How many grams of oxygen are in 50 liters of air?

Given:
V (air) = 50 l
Find: m (O2) -?
Decision:
1) Using the table, determine the O2 volume fraction:
φ (O2) = 21%;
2) Calculate the O2 volume:
V (O2) = φ (O2) * V (air) / 100% = 21% * 50/100% = 10.5 l;
3) Calculate the amount of substance O2:
n (O2) = V (O2) / Vm = 10.5 / 22.4 = 0.5 mol;
4) Using the periodic system of D.I.Mendeleev, calculate the molar mass of O2:
M (O2) = Mr (O2) = Ar (O) * N (O) = 16 * 2 = 32 g / mol;
5) Calculate the mass of O2:
m (O2) = n (O2) * M (O2) = 0.5 * 32 = 16 g.
Answer: The mass of O2 is 16 g.