Chemists dopers, please help.

Factual Questions
1

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

2

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.

3

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.

4

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

5

Thanks, Naski, change my answer to 9L.

6

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?

7

consider that the rate is the volume/time. since you know the “new” time, you multiply that by the rate to get the volume.

8

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.

9

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.

10

Thanks all!
g8guy, check your email!