Question

Which gas would you expect to behave more like an ideal gas; HCl, H2O, or He? Why?

Answer 1

He expect to behave more like an ideal gas

A fundamental assumption of the ideal gas law is that the individual particles of the gas do not interact. That is, there are no interparticle forces, nor do they collide. This implies infinitely small particles. Basically, any gas at a low enough pressure and high enough temperature will behave very close to an ideal gas. In fact, no real physical gas behaves exactly as an ideal gas.

Any gas will deviate from the ideal gas law if 1) the pressure is increased, or 2) the temperature is lowered. The pressure, or temperature at which deviation from ideal gas law behavior depends on the particles of the gas.

In a similar way, Helium will show the same behavior. Helium, however, is a very small particle (noble gas = atoms not molecules), is extremely non-polarizable, and has very low inter-atomic forces. This results in a very low boiling point, about 4 Kelvins. Thus, Helium will demonstrate ideal gas law behavior to temperatures much closer to its boiling point and to much higher pressures.

HCl is a liquid at high temperatures it deviates from ideal gas law. H2O possess hydrogen bonding and also at low pressure high temperature deviates from ideal gas law. both of these become solid at very low temperatures