# Avogadro's gas Law doesn't make sense? (it does actually)

It says equal volumes of all gasses under same conditions of temp. and pressure contains the same number of atoms/molecules.

Now suppose you have 1L of Chlorine in a box and 1L Hydrogen in another box , now the comparitively huge size of the Cl atom means less of it will fit in the box , whereas lots of hygrogen will. ( breaking the law)

But, well this should have an effect in pressure , how does all that boil down to avogadro being right ?

• Molecules are very small indeed so what you say about not fitting in is not true; there is plenty of space. If you don't believe me work out what fraction of the volume of 1 litre Avogadro's number of molecules of each of your gasses would occupy. You can easily find the bond lengths and atomic radii needed, perhaps even molecular volume. May 9, 2018 at 16:49
• The gas laws assume ideal gases which thus ignore atom/molecule size and atom/molecule interactions.
– MaxW
May 9, 2018 at 16:58
• I dint get it But if you scale that up many many times you should actually "see" the difference @porphyrin May 9, 2018 at 17:20
• @porphyrin we can fit more chlorine but that will lead to a higher pressure in the chlorine box ! Jul 14, 2018 at 19:02

Oh it definitely makes sense since it allows us to describe a lot of systems with very simple formulas.

The problem is that this law assumes "ideal gases" (just like the ideal gas law) where atoms/molecules are "points" (no volume) and they don't interact with each other. This works well at rather low temperatures, low pressure and for gases without strong intermolecular interactions.

But you are correct, there are problems and the law doesn't work in a lot of cases. For example if you go to high pressures the volume of the indiviual atoms/molecules does play a role, since if they are bigger the available space will be smaller and the pressure will be higher than predicted by the ideal gas law.

• Exactly this was what i was looking for it had to have that point molecule assumption, I've been occasionally thinking about this for like a year ! May 9, 2018 at 17:24
• our teacher never mentioned that ! May 10, 2018 at 15:43

You're right that it should have an effect on pressure-and it does. It's just that it's practically a negligible amount.

One of the assumptions of the Ideal Gas Model is that the volume of the molecules is negligibly small compared to the volume occupied by the gas. This assumption allows us to simplify the complexity in the derivation. Note how the only mention of the atom's volume is in the assumption and left completely out of the mathematics.

• Exactly this was what i was looking for it had to have that point molecule assumption, I've been occasionally thinking about this for like a year ! May 9, 2018 at 17:23