Question about DIFFUSION OF GASES AND GRAHAM'S LAW?

1. A gas of unknown molecular weight is found to diffuse at a rate of 0.19cm/s compared to 0.59 cm/s for helium gas. Calculate the molecular weight of the unknown gas.

2. Which of all of the possible gases should have the highest rate of diffusion?

3. A balloon inflated with helium gas will rise to the ceiling of a room. However, after a certain period of time, the balloon will descend to the floor. Why does the balloon eventually descend?

4. If the cork containing the concentrated NH3(AMMONIA) solution were inserted into the glass tube several seconds after the insertion of the cork containing HCl, what effect would this have on the calculated value of the molecular weight of HCI?

Answer:
Graham's law holds for effusion, but it is only approximately true for diffusion.

1. rate1 / rate 2 = (Mol Wt 2)^1/2 / (Mol Wt 1)^1/2

substitute in and solve

2. Let's see the lowest MW for a gas is 2.02 g/mole for H2 so it should effuse the fastest (and probably diffuse also)

3. The He gas will effuse (sneak through a small opening) through the pores in a rubber type balloon quite fast and the volume will get smaller, so the density goes up and the balloon sinks.

4. NH3 = 17 g/mole HCl = 36.5 g/mole

You give the HCl a head-start, so it will come across as being faster which translates as being lower MW.

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