How much oxygen is required to obtain 71 g of phosphorus (V) oxide?

Given:
m (P2O5) = 71g
Find:
V (O2) -?
Solution:
1) First, let’s compose the reaction equation:
4P + 5O2 = 2P2O5
2) Now let’s define the formulas with which we can solve the problem:
v (number of things) = m / M
V = v × 22.4 l / mol
3) Then we substitute the known values into the formulas:
v (P2O5) = 71g / 142g / mol = 0.5 mol
Because P2O5 and O2 ratios are 1 in 2.5:
0.5 × 2.5 = 1.25 mol
V (O2) = 1.25 mol × 22.4 l / mol = 28 l
Answer: 28L