Chemical Forums
Specialty Chemistry Forums => Citizen Chemist => Topic started by: billnotgatez on June 15, 2005, 08:33:03 AM
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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.
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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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Also, I do not see the density of Aluminum on the periodic table.
We'll get to it eventually.
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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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Aluminum is 27 g/mole regardless of STP. It has a density of 2.75 g/cm3
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Only for gases will the molar volume change -- 22.4 L/mol @ STP, 24.8 L/mol @ SATP.
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Hmmm...I just checked in my CRC handbook out of curiosity and 13Al27 has a natural abundance of 100%.
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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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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.
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I don't have the densities @ STP on me
try molar mass / 22.4 ;)
Only for gases will the molar volume change
What about thermal expansion?
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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?
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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
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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Aye, poorly worded then I guess. Just not on the ball today. ;)