Yellow phosphorus is an impure form of white phosphorus (P4 ).

 Molecular mass of yellow phosphorus (P4 ) = ´ = 4 x 30 =120 g/mol

According to Avogadros hypothesis,

Mass of 6.022 x 10²³ molecules of P4 = 120 g

and mass of 1 molecule of P4 =120 x 1/ 6.022 x10²³ = 19.926 x10²³ g

                                                                                             =1.993 x10^-22g

                                                                                              =1 993x10^-19mg