# Non-Ideal Gas Behavior

I obtained measurements of temperature and pressure at various temperatures ($-10, 0, 20, \text{ and } 50^\circ \mathrm{C}$) and plotted them on a graph. Projecting backwards, the best-fit line predicts 0 pressure at significantly above $-273.15^\circ \mathrm{C}$(0 Kelvin). I am wondering why this is.

My understanding is that a true linear relation assumes "Ideal Gas Behavior", which is predicated on the false assumptions that molecules have no attraction for each other and that they take up no space. I know that at high/low temperatures and pressures the realities about volume and attraction begin to make a difference. But I don't understand how these account for my results.

The pressure for all measurements fell between $680$ and $820$ torr. My understanding is that that is not a significant amount of pressure. In terms of predicting pressures at lower temperatures, I understand that the less kinetic energy of the gas the more relevant the attractive forces are...but I would think that would lower the pressure which makes things worse--if the pressure actually falls faster and faster as temperature drops, then the predicted zero-pressure point would be even higher than the predicted values I got. I need it to be lower!

I am under the (possibly incorrect) impression that both the realities of volume and attraction are supposed to be causing my incorrect prediction...but I don't understand how either does...

Thanks for any help!

• That's a pretty narrow range of temperatures and pressures. Maybe extrapolating all the say back to 0 K is too inaccurate. Show us your data. Feb 4, 2016 at 22:36
• Yeah, I guess it is too narrow. But that's the point of the experiment I think...to get data that doesn't extrapolate properly and then have to explain the deviations :)
– Jo.P
Feb 8, 2016 at 20:37