Why is this that the gases with small molecular masses like hydrogen and helium show positive deviatioon from ideal behaviour.

 

In Hydrogen and Helium , the molecular masses are very small and hence Vanderwaal forces of attraction are very poor in these cases. so from the Vanderwaal equation, the term having factor 'a'  corresponding to the attraction forces is eliminated.

(P + a/v²)  (V-b)  =RT  =P' V'

PV- Pb  =P' V'

PV  = P'  V'  +Pb

Therefore, PV is more than P' V'

Hence positive deviation.