When ${\mathrm{PbO}}_2$ reacts with concentrated ${\mathrm{HNO}}_3$, the gas evolved is

 we know that ${\mathrm{PbO}}_2$ is a powerful oxidising agent and it liberates ${\mathrm{O}}_2$ when treated with concentrated. ${\mathrm{HNO}}_3$.

The chemical reaction is as follows:

${\mathrm{PbO}}_2+\text{ conc }·{\mathrm{HNO}}_3\to \mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$