The volume of unreacted Oxygen = 1.25 L = 1.25 dm³ = 1250 cm³
Further explanation
Given
Combustion of propane
3.6 L CO2
7.25 L O2
Required
The volume of unreacted oxygen
Solution
Reaction
C₃H₈ + 5O₂ ⇒ 3CO₂ + 4H₂O
Avogadro's :
In the same T,P and V, the gas contains the same number of molecules
So the ratio of gas volume will be equal to the ratio of gas moles
Vol O₂ = 5/3 x vol CO2 = 5/3 x 3.6 = 6 L
The unreacted : 7.25 - 6 = 1.25 L