How much phosphorus needs to be burned to get 22g of phosphorus oxide?

Phosphorus combustion reaction:

4P + 5O2 = 2P2O5.

From the reaction equation it follows that 4 mol of phosphorus enters the reaction with the formation of 2 mol of oxide, which means:

n (P) = 2 • n (P2O5).

Find the practical number of moles:

n (P2O5). = m / Mr = 22/142 = 0.15 mol.

n (P) = 2 * 0.15 = 0.3 mol.

Then the mass of phosphorus:

m (P) = n • Mr = 0.3 • 31 = 9.6 g.