# Chemists dopers, please help.
**URL:** https://boards.straightdope.com/t/chemists-dopers-please-help/91793
**Category:** Factual Questions
**Created:** [November 6, 2001, 3:38pm UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793 "2001-11-06T15:38:26Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@KarlGrenze](https://boards.straightdope.com/u/KarlGrenze)
#### Post date: [November 6, 2001, 3:38pm UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793/1 "2001-11-06T15:38:26Z")
Ok, I have just taken a chemistry quiz, and would like to know the answer to one of the questions. The question is:
90.0 L of H[sub]2[/sub] effuse through a porous membrane in 30 min at a certain temperature. What is the volume of CF[sub]4[/sub] that would pass through the same membrane in 20 min at the same temperature?
My answer: Less than 90.0 L (and a very bad use of Graham’s Law, just so my TA could have an excuse to give me partial credit.)
Formulas: Well, Graham’s Law [rate A / rate B = square root( FormulaMass A / FM B )]
There was also another formula that was presented, and I haven’t seen or put to use with the textbook exercises. Something like: I don’t know if M means molar mass or molarity.
u= SquareRoot[(3 RT) / (M)]
_Please note that this is a first semester General Chemistry Course_
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### Author:  [@Cardinal](https://boards.straightdope.com/u/Cardinal)
#### Post date: [November 6, 2001, 3:55pm UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793/2 "2001-11-06T15:55:41Z")
I would assume that the answer is:
Root of (FM CF4/FM H2), which will give you the ratio for the same amount of time. Find that amount, then realize that you only have 2/3 the time.
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### Author:  [@Spritle](https://boards.straightdope.com/u/Spritle)
#### Post date: [November 6, 2001, 4:37pm UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793/3 "2001-11-06T16:37:56Z")
I think you need a reciprocal there. the more massive (big M has a slower rate (little r) so:
rate A/rate B = root(mass B/mass A)
I roughed out some numbers (I don’t recall the mw of Fluorine) and I came up with somewhere in the neighborhood of 12L of carbon tetrafluoride.
CF[sub]4[/sub] is approx. 25 times as massive so the rate would be one fifth (root of 25). Of course, you are only allowing diffusion for 2/3 the time.
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### Author:  [@Naski](https://boards.straightdope.com/u/Naski)
#### Post date: [November 6, 2001, 5:02pm UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793/4 "2001-11-06T17:02:49Z")
i think this is a more chemiical enginerring question so i’m afraid i’m neither use nor ornament on this one
Exccepting that the formula mass of H2 is 2
and that the Formula mass of CF4 is 88 (F = 19, C = 12)
Nask
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### Author:  [@Spritle](https://boards.straightdope.com/u/Spritle)
#### Post date: [November 6, 2001, 5:22pm UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793/5 "2001-11-06T17:22:45Z")
Thanks, Naski, change my answer to 9L.
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### Author:  [@KarlGrenze](https://boards.straightdope.com/u/KarlGrenze)
#### Post date: [November 6, 2001, 5:25pm UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793/6 "2001-11-06T17:25:36Z")
Eh, thanks for your questions. May major concern with the problem is not how to get the rates of effusion, but how to find the volume. I think that is the part that my mind blocks out, since it wasn’t part of the homework nor of the examples she gave.
So, once you get the effusion rate(not a big problem there), how do I get the volume?
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### Author:  [@Spritle](https://boards.straightdope.com/u/Spritle)
#### Post date: [November 6, 2001, 6:34pm UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793/7 "2001-11-06T18:34:53Z")
consider that the rate is the volume/time. since you know the “new” time, you multiply that by the rate to get the volume.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [November 7, 2001, 5:48am UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793/8 "2001-11-07T05:48:06Z")
Also remember that for gasses at a given temperature and pressure, volume is proportional to the number of molecules. At STP, one mole of any gas at all will take up 22.4 liters.
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### Author:  [@g8rguy](https://boards.straightdope.com/u/g8rguy)
#### Post date: [November 7, 2001, 6:51am UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793/9 "2001-11-07T06:51:43Z")
Right. So 90 L of H[sub]2[/sub] in 30 minutes gives you a rate of 3 L/min. Call this R[sub]H2[/sub].
Then you know that R[sub]CF4[/sub] = R[sub]H2[/sub]\*sqrt(M[sub]H2[/sub]/M[sub]CF4[/sub]).
Stick the numbers in, and get R[sub]CF4[/sub] = 3 L/min \* sqrt(2/88) = 0.45 L/min.
So the rate at which CF[sub]4[/sub] diffuses is 0.45 L/min, and thus the volume of CF[sub]4[/sub] is 0.45 L/min \* 20 min = 9 L, as **Spritle** correctly stated.
BTW, if you wish, my offer of tutoring is still open for the moment.
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### Author:  [@KarlGrenze](https://boards.straightdope.com/u/KarlGrenze)
#### Post date: [November 7, 2001, 11:21pm UTC](https://boards.straightdope.com/t/chemists-dopers-please-help/91793/10 "2001-11-07T23:21:21Z")
Thanks all!
**g8guy** , check your email!