Originally posted by Beagle The diffusion rate of methane gas compared to that of helium is:
A. twice as fast
B. 4 times as slow
C. 4 times as fast
D. half as fast
E. 12 times as fast
Remember as a gas gets heavier and heavier it makes it more slower overall. So the first thing you should wonder is what is different. Helium is a very light gas. Use common sense and remember all those helium balloons at your 5th birthday party. Then look at methane. It is the most simplest alkane on earth. But it is more complex than helium. Thus, I would eliminate choices A, C, and E immediately. The rest is guessing. I'm sorry I don't know the exact way to solve this solution, but my method gives you a 50/50 shot. Maybe it has to do with molecular weight. Since Helium is a diatomic gas (CH4 (methane is 2x bigger). since you know bigger molecules are slower, then it means that He is twice as fast. But the answer is not A, we are dealing with Methane compared to Helium not the reverse. So we know that methane is twice as slow. The answer is D. Hey, I just solved it by using common sense. Great eh???
I wonder if this has anything to do with Graham's law of gas effusion, which was:
(Rate of diffusion of gas A)(density of gas A)^1/2 = (rate of gas B)(density of gas B)^1/2
(the ^1/2 means square root).
At STP conditions, 1 mole of methane should have the same volume as 1 mole of helium, right? (this I'm not sure). If that's the case, then methane is 4x denser than helium (i.e. 4x heavier for the same volume). If that's the case, the Graham's law equation works out to methane diffusing half as fast as helium.
Maybe that's why the answer is D.. Anyway, being that it's been 7 years since I took general chem, somebody might want to check up on this because I might be wrong.
P.S. BTW Helium is a noble gas and does NOT normally exist in diatomic form-- Hydrogen does.
Thanks I totally forgot that since I didn't have a periodic table in front of me. But still, by using common sense you can work these out even without knowing every formula. I believe the DAT will be the same way. But the more you know the more you can apply.