Calculate the number of molecules of phosphorus [P4] present in 31 gram of phosphorus.
Molecular mass of P4 = 124 g/mol.
i.e.
Mass of 1 mole ( 6.023 X 1023) P4 molecules = 124 g
Hence..
124 g P4=6.023 x 1023 P4 molecules
so
1g P4 =( 6.023 x 1023)/(124) =0.25 x 1023molecules
and
31 g P4 = 0.25 x 1023 x 31 = 1.5 x1023 molecules.