Diffusion Rate Comparison (Graham's Law)

Comparing diffusion rates of HCl and NH₃ gases

Gas Laws Intermediate (High School) 30 min ~$10.00

Objective

Verify Graham's law of diffusion by comparing the rates at which HCl and NH₃ gases travel through a glass tube.

Background

Graham's law states that the rate of diffusion of a gas is inversely proportional to the square root of its molar mass. When HCl (36.5 g/mol) and NH₃ (17 g/mol) diffuse toward each other in a tube, they form a white ring of NH₄Cl where they meet. The ring forms closer to the HCl end because NH₃ diffuses faster.

Safety Warnings

  • Concentrated HCl fumes are corrosive and irritating
  • Concentrated ammonia fumes are irritating
  • Perform this experiment in a fume hood or well-ventilated area
  • Do not inhale either gas directly

Required PPE

goggles gloves lab_coat

Materials

  • Concentrated hydrochloric acid (5 mL)
    For HCl fumes
  • Concentrated ammonia solution (5 mL)
    For NH₃ fumes
  • Cotton wool (2 pieces)
    For soaking

Equipment

Glass tube (1 m long, 2 cm diameter) Rubber stoppers (2) Ruler Retort stand and clamps Stopwatch

Procedure

1

Clamp the glass tube horizontally on the retort stand.

3 min
2

In a fume hood, soak one cotton ball in concentrated HCl and another in concentrated ammonia.

2 min Fume hood — both gases are irritating
3

Simultaneously insert the HCl cotton ball into one end and the NH₃ cotton ball into the other end. Stopper both ends.

2 min Work quickly
4

Start the timer. Watch for a white ring of NH₄Cl to form where the two gases meet.

10 min
5

When the white ring appears, stop the timer. Measure the distance from each end to the ring.

3 min
6

Calculate the ratio of distances: d(NH₃)/d(HCl) should equal √(M(HCl)/M(NH₃)) = √(36.5/17) = 1.46.

5 min

Expected Results

The white NH₄Cl ring should form approximately 60% of the way from the HCl end (40% from the NH₃ end), since NH₃ diffuses 1.46 times faster than HCl. The ratio d(NH₃)/d(HCl) should be approximately 1.4-1.5.

Cleanup

Remove cotton balls and dispose in fume hood. Rinse the glass tube with water. Ventilate the area.

Details

Category
Gas Laws
Difficulty
Intermediate (High School)
Duration
30 min
Est. Cost
$10.00
Steps
6
Materials
3

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Frequently Asked Questions

What is the objective of Diffusion Rate Comparison (Graham's Law)?
Verify Graham's law of diffusion by comparing the rates at which HCl and NH₃ gases travel through a glass tube.
How difficult is Diffusion Rate Comparison (Graham's Law)?
This experiment is rated as Intermediate (High School). It takes approximately 30 minutes to complete.
What safety precautions are needed for Diffusion Rate Comparison (Graham's Law)?
Key safety precautions include: Concentrated HCl fumes are corrosive and irritating; Concentrated ammonia fumes are irritating; Perform this experiment in a fume hood or well-ventilated area.
What materials are needed for Diffusion Rate Comparison (Graham's Law)?
The main materials required are: Concentrated hydrochloric acid, Concentrated ammonia solution, Cotton wool.
What results should I expect from Diffusion Rate Comparison (Graham's Law)?
The white NH₄Cl ring should form approximately 60% of the way from the HCl end (40% from the NH₃ end), since NH₃ diffuses 1.46 times faster than HCl. The ratio d(NH₃)/d(HCl) should be approximately 1.4-1.5.