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Topic: chemistry  (Read 6004 times)

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yummy

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chemistry
« on: July 14, 2005, 03:19:27 PM »
please can someone help with this question.. what is the volume of gas at -30C and 0.75 atm if it occupies a volume of 255L at 40.0C and 1.25atm? thank you all
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arnyk

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Re:chemistry
« Reply #1 on: July 14, 2005, 03:53:16 PM »
I can help you if you show me some workage.
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Offline Donaldson Tan

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Re:chemistry
« Reply #2 on: July 14, 2005, 09:00:50 PM »
1. convert all temperature to kelvin
2. solve using the general gas law, ie. PV = nRT (assume perfect gas)
3. n is constant since amount of gas remains unchanged.
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"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

arnyk

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Re:chemistry
« Reply #3 on: July 14, 2005, 11:15:29 PM »
Alrighty then, let's see what you are given and what you require.

Given:

P1 = 1.25 atm (convert to kPa)
V1 = 255 L
T1 = 40C (convert to K)

P2 = 0.75 atm (convert to kPa)
V2 = ?
T2 = -30C (convert to K)

Now what formula could you possibly use which utilizes all givens and solves for the variable?

As geodome said, n is constant. ;)

Combined
Gas
Law
=
Combined
Gas
Law

 ::)
« Last Edit: July 14, 2005, 11:15:50 PM by arnyk »
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Offline xiankai

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Re:chemistry
« Reply #4 on: July 15, 2005, 12:54:56 AM »
a basically summarised version of arnyk's formula is the combined gas law, visually represented as:

P1V1/T1 = P2V2/T2.

n is the universal gas constant while R is the molar mass. molar mass is also a constant, so u can safely ignore it
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arnyk

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Re:chemistry
« Reply #5 on: July 15, 2005, 09:55:55 AM »
I always thought n = moles and R = universal gas constant and molar mass usually isn't in the gas law formulas?
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Offline xiankai

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Re:chemistry
« Reply #6 on: July 15, 2005, 10:18:18 AM »
yeah thats the problem i thought too. there are websites that say one thing, but there are websites that say the opposite. while both are reputable, i have no choice but to follow up on geo's statement that n is the constant ::)

just to note, these shouldnt be considered too seriously as they can be used alternatively anyway, their positions in the formula are basically the same.

yeah i think molar mass usually isnt in the gas law formulas. i brought it up in case yummy is curious :P
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Offline Donaldson Tan

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Re:chemistry
« Reply #7 on: July 15, 2005, 10:33:57 AM »
Quote
n is the universal gas constant while R is the molar mass. molar mass is also a constant, so u can safely ignore it

The above quote is wrong. Reference to any physical chemistry textbook will tell that R refers to molar gas constant, and n refers to number of moles. Since n is constant, then nR must be constant, and given that PV = nRT, then

P1V1/T1 = P2V2/T2
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"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

arnyk

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Re:chemistry
« Reply #8 on: July 15, 2005, 11:50:13 AM »
Woohoo!  For once in my life, I got something right lol.  :D

R = 8.31 kPaL/molK
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Offline xiankai

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Re:chemistry
« Reply #9 on: July 15, 2005, 06:52:53 PM »
ok thanks for clarifying that  :)

hey u've been getting everything right so far  :P
« Last Edit: July 15, 2005, 06:55:09 PM by xiankai »
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