chloride dust begins to form immediately over the flask containing the hydrochloric acid. 

Curriculum Notes 

This demo is usually performed when Graham's law is being introduced. However, this is also a good example of an irreversible gas phase acid-base neutralization. Allow about 5 minutes to perform this demo.

Lead Time 
One day of lead time is required for this project.

Graham's law states that the rate of diffusion of a gas is inversely proportional to the square root of the molar mass. Since the molar mass of hydrogen chloride is about twice that of ammonia, that means that ammonia diffuses about forty percent faster than hydrogen chloride. In this demo, that means that the ammonia makes it to the hydrogen chloride flask before the hydrogen chloride has a chance to make it to the ammonia flask and we observe the reaction taking place above the HCl. The reaction is: NH3(g) + HCl(g) ==> NH4Cl(s

  • two stoppered 125 mL Erlenmeyer flasks, one containing about 15 mL of concentrated ammonia solution and the other containing about 15 mL of concentrated hydrochloric acid.
  • a small aquarium
  • a small cart to perform the demo on
  • a dark background box (optional)
  • the two flasks should be about 25 cm apart on a small cart
  • remove the stoppers from the flasks
  • immediately invert the aquarium over the flasks
  • a white cloud of begins to form over the flask containing HCl
Safety Precautions 
  • Both concentrated ammonia solution and concentrated hydrochloric acid and the vapors that they emit are extremely caustic and corrosive. Avoid breathing the vapors. If either of these liquids comesinto contact with your skin, flush with copious amounts of water. If you get some in you eyes, flush with water for at least 15 minutes and seek medical attention. Wear goggles.
  • Make sure that the aquarium makes a good seal with the surface of the cart and that there is no pace for the vapors to escape. Do not remove the aquarium to replace the stoppers until you are either outside or in front of a fume hood.
Prep. Notes 

When breaking down this demo, make sure not to remove the aquarium in the classroom. Wait until you're back at the lab in front of the fume hood.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group