What mass of phosphorus reacted with oxygen if 7.1 g of phosphorus oxide (v) was formed?

To solve the problem, we write down given: m (P2O5) = 7.1 g. Phosphorus reacted with oxygen. Find: m (P) -?
Solution:
To solve this problem, we write down the reaction equation:
P + O2 = P2O5
Let’s arrange the coefficients and get:
4P + 5O2 = 2P2O5
Over phosphorus (V) oxide (P2O5), write 7.1 g, and over phosphorus – x g. Calculate the molar mass of phosphorus (V) oxide.
M (P2O5) = 31 * 2 + 16 * 5 = 142 g / mol, since 2 mol of phosphorus (V) oxide was formed, we must multiply the molar mass by the number of moles and get 284 g.
Let’s calculate the molar mass of phosphorus.
M (P) = 31 g / mol, since according to the equation for the reaction of phosphorus 4 mol entered, we must multiply the molar mass by the number of moles and get 124 g.
Let’s compose and solve the proportion. We write x g over phosphorus, and 7.1 g over phosphorus (V) oxide, under phosphorus we write 124 g, and under phosphorus (V) oxide – 284 g.
Hence x = 124 * 7.1 / 284 = 3.1 g
Answer: 3.1 g