The mass of phosphorus required to obtain 100 g of phosphorus (V) oxide is …

Given:
m (P2O5) = 100 g
To find:
m (P) =?
Decision:
1) Let’s compose the equation of the chemical reaction, we know the mass of P2O5, and let the mass of phosphorus be x.
4P + 5O2 = 2P2O5
2) Find the molecular weight of phosphorus
M (P) = 31, since 4 molecules 31 x 4 = 124 g / mol
3) Find the molar mass of P2O5
M (P2O5) = (31 x 2) + (16 x 5) = 142 g / mol, since there are two P2O5, then 142 x 2 = 284 g / mol
4) x: 124 = 100: 284
x = (124 g / mol x 100 g): 284 g / mol = 43.67 g.
Answer: m (P) = 43.67 g.