(a) For $1\ g$ gas $PV = rT = \left( \frac{R}{M} \right)T$
Since $P$ and $V$ are constant $\Rightarrow T \propto M \Rightarrow \frac{T_{N_{2}}}{T_{O_{2}}} = \frac{M_{N_{2}}}{M_{O_{2}}}$
$\Rightarrow \frac{T_{N_{2}}}{(273 + 15)} = \frac{28}{32} \Rightarrow T_{N_{2}} = 252K = - 21℃$