Q=ΔU−Uf−Uu= [internal energy of 4 moles of a monoatomic gas + internal energy of 2 moles of a diatomic gas] – [internal energy of 4 moles of a diatomic gas]
=(4×
3
2
RT+2×
5
2
RT)−(4×
5
2
RT)=RT
Note : (a) 2 moles of diatomic gas becomes 4 moles of a monoatomic gas when gas dissociated into atoms. (b) Internal energy of µ moles of an ideal gas of degrees of freedom F is given by U=
f
2
µRT,F=3 for a monoatomic gas and 5 for diatomic gas