c
$P \propto n \Rightarrow \frac{P_{1}}{P_{2}}=\frac{n_{1}}{n_{2}}$
$n_{1}=\frac{35}{28} \times 1000=1250$
$\Rightarrow n_{2}=\frac{1250}{6} \times 9=1250 \times \frac{3}{2}=1875$
$n_{2}=n_{N_{2}}+n_{\circ_{2}}=1875=1250+n_{\mathrm{o}_{2}}$
$n_{\mathrm{o}_{2}}=625$ mole
mass $=$ no. of mole $\times$ molecular weigh $20 \mathrm{kg}$