Graham's Law: Diffusion


  1. To measure the relative rates of diffusion of ammonia gas and hydrogen chloride gas.
  2. To verify Graham's law of diffusion.


  1. 12M HCl (aq)
  2. 15M NH3
  3. NaHCO3
  4. Glass tube (1 cm * 70 cm)
  5. (2) 50-mL beakers
  6. (2) Medicine droppers
  7. (2) Rubber stoppers
  8. (2) Ring stands
  9. Meter stick
  10. Cotton balls
  11. Stopwatch


  1. Fit cotton plugs snugly into the ends of the diffusion tube. Close each end loosely with a rubber stopper.
  2. Prepare one small beaker of concentrated HCl and another with NH3.
  3. Place a few drops of HCl on one side of the diffusion tube and place a few drops of NH3 on the other side of the diffusion tube.
  4. Immediately replace the stoppers and record the time.
  5. A white deposit will form in the tube. As soon as it appears, record the time. Measure to the nearest 0.1 cm the distance from the inside end of each cotton plug to the center of the white deposit.

Data & Information:

Distance from HCl to product min:sec
Distance from NH3 to product cm
Rate of diffusion of HCl (Distance/Time) cm/sec
Rate of diffusion of NH3 (Distance/Time) cm/sec
Experimental ratio of rates (NH3/HCl) (NH3/HCl)
Theoretical ratio of rates (NH3/HCl) (NH3/HCl)


  1. Calculate the rate of diffusion for each gas by dividing the distance traveled (cm) by the time required (sec) for the appearance of the white deposit. Enter the calculated rates above.
  2. Calculate the ration between the rate of diffusion of NH3 and the rate of diffusion of HCl, using the rates calculated above. Enter the value for this ratio above.
  3. Using the molecular masses of NH3 and HCl, calculate the theoretical ratio between the rates of diffusion of these gases. Enter the value above. (Rate of diffusion = 1/sq. root of molecular mass)
  4. Calculate the % error in your experimentally determined value for the ratio of the rates of diffusion of NH3 and HCl. Use the theoretical ratio calculated in question 3 as the accepted value for the ratio. (% error = absolute value of (theoretical ratio - experimental ratio/ theoretical ratio))