The atomic weight for a triatomic gas is a. The correct formula for the number of moles gas in its$\mathrm{\omega } \mathrm{g}$ is :

Atomic weight of an element $=\mathrm{a}$

Now, triatomic gas suggest that there are 3 atoms in 1 molecule.

Thus, molecular weight $=3$ times atomic weight $=3a$
Number of moles of gas in '$\mathrm{\omega }$' g is $=\frac{\mathrm{\omega }}{ \mathrm{molecular} \mathrm{weight}}=\frac{\mathrm{\omega }}{3\mathrm{a}}$