graham?s law

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DieselPetrolGrl

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What is graham?s law good for? Can anyone supply a sample MCAT style Q?
I am sorry ..i dont mean to seem lazy its just that after reading it in kaplan all i absorbed was the formula and i cant begin to imagine it in Q format..as always thank you for your time :thumbup:

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"The rate of effusion of a gas is inversely proportional to the square root of either the density or the molar mass of the gas."
http://www.chem.tamu.edu/class/majors/tutorialnotefiles/graham.htm

but i am so fried i cant even begin to desipher the whole KE and Temp and diffusion correlation at all ...all i know is mass dont matter in KE but ..it does in graham..then how can they be related..cant fne..logical...correlation..ahH!
 
If the valve of a flask contianing an equal mole mixture of CH4 and SO2 is opened for a brief moment, allowing a portion of the gas to escape, what is observed for the gas remaining in the flask?

  • A. Partial pressure of CH4 goes down; the mole fraction of SO2 goes down.
    B. Partial pressure of SO2 goes down; the mole fraction of CH4 goes down.
    C. Partial pressure of CH4 goes up; the mole fraction of SO2 goes up.
    D. Partial pressure of SO2 goes up; the mole fraction of CH4 goes up.

OR

How much faster does helium (MW = 4) escape from a balloon than neon (MW = 20) after the tempature is raised from 300K to 310K?

  • A. 5.2 times faster
    B. 5.0 times faster
    C. 2.2 times faster
    D. They both escape at the same speed.
 
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PRamos said:
If the valve of a flask contianing an equal mole mixture of CH4 and SO2 is opened for a brief moment, allowing a portion of the gas to escape, what is observed for the gas remaining in the flask?

  • A. Partial pressure of CH4 goes down; the mole fraction of SO2 goes down.
    B. Partial pressure of SO2 goes down; the mole fraction of CH4 goes down.
    C. Partial pressure of CH4 goes up; the mole fraction of SO2 goes up.
    D. Partial pressure of SO2 goes up; the mole fraction of CH4 goes up.

OR

How much faster does helium (MW = 4) escape from a balloon than neon (MW = 20) after the tempature is raised from 300K to 310K?

  • A. 5.2 times faster
    B. 5.0 times faster
    C. 2.2 times faster
    D. They both escape at the same speed.



So... the answers?? and an explanation of partial pressure and mole fraction changes would be nice.
 
for the first question, the rate at which CH4 leaves the flask is greater than SO2 so therefore, the mole fraction of SO2 will increase while the mole fraction of CH4 decreases. So it's gotta be B or C. Now here's the problem....if the mole fraction of SO2 is increasing, shouldn't the partial pressure of SO2 increase also? And the partial pressure of CH4 should be decreasing.....I don't know...

But willthatsall is right about the second one....I say C too

edit: bah......nm I get it now....it's B for the first one.
 
For the first one, the partial pressure of both gases decreases because both gases are escaping. However, CH4 escapes more quickly, so the mole fraction of CH4 decreases and the mole fraction of SO2 increases.
 
Oh boy....shows how much I know.... :(

Hey....maybe we should put up more random questions since it's getting close to D-Day...What do you guys think?
 
I like random problems, but the company that created the problems probably doesn't appreciate them as much. :D
 
PRamos said:
If the valve of a flask contianing an equal mole mixture of CH4 and SO2 is opened for a brief moment, allowing a portion of the gas to escape, what is observed for the gas remaining in the flask?

  • A. Partial pressure of CH4 goes down; the mole fraction of SO2 goes down.
    B. Partial pressure of SO2 goes down; the mole fraction of CH4 goes down.
    C. Partial pressure of CH4 goes up; the mole fraction of SO2 goes up.
    D. Partial pressure of SO2 goes up; the mole fraction of CH4 goes up.

Both gases escape the flask, but because CH4 is lighter than SO2, it escapes faster. Over time, the partial pressure of both gases decrease (each gas is losing moles from the flask). But, because SO2 is lost at a slower rate than CH4, the system becomes richer in SO2. The mole percent of SO2 increases. The correct answer is B.

PRamos said:
How much faster does helium (MW = 4) escape from a balloon than neon (MW = 20) after the tempature is raised from 300K to 310K?

  • A. 5.2 times faster
    B. 5.0 times faster
    C. 2.2 times faster
    D. They both escape at the same speed.

The temperature doesn't matter, because both gases are being heated to the same temperature. All that matters is the mass. The speed depends on the square root of mass, so the relative speed is square root of 5, which is a little more than 2. C is our winner.

willthatsall said:
I like random problems, but the company that created the problems probably doesn't appreciate them as much.

These were on an worksheet I got during the BR gas lecture. I have no idea if there's a copyright issue, but if there is, then I'll eliminate my posts.
 
I don't think it's a big deal, we have discussed some different problems on here in the past if people were having problems. But I don't think it's a good idea to start posting tons of questions because then the copyright might become an issue. I'm not sure if we are really allowed to share any material according to SDN rules, but I'm sure they can draw a distinction between getting help and advice and illegally distributing materials.
 
u all are a gem - i guessed A and C but i can see why i was wrong... * squints * :)
 
Would it be easier to apply the equipartition theory, determine the number of modes. Then you would know which gas would be actually moving faster?
 
well u know that the lighter gas is moving faster...
i think...
that would be ....methane
what diff will ur new fangled formula make! * is a hard sale :)*


(please correct me if i am wrong)
 
Graham's law:

RMS MW of Gas A/RMS MW of Gas B = Velocity of Gas B/ Velocity of Gas A

The MW of a gas in inversley prop to velocity.
Important to keep in mind: Different Gases at same temp have the same amount of kinetics energy but the velocities are related to MW therefore might is different. N
 
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