Diffusion of Gases

Purpose:

To demonstrate the relationship between molar mass and diffusion rate of a gas.

Materials:

• 2 glass petri dishes
• Cotton balls
• Ring stand and clamp
• 2 rubber stoppers (to fit tube, with no holes)
• Long glass tube open at each end

Reagents:

• Conc. ammonia (NH₃)
• Conc. hydrochloric acid (HCl)

Hazards and PPE:

• Hydrochloric acid is acutely corrosive and toxic. Avoid skin contact and inhalation of fumes.
• Concentrated ammonia can cause severe skin burns and respiratory irritation, and is an extreme aquatic toxin. Store and handle with care.
• Wear approved safety goggles and gloves.

Protocol:

Preparation

1. Clamp tube horizontally on ring stand.
2. Fill petri dishes with NH₃ and HCl respectively and add cotton balls. Cover with parafilm and label.

Demonstration

1. Take cotton ball from NH₃ petri dish, place in end of tube, and stopper tube.
   1. Fully cover Petri dish.
2. Quickly repeat with HCl cotton ball, place at opposite end of tube and stopper tube end.
   1. Fully cover Petri dish.
3. Observe a white ring (NH₄Cl) form after a few minutes. It should be closer to the side in which you place the ammonia cotton ball.

Additional Notes:

• Results are only semi-quantitative due to air.
• Results do not follow Graham’s Law of Effusion because the gases are diffusing through air.

Disposal:

• Rinse out tube in a fume hood with water, collecting runoff in a large beaker already containing water. Tube can then be cleaned normally.
• Any remaining solutions, including runoff beaker, can be poured down sink with excess water.
• Cotton balls can be thrown in trash.

Reactions:

• HCl_(g) + NH_3(g) → NH₄Cl_(g) + H_2(g)

Citations:

1. This demo adapted from the demo library of the Chemistry department at the University of Illinois at Urbana-Champaign.

Videos:

• Ammonia and hydrogen chloride diffusion experiment

  Nigel Baldwin