Calculate the mass of burnt red phosphorus if 22.72 g of phosphorus oxide +5 are formed.

Given:
m (P2O5) = 22.72 g
To find:
m (P) -?
Decision:
1. Let’s compose the equation of the phosphorus combustion reaction:
4 Р + 5 О2 = 2 Р2О5
According to the equation:
4 mol of phosphorus enters into the reaction;
2 mol of phosphorus (V) oxide is formed.
Let’s find the mass of phosphorus by the formula:
m (P) = n * M = 4 mol * 31 g / mol = 124 g
Let’s find the mass of phosphorus oxide (V) by the formula:
m (P2O5) = n * M = 2 mol * (31 * 2 + 16 * 5) = 284 g
2. Let’s calculate the mass of burnt red phosphorus, making the proportion:
x g P – 22.72 g P2O5
124 g R – 284 g P2O5
Hence, x = 124 * 22.72 / 284 = 9.92 g
Answer: m (P) = 9.92 g