Find the number of millimoles of the indicated species in 64mg of P2O5.?

Answer 1

molar mass P2O5 = 142 g/mol = 142 mg/mmole

64 mg x 1 mmole/142 mg = 0.45 mmoles (2 significant figures)

Answer 2

64 / 283.9  =  0.22543148996 mmol  << round for sig figs