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Reallllly stumped!!!!!


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#1 macscigirl

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Posted 3 November 2008 - 10:41 PM

Ok so I've been working on this problem for what seems like forever now and I can't get it. Can someone PLEASE help me?!

It has a picture of a flask that has a bulb on each end, these two bulbs are connected.
Question: Consider the flasks in the following diagram. What are the final partial pressures of H2 and N2 after the stopcock between the two flasks is opened? (Assume the final volume in 3.00L) What is the total pressure in torr?

One side of the bulb is H2 and it has 2.0L H2 and 475 torr.
The other side is N2 and has 1.0L N2 and .200 atm.


Please someone help me!!!!! This is due tomorrow and I have no clue how to work this out, I've tried everything I know!!!
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#2 big314mp

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Posted 4 November 2008 - 12:09 AM

Remember that total pressure is the sum of all of the partial pressures.

So (stuff in () is subscript):

P(H2) + P(N2) = P(T)

What this means is that you can calculate the pressure of each gas as if the other wasn't there, and then add them together to get the total pressure.

So the problem becomes:

1) You have 2.0L of gas A, and it is at 475 torr. You let it expand to 3.0L. What is the pressure?

2) You have 1.0L of gas B, and it is at .2atm. You let it expand to 3.0L. What is the pressure?

3) Add the answers from 1 and 2, and you will get total pressure.
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#3 macscigirl

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Posted 4 November 2008 - 12:16 AM

so what formula would I use? I used ideal gas (I think) and that didn't come up with the answer I was looking for.
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#4 big314mp

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Posted 4 November 2008 - 12:20 AM

Does the question give you a temperature? You should be able to use the ideal gas law.
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#5 macscigirl

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Posted 4 November 2008 - 01:06 AM

No it didn't give me temp!!! Thats why I was SO frustrated! I figured it out though, I took P1xV1/Pfinal

Then of course I added the two together to get the total pressure.
That was sheer luck that I happened to punch that into my calc!!!
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#6 CaptainPanic

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Posted 4 November 2008 - 08:02 AM

Gases that mix don't change temperature (unlike liquids, that can have a significant temperature change when you mix). So, the temperature is irrelevant.

Step 1.
Convert everything to normal SI units (Pascal for pressure). The volume is preferred in m3 (cubic meters).

Step 2.
Realize that if you have 1 bar pressure in 1 m3, and you expand that to 2m3, the pressure will be 0.5 bar. P*V = constant in this case.

Step 3.
Realize that partial pressures can be added up to forms the total pressure. If, in the same volume, you have 0.5 bar of gas A, and 0.6 bar of gas B, the total will be 1.1 bar of gas A+B.

Step 4.
Convert your answer back into the units (torr, atm, psi, whatever) asked by the teacher.

Edited by CaptainPanic, 4 November 2008 - 08:11 AM.

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#7 big314mp

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Posted 4 November 2008 - 08:38 AM

Temperature is relevant, as the moles of gas is dependent on pressure, which is dependent on temperature. The total pressure is also dependent on temperature.

Maybe they cancel out somewhere and I'm missing something.
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#8 macscigirl

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Posted 4 November 2008 - 03:00 PM

Yeah I have no clue how I came out with the correct answer but I did. It would have helped A LOT if I would have had temp!!! I could have found the moles of the gasses and then used The Ideal Gas Law. But I just used what I had and it worked!
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#9 CaptainPanic

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Posted 4 November 2008 - 03:14 PM

Temperature is relevant, as the moles of gas is dependent on pressure, which is dependent on temperature. The total pressure is also dependent on temperature.

Maybe they cancel out somewhere and I'm missing something.


Looking first at only H2:

I'm assuming that temperature doesn't change.

If you want, you can assume any temperature and use the ideal gas law. That works too.

Before opening stoplock:
P_1(H_2)\cdot{V_1(H_2)}=n(H_2)\cdot{R\cdot{T}}
After opening stoplock:
P_2(H_2)\cdot{V_2(H_2)}=n(H_2)\cdot{R\cdot{T}}

Obviously, n does not change (you have no reactions, and you cannot create new molecules from nothing).
T was assumed constant. Therefore:

P_1(H_2)\cdot{V_1(H_2)}=n\cdot{R\cdot{T}}=P_2(H_2)\cdot{V_2(H_2)}
Or:
P_1(H_2)\cdot{V_1(H_2)}=P_2(H_2)\cdot{V_2(H_2)}
Or:
P\cdot{V} = constant


Same equations are used for N2.

Then you add up the partial pressures P_2(H_2)+P_2(N_2)=P_2(total).



The point that got me puzzled now is that mixing the gases will have an entropy effect. Furthermore, one side will have expansion, the other compression. The question asked doesn't seem to indicate that these factors should be included...

Edited by CaptainPanic, 4 November 2008 - 03:58 PM.
more formulas, and more pondering

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