[hellothere1]

16. (2002) When the temperature of a sample of H2S gas is lowered,
the pressure decreases more than predicted by the ideal
gas equation. To what is this deviation from expected
behavior due?

1. attractive forces between molecules
2. mass of the molecules
3. volume of the molecules
(A) 1 only (B) 2 only
(C) 1 and 3 only (D) 2 and 3 only

The answer is (A) - 1 only.

According to the ideal gas law... PV=nRT.
If T is decreased, P and V must decrease (or at least one of them). I predicted that if the pressure decreased more than expected, then the volume must have increased, so answer (C). However, by the answer, it shows that 3.(volume of the molecules) is not correct. Could you please explain what I am missing? Thank you.

[Brian Lin]

I rearranged the PV=nRT equation into P=(nR/V)T. Since we know nothing about the volume, we can close our eyes and assume that V is constant. So now we have Pressure is proportional to Temperature.

Also, we know that the sample of H2S remains the same amount from the beginning to the end (law of conservation of mass), so n is the same.

Using the Van der Waals equation.. 

[P(obs) + a(n/v)^2]*(v-nb)=nRT

we know n and v are constant, leaving the only possible source of change at the letter "a".

"a" depends on the IMF forces and the boiling points of the molecule.
Therefore, we can deduce that only the intermolecular forces are responsible for the decrease in pressure.

I hope this helps and hope you make it to the Study Camp. 

[hellothere1]

Thank you very much for your solution. I understand now . Also, thanks a lot for the encouragement!