"PARAGRAPH II"

A trinitro compound, 1, 3,5 tris-(4-nitrophenyl) benzene, on complete reaction with an excess of Sn/HCl gives 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 474 mol -1 and between compounds P and S is 172.5 gmol -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]