What is the total mass of 1.5 moles of H(g)?

Well, $1.5 \cdot m o l$ of hydrogen ATOMS would have a mass of $1.5 \cdot g$.
Of course, hydrogen normally exists as the dihydrogen molecule, ${H}_{2}$, i.e. a binuclear species. In fact, ALL the elemental gases, SAVE the NOBLE GASES, are binuclear; cf. ${H}_{2}$, ${N}_{2}$, ${O}_{2}$, ${F}_{2}$, and $C {l}_{2}$.
And thus $1.5 \cdot m o l$ of $\text{dihydrogen gas}$ has a mass of $3.0 \cdot g$.