[ajax0604]

Could someone tell me whether I am on the right track?
Avogadro's Law states that the same number of molecules of different gases will occupy the same volume (provided that the temperature and pressure remain constant). This is based on the assumption that all gases are ideal and all behave in exactly the same way. However, in reality, if there was an equal number of water vapour molecules and helium molecules (in separate balloons for example), the water molecules would occupy a smaller volume because there is stronger intermolecular attraction compared to that existing between helium atoms??   

[Borek]

Sounds OK to me.

Just remember the intermolecular forces play a role when the distances between molecules are small. The denser the gas the higher the differences between ideal and real gas behavior.

[yasas.w]

Could someone tell me whether I am on the right track?
Avogadro's Law states that the same number of molecules of different gases will occupy the same volume (provided that the temperature and pressure remain constant). This is based on the assumption that all gases are ideal and all behave in exactly the same way. However, in reality, if there was an equal number of water vapour molecules and helium molecules (in separate balloons for example), the water molecules would occupy a smaller volume because there is stronger intermolecular attraction compared to that existing between helium atoms??

To me it sounds like it should be vice versa because,
H₂O_(g)'s radius > He_(g)'s radius

[formaldehyde23]

Acutally yasas, the OP is correct.

H₂O occupies a smaller volume in the container because there are attractive forces due to the H-bonds.
He just has London dispersion forces between each other, and H-bonds are much stronger than London forces.

A gas molecule's radius does not imply that there will be a larger volume.

[yasas.w]

Acutally yasas, the OP is correct.

H₂O occupies a smaller volume in the container because there are attractive forces due to the H-bonds.
He just has London dispersion forces between each other, and H-bonds are much stronger than London forces.

A gas molecule's radius does not imply that there will be a larger volume.

Well lets think there is a fish tank and the volume is V m³. Volume of a fish is a m³.
If there are an n number of fish in the tank, the total volume occupied by fish are an m³ right? So the net volume they can swim inside the tank is V-an m³. Despite how attractive or vice versa they're to each other the total volume occupied by them will be an m³. That is what which made me to think that the net volume occupied by H₂O_(g) should be greater than the net volume occupied by He_(g)

[billnotgatez]

Does our discussion depend on if we are doing
Ideal Gas Law
or
Van der Waals equation

Ideal gas law
http://en.wikipedia.org/wiki/Ideal_gas_equation
Van der Waals equation
http://en.wikipedia.org/wiki/Van_der_Waals_equation

[yasas.w]

However, in reality, if there was an equal number of water vapour molecules and helium molecules (in separate balloons for example), the water molecules would occupy a smaller volume because there is stronger intermolecular attraction compared to that existing between helium atoms??

so it should be Van der Waals =)

[Borek]

There are two things responsible for the differences between the real gas and the ideal gas. One is the interaction between molecules (much larger in the case of water), the other is the molecule volume (again, much larger in the case of water). However, their effects go in the opposite directions, and the final result depends on which of these effects dominate.