Diffusion/effusion question

[virajpatel]

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Could someone help me figure this out? don't quite understand the explanation in examkrackers.

 

[chemkay]

Is the answer D?

 

[jwan14]

In my opinion the right answer isn't one of the choices. My thinking is that because it takes 40% more time to diffuse the same distance that just means that it is 40% heavier than Neon. Neon is 20 g/mol so you would expect CaHb to be 28 g/mol. 2 C's = 24 g/mol and 4 C's = 4 g/mol. C2H4 seems right to me. Is that what you got? I could have made an easy mistake. I've been doing the DESTROYER lately and ofttimes I think that I killed a problem then look at the answer and realize I missed an important step or made a dumb mistake haha.

 

[virajpatel]

The Answer is C, I have attached the explanation but it went way over my head. @jwan14 That happened to me yesterday lol I had just watched Chad's videos on gases and started doing these questions, I swear I thought I was getting everything right until I went through and checked my answers lol but then again I have discovered a few errors in this examkrackers book already

 

[discensdentibus]

I got you. Cool problem. Try putting some d=rt into it and you are gold. Let me know if you can't see this picture or if you cant read my handwriting

 

[201Blue]

I got you. Cool problem. Try putting some d=rt into it and you are gold. Let me know if you can't see this picture or if you cant read my handwriting

Where did you get 10 for TNe?

 

[RamsFan1991]

Please don't tell me they actually have questions like this on the DAT, holy crap!

 

[discensdentibus]

I arbitrarily chose 10 (could be minutes seconds w.e) for the time it took to diffuse 10 (chosen, once again, arbitrarily and for simplicity with calculations). The important part is to keep the distance the same for both equations and make the CH compounds diffusion TIME 1.4x longer. I chose 10 because 1.4x10 = 14 and would pretty up the calculations. Try it for yourself. You can choose "5" for T Ne and subsequently (5x1.4) "7" for T CH cpd and stuff will still work itself out.

 

[discensdentibus]

You cannot equate 40% slower as 40% heavier. The key takeaway from the problem statement is that they say they travel the same DISTANCE. They only give you TIME. You gotta find RATE. There is probably another way to solve this problem, but I thought this would be a good way to illustrate it logically.

 

[virajpatel]

Ohh that makes soo much sense. Can you relay the relation of KE to mass, temperature and speed to me? I think that's what ends up confusing me the most on these question? But thanks so much @discensdentibus

@RamsFan1991 I honestly don't know if they have questions like this on the DAT but if it helps this question is out of a practice MCAT book but a friend of mine recommended I use this as my resource for gen chem and orgo

 

[discensdentibus]

its pretty much a manipulation of a familiar physics equation. Ek=1/2mv^2. (Ek is a measure of the average kinetic energy which is the same for all molecules at 1 mol and same temp). You set them equal to each other and solve for the ratio of "v" you want.

The fastest way to solve this problem (I'll say it now that it seems like you make more sense of it) is to kinda go with what the solutions manual says; they are a bit wordy, though. 40% slower means Neon is 1.4 times as fast as CH. Squaring 1.4 and multiplying by 20 gets you your answer quickly.

You get led a astray a little if you try to say "well, 40% slower means .6 times the rate of Ne". The fast way is not really that straightforward and is more error prone