What is the mass of 0.2 mol of phosphorus (V) oxide P2O5?

Let’s compose the reaction equation:
4P + 5O2 = 2P2O5.
Let’s find the amount of phosphorus oxide (V):
n = m / Mr
Mr (P2O5) = 2 * 31 + 5 * 16 = 142 g / mol
n = 15 g / 142 g / mol = 0.1 mol
Let’s calculate the amount of phosphorus from the proportion:
x mol / 4 mol = 0.1 mol / 2 mol; x = 0.2 mol
Let’s find the mass of phosphorus:
m = n * Mr = 0.2 mol * 31 g / mol = 6.2 g
Answer: m (P) = 6.2 g