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Topic: Oxygen and hydrogen: i need help on my pre laboratory exercise...  (Read 16602 times)

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Offline ^^Samantha**

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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




Offline Borek

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #1 on: August 29, 2006, 04:49:17 AM »
Please read forum rules.

Questions 2 & 3 can be easily answered after minute of googling.
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Offline ^^Samantha**

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #2 on: August 29, 2006, 05:16:53 AM »
oh sorry about that...

uhm about number 1?...
« Last Edit: August 29, 2006, 05:29:57 AM by ^^Samantha** »

Offline sdekivit

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #3 on: August 29, 2006, 06:23:06 AM »
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.

Offline ^^Samantha**

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #4 on: August 29, 2006, 07:30:53 AM »
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?

Offline ^^Samantha**

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #5 on: August 29, 2006, 07:35:34 AM »
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???

Offline Will

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #6 on: August 29, 2006, 07:41:37 AM »

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.

Offline ^^Samantha**

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #7 on: August 29, 2006, 08:29:06 AM »
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?
 ???


Offline Borek

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #8 on: August 29, 2006, 08:32:28 AM »
Oxygen = 32 x .2 = 25.6 g/mol

Check your math.
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Offline ^^Samantha**

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #9 on: August 29, 2006, 09:01:20 AM »
oh thanks  ;D

so tell me, am i in the correct path? so that i can proceed now...

Offline Will

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #10 on: August 29, 2006, 09:04:27 AM »
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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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #11 on: August 29, 2006, 01:40:28 PM »
3.) List down important uses of:

a.) Hydrogen

b.) Oxygen

The real answer: fuel cells.
Pierre.

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Offline english

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #12 on: August 31, 2006, 05:44:32 AM »
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

Offline Borek

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #13 on: August 31, 2006, 06:17:29 AM »
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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Offline english

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Re: Oxygen and hydrogen: i need help on my pre laboratory exercise...
« Reply #14 on: August 31, 2006, 12:32:14 PM »
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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