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#### tylerxx26

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##### Gas Laws and Reactions Difficulty
« on: December 24, 2012, 09:12:38 AM »

"What would the mass of air be inside a hot air balloon that has the volume of 5.25 x 10^4 L. The temperature of the air inside is 30 degrees celsius and the pressure is 95.5 kPa. Assume the composition of air is 80% nitrogen gas and 20% oxygen gas to determine the molar mass of air."

any input on how to do this would be appreciated, it's the last question on my assignment.

Similar questions that i've done like this are: What mass of oxygen, is available for a combustion reaction in a volume of 5.6L at STP
vO2 = 5.6L
vSTP = 22.4L/mol (22.4l/mol because of STP)
therefore
nO2 = 5.6 x (1mol/22.4l)
= 0.25 mol
mO2 = 0.25mol x (32.00g/1mol)
= 8 grams.

Idk how to determine the mass of the original question because it's a composition and not at STP or SATP
« Last Edit: December 24, 2012, 10:10:34 AM by tylerxx26 »
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#### billnotgatez

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##### Re: Gas Laws and Reactions Difficulty
« Reply #1 on: December 24, 2012, 09:55:03 AM »

Forum rules require you show some work or at least what you understand and what you do not understand
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#### tylerxx26

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##### Re: Gas Laws and Reactions Difficulty
« Reply #2 on: December 24, 2012, 10:38:57 AM »

What i have so far (not sure if i'm on the right track at all)
v = 52500 L
p = 95.5 kPa
T = 303 K

n=pv/RT
= 95.5 x 52500/ (8.31 x 303)
= 1991.218977 mol
nN2 = 1991.21877mol x (0.80)
= 1592.975182 mol
nO2 = 1991.21877 mol x (0.20)
= 398.2437955

mN2 = 1592.975182 mol x (28.02g/1mol)
= 44635.1646 g
mO2 = 398.2437955 mol x (32.00g/1mol)
= 12743.80146
mTotal = 44635.1646 + 12743.80146 = 57378.96608 grams

But imo, i feel like there's no way that this is right

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#### Hunter2

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##### Re: Gas Laws and Reactions Difficulty
« Reply #3 on: December 24, 2012, 12:47:56 PM »

Use in the calculations the SI units. The numeric result sounds correct.
« Last Edit: December 24, 2012, 01:00:36 PM by Hunter2 »
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#### fledarmus

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##### Re: Gas Laws and Reactions Difficulty
« Reply #4 on: December 24, 2012, 03:34:12 PM »

What don't you like about your answer? What would you expect for the density of air at 30°C? Is that anywhere close to your answer?

If you don't know what to expect, you might look here... http://en.wikipedia.org/wiki/Density_of_air

It's always helpful to have at least some physical understanding of how large a number you expect for an answer. it helps to tell you if you screwed up someplace in the problem. Fortunately, with the internet, it's really easy to look up the answers for something reasonably close to most problems
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