1. Diffusion -Research of gas diffusion
Date：May.26th, 2011
Speaker：Chen,Yang, Deng

2. Purpose
Improve the diffusion experiment in high school chemistry course, to prevent the excessive inhalation of irritant gases
Measure the gas diffusion rate to calculate the molecular weights of HCl and NH3 gas, and compare them with the theoretical values
Discuss the influent
factors of gas diffusion
rate

3. Principle of gas diffusion experiments (1)
＊ Gas diffusion rate (R) : the volume or mole of the diffusion gas per second 
R = volume of diffusion gas / time
or R = mole of diffusion gas / time

4. Principle of gas diffusion experiments(2)
＊ Graham’s law ：Under the condition of the same temperature and pressure, the gas diffusion rate is proportional to the reciprocal of the square root of the gas molecular weight.
R1/ R2= /
R1：The diffusion rate of gas  R2：The diffusion rate of gas  M1：The molecular weight of the gas  M2：The molecular weight of the gas 2

5. Principle of gas diffusion experiments(3)
＊ When the temperature and pressure of gas are
different, the diffusion rate  can be derived as
＊ Two different gases (assumed to be ideal gas) are compared：
or = constant
P1：Partial pressure of gas  P2：Partial pressure of gas 1
t1：Diffusion time of gas  t2：Diffusion time of gas 1

6. Original experiment of gas diffusion
NH3(g) + HCl(g) → NH4Cl(s)
In this experiment, the gas diffuses in the glass
tube, so the diffusion rate can be expressed as
R = diffusion distance(r) of gas / time(t)
The diffusion times of the two different gases are the same.
NH4Cl(s)
NH4OH(aq)
HCl(aq))

7. Gas diffusion rate M：Molecular Weight According to the Graham’s law：
R1/ R2= /
and
R = diffusion distance(r) of gas / time(t)
t is the same
M1/2 r = constant
M：Molecular Weight

8. Improved experiment Experimental diagram universal indicator paper
volumetric flask
10cm
universal indicator paper
Glass tube (30cm)↑

9. Result

10. Ammonia Result at 15 ℃ t(s) vaper P M √M Pt/√M 16 580 17 4.123106 12 10 18

11. Hydrochloric acid Result at 15 ℃ t(s) vaper P M √M Pt/√M 168 100 36.5
106 85
162

12. Ammonia Result at 15.5 ℃ t(s) vaper P 5 580 17 4.123106 703.3533 10
Pt/√M 5
580 17 10 14 15

13. Hydrochloric acid Result at 15.5 ℃ t(s) vaper P M √M Pt/√M 54 100 36.5 70 74 95

14. Ammonia Result at 17 ℃ t(s) vaper P M √M Pt/√M 14 580 17 4.123106 12 11 9

15. Hydrochloric acid Result at 17 ℃ t(s) vaper P M √M Pt/√M 80 100 36.5 95 73 74

16. Comparison

17. Conclusion
We try to improve the diffusion experiment, but the experimental results although are slightly improved, but still not as good as expect, maybe some factors are not taken into account.
It takes about 158 seconds for HCl(g) to pass 10cm of the tube  at 17 ℃, but only 14 seconds for NH3(g). The experimental data do not match the Graham’s law.

18. Conclusion
The saturated vapor pressures of concentrated HCl(aq) and NH3(aq) are much different, so they will affect the diffusion rate in the glass tube in the experiments.
So, adding the factor of saturated vapor pressure to the diffusion law, the experimental data are more consistent with the theoretical data.

19. Thank for your listening!