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Topic: Atomic Weight of Aluminum  (Read 14149 times)

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Online billnotgatez

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Atomic Weight of Aluminum
« on: June 15, 2005, 08:33:03 AM »
Atomic Weight of Aluminum: 26.98154

Does this mean that Aluminum weighs about 27 grams per mole at standard temperature and pressure?

Also, I do not see the density of Aluminum on the periodic table.
« Last Edit: June 15, 2005, 08:51:38 AM by billnotgatez »

Offline jdurg

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Re:Atomic Weight of Aluminum
« Reply #1 on: June 15, 2005, 01:31:22 PM »
Yes.  In fact, a mole of aluminum will weigh almost 27 grams no matter what temperature and pressure it's at!   :o ;) ;D  (Atomic mass doesn't change with temperature and pressure).  The atomic masses you see on the periodic table are actually the weighted atomic masses of the elements.  It's an average of the masses of the naturally occuring isotopes of each element.  So if Element X has two naturally occuring Isotopes, one which has a mass of 200 g/mole and another with a mass of 203 g/mole, and X-200 makes up 74% of all naturally occuring X, the atomic mass you would see on the periodic table for element X would be (200*.74)+(203*.26) = 200.78 g/mole.  So one mole of naturally occuring, non isotopically purified Element X would weigh 200.78 grams per mole.
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Re:Atomic Weight of Aluminum
« Reply #2 on: June 15, 2005, 11:01:46 PM »

Also, I do not see the density of Aluminum on the periodic table.


We'll get to it eventually.
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Offline jdurg

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Re:Atomic Weight of Aluminum
« Reply #3 on: June 16, 2005, 09:30:06 AM »
Yeah, my apologies for being really lazy with the periodic table.  I've just been so freaking busy with Golf, *Ignore me, I am dishonest*, my baseball card collection, etc. etc.
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Re:Atomic Weight of Aluminum
« Reply #4 on: June 16, 2005, 10:17:56 PM »
Aluminum is 27 g/mole regardless of STP. It has a density of 2.75 g/cm3

arnyk

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Re:Atomic Weight of Aluminum
« Reply #5 on: June 16, 2005, 11:08:15 PM »
Only for gases will the molar volume change -- 22.4 L/mol @ STP, 24.8 L/mol @ SATP.

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Re:Atomic Weight of Aluminum
« Reply #6 on: June 17, 2005, 12:12:51 AM »
Hmmm...I just checked in my CRC handbook out of curiosity and 13Al27 has a natural abundance of 100%.

Offline Borek

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Re:Atomic Weight of Aluminum
« Reply #7 on: June 17, 2005, 03:10:06 AM »
Only for gases will the molar volume change -- 22.4 L/mol @ STP, 24.8 L/mol @ SATP.

Colud you elaborate?
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arnyk

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Re:Atomic Weight of Aluminum
« Reply #8 on: June 17, 2005, 03:20:49 PM »
All gases have different densities, but under constant conditions (T, P, n) all gases will occupy the same volume.  

molar volume = molar mass / density

molar volume of He = (4.00 g/mol) / (0.162 g/L @ SATP)
                             = 24.8 L /mol

molar volume of Ne = (20.2 g/mol) / (0.815 g/L @ SATP)
                             = 24.8 L/mol

Do that for any gas and the molar volume will be quite close to 24.8 L/mol @ SATP.  

I don't have the densities @ STP on me, but when you do the same calculations with those the molar volume will be 22.4 L/mol.

Offline Borek

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Re:Atomic Weight of Aluminum
« Reply #9 on: June 17, 2005, 04:05:22 PM »
Quote
I don't have the densities @ STP on me

try molar mass / 22.4 ;)

Quote
Only for gases will the molar volume change

What about thermal expansion?
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arnyk

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Re:Atomic Weight of Aluminum
« Reply #10 on: June 17, 2005, 04:16:11 PM »
Right, you can rearrange to find the densities at STP. :D

Thermodynamics?  That's a bit over my head actually.  But wouldn't that only occur if the temperature were to vary, and since molar volume depends on constant conditions that wouldn't be a problem?  
« Last Edit: June 17, 2005, 04:16:32 PM by arnyk »

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Re:Atomic Weight of Aluminum
« Reply #11 on: June 17, 2005, 05:06:16 PM »
But wouldn't that only occur if the temperature were to vary, and since molar volume depends on constant conditions that wouldn't be a problem?

What I am pointing at is that your phrase

Quote
Only for gases will the molar volume change

is incorrect - for gases the changes are the largest, but every substance changes it molar volume when the temperature or pressure change. For most practical purposes most solids don't change volume with pressure, but this effect is perfectly measurable and can be taken into account when making deep sea equipment (very deep sea :) ).

Additionally you are slightly inconsistent. If we are at constant conditions  substance can't change it's molar volume. If molar voume changes we are not at constant conditions. Either-either.  ;)
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arnyk

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Re:Atomic Weight of Aluminum
« Reply #12 on: June 17, 2005, 06:14:31 PM »
Aye, poorly worded then I guess.  Just not on the ball today. ;)

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