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

The equation for the reaction of phosphorus with oxygen:

4P + 5O2 = 2P2O5

The amount of substance P2O5:

v (P2O5) = m (P2O5) / M (P2O5) = 71/142 = 0.5 (mol).

According to the reaction equation, 2 mol of P2O5 is formed from 4 mol of P, therefore:

v (P) = v (P2O5) * 4/2 = 0.5 * 4/2 = 1 (mol).

Thus, the required mass of phosphorus:

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

Answer: 31 g.

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