Question

At lower temperature, all gases except ${{\text{H}}_2}{\text{, He}}$ shows:
A.Negative deviation
B.Positive deviation
C.Positive and negative deviation
D.None of the above.

Answer 1

The positive and negative deviation is determined with the help of compressibility factor. The compressibility factor is greater than 1 then the gas shows positive deviation.
Formula used: ${\text{Z}} = \dfrac{{{\text{PV}}}}{{{\text{RT}}}}$
Here Z is compressibility factor, P is pressure, V is molar volume, R is universal gas constant and T is temperature.

Complete step by step answer:
The ideal gas equation is not applicable to real gas. Hence, we use the real gas equation which involves the compressibility factor. The value of compressibility factor is as follow:
${\text{Z}} = \dfrac{{{\text{PV}}}}{{{\text{RT}}}}$
Hydrogen and helium are very lighter gas. The attraction between these gases is very less. The value of van der waal coefficient is very less for hydrogen and helium. Hence the pressure exerted is high. So the value of Z that is compressibility factor is greater than 1 even at lower temperature. Hence it shows positive deviation. For other gases there is considerable attraction and hence the compressibility factor is less that 1 so they show the negative deviation. Due to lower temperature the kinetic energy lowers and hence the attraction in between gases increases.
For an ideal gas equation the value of compressibility factor is 1. So the equation becomes ${\text{PV}} = {\text{RT}}$ which is an ideal gas equation. That is the gas behaves as ideal gas.

Hence, the correct option is A.

Note:
Ideal gas considers that the attraction in between the gas molecule is zero. If this is considered as true so a gas should not liquefy but we see that in actual practice gases liquefy. This leads to the failure of the ideal gas equation.