How much phosphorus is needed to produce 71 grams of phosphorus oxide?

Let’s find the amount of phosphorus oxide (V) substance by the formula:

n = m: M.

M (P2O5) = 142 g / mol.

n = 71 g: 142 g / mol = 0.5 mol.

Let’s find the quantitative ratios of substances.

4 P + 5 O2 = 2 P2O5

There are 2 mol of P2O5 salt per 4 mol of P.

Substances are in quantitative ratios 4: 2 = 2: 1.

The amount of phosphorus substance is 2 times greater than the amount of oxide substance.

n (P2O5) = 2n (P) = 0.5 × 2 = 1 mol.

Let’s find the mass P.

m = n × M.

M (P) = 31 g / mol.

m = 31 g / mol × 1 mol = 31 g.

Answer: 31 g.