A trinitro compound, 1, 3, 5 -tris-( 4 -nitrophenyl) benzene, on...

A trinitro compound, $1, 3, 5$-tris-($4$-nitrophenyl) benzene, on complete reaction with an excess of $Sn / HCl$ gives a major product, which on treatment with an excess of ${NaNO}_{2} / HCl$ at $0 ° C$ provides $P$ as the product. $P$, upon treatment with excess of ${H}_{2}O$ at room temperature, gives the product $Q$. Bromination of $Q$ in aqueous medium furnishes the product $R$. The compound $P$ upon treatment with an excess of phenol under basic conditions gives the product $S$. The molar mass difference between compounds $Q$ and $R$ is $474g{mol}^{- 1}$ and between compounds $P$ and $S$ is $172 . 5g{mol}^{- 1}$.
The number of heteroatoms present in one molecule of $R$ is [Use: Molar mass (in g mol-1): $H = 1, C = 12, N = 14, O = 16, Br = 80, Cl = 35 . 5$ Atoms other than $C$ and $H$ are considered as heteroatoms]