What mass of phosphorus is needed to obtain 71 g of phosphorus (V) oxide.

From the reaction equation: 4P + 5O2 = 2P2O5, it can be seen that the formation of 1 mol of phosphorus (V) oxide requires 2 mol of phosphorus (n (P) = 2n (P2O5))

We know the mass of phosphorus oxide (V) m (P2O5), knowing it and the molar mass of phosphorus M (P2O5), we find the amount of phosphorus oxide substance that has reacted n (P2O5) = m (P2O5) / M (P2O5) = 71/142 = 0.5 mol.

Then n (P) = 2 * 0.5 = 1 mol

Find the mass of phosphorus m (P) = n (P) * M (P), where M (P) is the molar mass of phosphorus.

m (P) = 1 * 31 = 31 g.