How many moles are there in 71 g of P205.

Given:
m (P2O5) = 71 g

To find:
n (P2O5) -?

1) Using the periodic system of D.I.Mendeleev, calculate the molar mass of phosphorus oxide (V):
M (P2O5) = Mr (P2O5) = Ar (P) * N (P) + Ar (O) * N (O) = 31 * 2 + 16 * 5 = 142 g / mol;
2) Calculate the amount of phosphorus oxide (V) substance:
n (P2O5) = m (P2O5) / M (P2O5) = 71/142 = 0.5 mol.

Answer: The amount of phosphorus (V) oxide substance will be 0.5 mol.