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Topic: short pressure problem  (Read 6176 times)

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slri

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short pressure problem
« on: October 16, 2005, 02:12:21 PM »
Hi, I am not sure how to proceed with this problem:
A 2.57 L balloon is filled with helium at room temperature (25°C). What is the volume (in L) of the balloon when it is taken outdoors on a cold Midwest morning (-12°).
first I figured it is a PV = nRT problem.  I figured I could apply this formula twice (once when balloon is outside and once when balloon is inside) and then calculate the differnece in pressure.  but then again, where would I go from there?  the other problem is, this is all there is to the problem, it doesn't specify the pressure, so I would end up with two variables in both cases.  am I even supposed to use the PV = nRT approach here?

help will be greatly appreciated!!
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Offline Borek

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Re:short pressure problem
« Reply #1 on: October 16, 2005, 02:50:35 PM »
pV=nRT

but you know that p = const, R = const, n = const, so

V/T = const

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slri

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Re:short pressure problem
« Reply #2 on: October 16, 2005, 06:33:56 PM »
? ? ? ?  ???
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Offline sundberg

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Re:short pressure problem
« Reply #3 on: October 16, 2005, 06:43:59 PM »
PV = nRT
And as Borek said the pressure, R constant and n (number of moles) is constant. The only factor that changes is the volume and the temperature. But the problem is that the number of moles is not given, so first use the formula to calculate this. Then you have all the parameters to use the formula to calculate what the volume would be outdoors.

1) PV/RT = n (where T is 25, but in Kelvin)
2) V = nRT/P (where T is -12, but in Kelvin)

The pressure is not specified because it is assumed to be one atmosphere (as it always is where we are, unless using pressure cannisters and things like that).
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Offline mike

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Re:short pressure problem
« Reply #4 on: October 16, 2005, 06:44:22 PM »
pressure is constant as Borek said. Also as Borek suggested you must use Charles' Law:

v1/t1 = v2/t2

(v)olume
(t)emperature
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Offline Borek

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Re:short pressure problem
« Reply #5 on: October 16, 2005, 06:49:23 PM »
Quote from: sundberg on October 16, 2005, 06:43:59 PM
But the problem is that the number of moles is not given, so first use the formula to calculate this.

Quote
The pressure is not specified because it is assumed to be one atmosphere (as it always is where we are, unless using pressure cannisters and things like that).

You are twice wrong. You don't need to calculate n and you don't need to assume anything about p. See mikes post - n, R and p cancels out, all you have to do is to compare starting and final conditions (indexes 1 and 2).
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Offline sundberg

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Re:short pressure problem
« Reply #6 on: October 16, 2005, 07:01:32 PM »
Yeah, you're right. I'm too tired for this. :o
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slri

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Re:short pressure problem
« Reply #7 on: October 16, 2005, 07:54:41 PM »
THANKS SO MUCH GUYS!! :D
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