Chemistry Question

[delusional]

Ya I know that's what Google is for. But, I can't find it anywhere.

So, I have to explain why Avogrado's Law is true. In that, why is it that the amount of molecules within a specific volume and pressure is the same for all gases. In other words, there will be the same amount of moles of oxygen in a set pressure and volume as there would be moles nitrogen.

All the websites just say this happens, but they don't really say why. I need helplife2.net.

 

[PJ]

I did Avogadro not that long ago...I should know this. Sorry.

 

[dfc05]

If you mean the PV=nRT equation... I'm not too comfortable explaining why most gases follow it... probably something with molecules in a gas ideally being about the same space apart at the same T & P. Maybe something to do with the amount of energy they have? I wish I had a textbook here.

I can however definitely tell you that it doesn't work for EVERYTHING -- there are lots of nonideal gas mixtures (and they're a pain in the butt to do calculations on). Here's some stuff copied out of my class notes:

There is no such thing as an ideal gas
There are conditions under which a gas can be adequately described by the ideal gas law:
1. molecules are far apart
2. molecules are small
3. neglect inter-molecular forces/molecular volumes
4. under different conditions, the same gas may not fit the ideal gas law — need a more realistic law

Edit: This may be helpful -- http://en.wikipedia.org/wiki/Ideal_gas_law
"The ideal gas law mathematically follows from a statistical mechanical treatment of primitive identical particles (point particles without internal structure) which do not interact, but exchange momentum (and hence kinetic energy) in elastic collisions.

Since it neglects both molecular size and intermolecular attractions, the ideal gas law is most accurate for monoatomic gases at high temperatures and low pressures. The neglect of molecular size becomes less important for larger volumes, i.e., for lower pressures. The relative importance of intermolecular attractions diminishes with increasing thermal kinetic energy i.e., with increasing temperatures. More sophisticated equations of state, such as the van der Waals equation, allow deviations from ideality caused by molecular size and intermolecular forces to be taken into account."

My interpretation: you have gas molecules with energy (which is related to temperature...). Assuming all this energy is going into making these molecules bounce around (not like atomic bonds wiggling and stuff, hence the "monoatomic" part) and the molecules don't want to stick to each other ("intermolecular attractions") and all size gas molecules move around the same regardless of how big they are ("molecular size"), on average they would bounce apart from each other at the same ideal distance, so the volume is proportional to the number of molecules you have.

 

[delusional]

That's perfect dfc05! Thank you very much. I might not understand it fully, but its what I was looking for.

 

[bbson john]

The factual gas equation is (P+a/V^2)(V-v) = nRT
where a has something to do with intermolecular attractive force, most likely Van der Waal's. v is the total volume of gas molecules. a and v varies with different molecules. So the molar volume of each gaseous substances are different. But you can see that a and v are always very small, the molar volume of various gases can be considered the same, especially under low pressure and high temperature.

 

[Gunner]

NEEEEEEEERDS!!!

 

[ríomhaire]

I haven't read the replies but I assume that it's because one of the many assumptions made in the law is that all gases are made of identical, infintismal particles.

I have a 3 hour chemistry exam in 15 minutes.

 

[Sam]

Them thar hard spheres.

I have a 3 hour chemistry exam in the afternoon.

 

[Danimal]

I haven't got a 3 hour chemistry test at all.

 

[bbson john]

I had a 6 hour chemistry exam last month.

 

[Dodo]

I haven't got a 3 hour chemistry test at all.

Ive got that too!! : O

-dodo

 

[Escaep]

I remember Avocado's number back in Chemistry. Wasn't that hard.

 

[dfc05]

The factual gas equation is (P+a/V^2)(V-v) = nRT
where a has something to do with intermolecular attractive force, most likely Van der Waal's. v is the total volume of gas molecules. a and v varies with different molecules. So the molar volume of each gaseous substances are different. But you can see that a and v are always very small, the molar volume of various gases can be considered the same, especially under low pressure and high temperature.

That's just one of many equations of state. I've also had to use Redlich-Kwong, RK-Soave, Peng-Robinson, and the virial EOS. http://en.wikipedia.org/wiki/Equations_of_state

But now I'm finished with my chemistry courses... all seven of them, and 19 chemical engineering courses. Most of which had three-hour final exams. Even our normal exams were so long, they made us come at night to take them because we wouldn't be able to finish them in class. :x They don't let you get out of this major without trying to kill you first.