Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: ^^Samantha** on August 29, 2006, 04:43:05 AM
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1.) W/c has a higher density, oxygen or air? Prove by calculations, knowing that air is approximately 20% oxygen and 80% nitrogen and that 1mol of any gas occupies 22.4 liters at STP.
2.) Give one method of industrial/ commercial preparation of:
a.) Hydrogen
b.) Oxygen
3.) List down important uses of:
a.) Hydrogen
b.) Oxygen
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Questions 2 & 3 can be easily answered after minute of googling.
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oh sorry about that...
uhm about number 1?...
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oh sorry about that...
uhm about number 1?...
well density is calculated as mass divided by volume: m/V. Maybe that will help you. You must take in to account the mass of nitrogen when calculating the mass of 1 L air.
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air = 20% oxygen & 80% nitrogen
Oxygen = 32.0g / mole
Nitrogen = 14.0 g / mole
1 mol of any gas occupies 22.4L at STP ... what does this mean?
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so 1 mol = 22.4L eh?
then Oxygen = 32.0g / 22.4L
= 1.4 g / L
then Nitrogen + Oxygen = 32.0g + 14g = Air = 46 g / 22.4L
Air = 2.0g / L
Air has higher density... AM I CORRECT???
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then Nitrogen + Oxygen = 32.0g + 14g = Air = 46 g / 22.4L
Air = 2.0g / L
N2 = 28.0g/mol, not that it'll make a difference to which has a higher density, as long as you know that the Mr of nitrogen is less than oxygen.
You are on the right track, but you have to remember to include the percentages (so use 32 x 0.2 and 28 x 0.8.) so you have the mass per one mole of gas, not two.
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Oxygen = 32 x .2 = 25.6 g/mol
Nitrogen = 28 x .8 = 22.4g/mol
__________
Air = 48g/mol
1 mole of any gas occupies 22.4L at STP
1 mol = 22.4L?
Density of Oxygen = 25.6 g / 22.4 L
= 1.14g /L
Density of Air = 48g / 22.4L
= 2.14 g / L
Air is more dense?
???
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Oxygen = 32 x .2 = 25.6 g/mol
Check your math.
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oh thanks ;D
so tell me, am i in the correct path? so that i can proceed now...
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so tell me, am i in the correct path?
Yeah, you have already correctly calculated the density of O2 as 1.4g/L so you only have to work out the density of air now.
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3.) List down important uses of:
a.) Hydrogen
b.) Oxygen
The real answer: fuel cells.
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Forget about the simple arithmetic for this very moment.
The initial question is asking what is more dense, air or oxygen. Air is a composite of several gases, depending on the system. Oxygen is of course 16.00 g/mol. We know that air must be more dense because it has several different atomic elements, not just oxygen.
By thinking this way you will realize that this question can simply be answered through understanding what a mole is, and of course a little common sense. Then you can go about the arithmetic. ;D
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The initial question is asking what is more dense, air or oxygen. Air is a composite of several gases, depending on the system. Oxygen is of course 16.00 g/mol. We know that air must be more dense because it has several different atomic elements, not just oxygen.
Sorry, but you are completely off. You forgot that these other elements have its volume too. Assuming constant pressure - if you mix 1 liter of oxygen with 1 liter of nitrogen you will not get a 1 liter of heavier mixture, but 2 liters of mixture with averaged density.
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Well I forgot to mention the percentage amounts to take into consideration. In this case we know there is more oxygen, which is 32.00 g/mol, and nitrogen molar mass plus oxygen molar mass is obviously greater. But I forgot the percentages, so this isn't as intuitive as I first thought.
We aren't adding 1 L, we're assuming densities at STP, which is 22.44 L. So the volume is 22.41, not 1.
But I see my mistake. Forgot the volumes too. I just try to rationalize through things before calculating.
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Well I forgot to mention the percentage amounts to take into consideration.
Exactly :)
We aren't adding 1 L, we're assuming densities at STP, which is 22.44 L. So the volume is 22.41, not 1.
That was just an example.
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Yeah sorry for the mistake, I just get a little imaginative sometimes ;D
I know it was just an example when you said 1 L, just trying to make you angry ::)
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Well I forgot to mention the percentage amounts to take into consideration. In this case we know there is more oxygen, which is 32.00 g/mol, and nitrogen molar mass plus oxygen molar mass is obviously greater.
It doesn't actually matter what percentage it is. As soon as you know air is a mixture of oxygen (Mr=32) and other gases with Mrs less than 32 then the density of air must be lower than that for oxygen. It wouldn't matter if air was 99.99% O2 or 0.01% O2 the density of air would still be lower. Obviously gases like Ar and CO2 raise the density but that is negligible compared to N2.
We aren't adding 1 L, we're assuming densities at STP, which is 22.44 L. So the volume is 22.41, not 1.
That was just an example.
And a perfectly legitimate example. The question told you that 1 mole of gas occupies x litres, when you are working with density it doesn't matter how much gas you have the density will be the same, you could have assumed there to be 100 billion litres of gas or 0.00001 moles of gas it won't make a difference.
Yeah sorry for the mistake, I just get a little imaginative sometimes ;D
I know it was just an example when you said 1 L, just trying to make you angry ::)
LOL ;D
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then the density of air must be lower than that for oxygen
You mean higher?
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then the density of air must be lower than that for oxygen
You mean higher?
No I mean lower. :P
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Of course. I have this nasty habit of not really reading the question closely.
20% oxygen, 80% nitrogen.
Sheesh... :-X
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I know it was just an example when you said 1 L, just trying to make you angry ::)
Not an easy task but doable. Don't try too hard ;)