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Topic: Chemistry GASES question, please help ?  (Read 8270 times)

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Offline Shahab Mirza

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Chemistry GASES question, please help ?
« on: October 03, 2014, 07:18:20 AM »
Comparative rates of diffusion of He and SO2 will be .

Answer is.  4 .

But I dont know how to solve it, please help me solve this question in simple steps, thanks

Offline mjc123

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Re: Chemistry GASES question, please help ?
« Reply #1 on: October 03, 2014, 08:16:52 AM »
Have you heard of Graham's Law?
Rate of diffusion is proportional to what power of the molecular weight?

Offline Enthalpy

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Re: Chemistry GASES question, please help ?
« Reply #2 on: October 06, 2014, 06:29:12 AM »
Shouldn't the molecule's size have some sort of influence too?

Offline Shahab Mirza

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Re: Chemistry GASES question, please help ?
« Reply #3 on: October 06, 2014, 07:47:56 AM »
Listen bro , i figure it out,

molecular weights of he and so2 = 4 and 64
64/4 = 16

√16 = √4X4 = 4 answer,

but i still wonder why our educational system prepare such questions for mcat lol

Offline mjc123

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Re: Chemistry GASES question, please help ?
« Reply #4 on: October 07, 2014, 05:00:43 AM »
"It is designed to assess problem solving, critical thinking, written analysis and knowledge of scientific concepts and principles" (wikipedia)
The problems may not be of direct medical relevance - that is not their purpose, it is to test whether you can apply the scientific principles you've learnt to solving problems - in other words, whether you really understand them. These are skills you would certainly need as a doctor, whether or not you ever need to use Graham's Law.

Offline Shahab Mirza

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Re: Chemistry GASES question, please help ?
« Reply #5 on: October 07, 2014, 06:16:51 AM »
"It is designed to assess problem solving, critical thinking, written analysis and knowledge of scientific concepts and principles" (wikipedia)
The problems may not be of direct medical relevance - that is not their purpose, it is to test whether you can apply the scientific principles you've learnt to solving problems - in other words, whether you really understand them. These are skills you would certainly need as a doctor, whether or not you ever need to use Graham's Law.

Yeah exactly ,  they do this , but here in our country they ask questions even out of text books/

Offline Borek

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Re: Chemistry GASES question, please help ?
« Reply #6 on: October 07, 2014, 06:22:09 AM »
in our country they ask questions even out of text books/

Good, in life and work you have to know how to deal with situations and things that were never mentioned in textbooks.

Besides, so far you have shown you have problems even with the basic material that for sure was covered by the course, so I am not convinced you judge the situation correctly.
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Offline Shahab Mirza

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Re: Chemistry GASES question, please help ?
« Reply #7 on: October 07, 2014, 06:37:26 AM »
in our country they ask questions even out of text books/

Good, in life and work you have to know how to deal with situations and things that were never mentioned in textbooks.

Besides, so far you have shown you have problems even with the basic material that for sure was covered by the course, so I am not convinced you judge the situation correctly.

yeah exactly, i have problems with basic concepts, but actually in real i have not problem with basic concepts i just get confused when it comes to apply these basic concepts to solve questions, in text books during our two years of high school , we were taught by teachers superficially means go through way , like if we move in periodic table from left to right electronegativity increases , metals are more electropositive, moles , avogadros numbers etc, but in our country the study structure is such weird that they only tell us defination but dont tell us what to do with these , they make us solve past papers and as in our country the same questions repeat in cycle year after year so in that way we cleared our high school , but when we reviewed MCAT , we were shocked , it was beyond our knowledge , but in MCAT, its not only chemistry or physics concepts, we have physical chemistry defination based questions too , we have english in mcat , some cramming material also comes in test , and afterall biology question are in majority which are easy to solve , btw thanks for your time.

Offline Corribus

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Re: Chemistry GASES question, please help ?
« Reply #8 on: October 07, 2014, 09:56:28 AM »
Shouldn't the molecule's size have some sort of influence too?
No, the primary influence is weight, because it is the kinetic energy of the moving particles (well, the velocity really) that determines how many gas molecules can pass through a hole for a given period of time. Higher mass particles move more slowly at the same temperature than smaller mass particles, so the rate of effusion is less. I guess if the hole was REALLY small, the molecular size would start to matter, but to a good approximation I believe the particles are treated as point masses anyway (or, that their volumes are negligible compared to the volume they occupy).
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Shahab Mirza

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Re: Chemistry GASES question, please help ?
« Reply #9 on: October 07, 2014, 05:27:01 PM »
Shouldn't the molecule's size have some sort of influence too?
No, the primary influence is weight, because it is the kinetic energy of the moving particles (well, the velocity really) that determines how many gas molecules can pass through a hole for a given period of time. Higher mass particles move more slowly at the same temperature than smaller mass particles, so the rate of effusion is less. I guess if the hole was REALLY small, the molecular size would start to matter, but to a good approximation I believe the particles are treated as point masses anyway (or, that their volumes are negligible compared to the volume they occupy).

Thanks, sir , i appreciated your theoritical and conceptual explaination but how we can solve it numerically? the way in which i did was right? above?

Offline mjc123

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Re: Chemistry GASES question, please help ?
« Reply #10 on: October 07, 2014, 05:54:38 PM »
Yes, rate is proportional to 1/sqrt(M)

Offline Shahab Mirza

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Re: Chemistry GASES question, please help ?
« Reply #11 on: October 07, 2014, 05:57:39 PM »
Yes, rate is proportional to 1/sqrt(M)

THANKS

Offline Shahab Mirza

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Re: Chemistry GASES question, please help ?
« Reply #12 on: October 07, 2014, 06:11:41 PM »
Yes, rate is proportional to 1/sqrt(M)

Sir, as you said that 1/sqrt (M) , so here i remember an other situation , see this.
The ratio of rate of diffusion of two gases is 1:3 , then ratio of their molecular weight is , "9:1" .

so as you told that 1/sqrt (M) so what ? here also this same formula is applicable 1:3^2 = 1:9 = 9:1 , or same situation or this is other case?

Offline Enthalpy

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Re: Chemistry GASES question, please help ?
« Reply #13 on: October 07, 2014, 06:51:20 PM »
I'm not convinced at all. In fact, I believe the "answer" 4 is plain wrong.

4 is only the ratio of the mean molecules' speed. To compute a diffusion coefficient, it also needs a mean free path or a mean collision frequency, which depends on the molecule size.

http://en.wikipedia.org/wiki/Diffusion_coefficient#Gases
"sigma is the average collision diameter"

Offline Corribus

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Re: Chemistry GASES question, please help ?
« Reply #14 on: October 07, 2014, 10:00:26 PM »
@Enthalpy

I grant you it's hard to tell from the ambiguous wording of the OP's question. But from the provided answer it's almost certain the question is referring to Graham's Law, which relates the rate of effusion of a single uniform gas through a pin-hole. This is different from the diffusion of one gas in another, where the cross-sectional area of the gas certainly matters, as it impacts the mean free path of translation (distance between collisions on average). In effusion, the pinhole is far smaller than the mean free path, such that only one gas molecule passes through at a time, and so collisions between gasses do not impact the rate of passage through the cross-sectional area. Here, only the velocity of the gas impacts the rate of passage through the hole, and the velocity is only related to the molecular weight.

http://en.wikipedia.org/wiki/Effusion
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

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