Re: Why does helium leak from a latex balloon faster than oxygen?

What you found in your experiments is just what would be expected - the helium leaked out faster. Your question is a very good one.
(By the way, do you really mean that you used an oxygen balloon, or do you mean a balloon filled with air, which is 20% oxygen and 80% nitrogen? I'll assume you meant air, though it doesn’t make any difference for my answer.)

1. Gases leak out of a balloon because the molecules of the gas go through tiny holes (pores) in the balloon material, and probably also around the knot.

2. They do this because the molecules are constantly moving, at great speeds, in random directions. Every once in a while, a molecule hits one of the pores in the balloon wall in just the right direction, and passes through - like this.

3. The faster the molecules are moving, the more often they collide with the walls of the balloon, and therefore the more often they will pass through a pore.

4. Now, here is the key to the answer: At any particular temperature, helium molecules are moving faster than molecules of oxygen. That is why they leak out more quickly.

5. Now the big question: Why are helium molecules moving faster than oxygen molecules at the same temperature?

The first part of the answer to this is that helium molecules are much lighter than oxygen molecules. On the atomic mass scale, helium molecules weigh 2 units, oxygen molecules weigh 32 units. Big difference! [If we were talking about air instead of oxygen, the average weight of molecules of air is 29.]

6. Why do lighter molecules move faster than heavier molecules at the same temperature?

A. All molecules have energy, which comes from their constant motion. This energy of motion depends on two things: (1) the molecule's speed, and (2) the molecule's mass. At any temperature, different molecules have the same energy. Since helium is so much lighter than oxygen, at the same temperature the helium molecules must be moving faster in order to have the same energy.

Imagine a heavy truck and a light car next to one another, motionless, as in the first picture.	1
Now suppose a big train rams into both of them at the same time, as in the second and third pictures.	2
The train transfers some of its energy to the truck and the car. It will transfer the same amount of energy to both. (This makes sense: Why should it transfer more energy to one than to the other?)	3
Now, what does your common sense tell you about which will wind up moving faster, the little car or the big, heavy truck? Well, if we were to actually do this, we would find that the car winds up moving faster than the truck. Think of it this way: the same push makes the little car go faster than the big heavy truck. Both car and truck have the same energy, but the car is moving faster because it is lighter.	4

It's the same ideas with the molecules: both helium and oxygen have the same amount of energy, helium is lighter so it must move faster. Therefore, the helium molecules leak out of the balloon faster.