The mass of oxygen required for the complete combustion of 6.72 liters of hydrogen sulfide is

Reaction equation:
2H2S + 3O2 = 2SO2 + 2H2O
From the condition we find the amount of hydrogen sulfide substance:
n (H2S) = 6.72 / 22.4 = 0.3 mol;
In the reaction equation, oxygen has a coefficient of 3 and hydrogen 2. It follows that the amount of substance that has reacted with oxygen through the amount of hydrogen sulfide is expressed as follows:
n (O2) = n (H2S) ⋅ 3/2 = 0.45 mol.
Hence:
m (O2) = M (O2) ⋅ n (O2) = 32 * 0.45 = 14.4 g