express this mass as a percentage of the weight of your sample of plant food. this is the % of elemental phosphorous in the plant food.

Question

I'm not sure what the question is asking for. The mass its referring to is 0.06438g Phosphorus and the weight of the plant food is 1.007g.

Answer 1

(Mass P/mass sample)*100 = %

Answer 2

ok and i had another question:

convert that to the % phosphorus present as phosphorous pentoxide (P2O5)

Answer 3

%P2O5 = %P x (molar mass P2O5/2*atomic mass P) = ??