Calculate the amount of oxygen required to burn 100 g of propane.

Given:
m (C3H8) -100g
To find:
n (O2) -?
Decision:
1) carry out the combustion reaction:
C3H8 + 3O2 = 3CO2 + 4H2O (we are not surprised where carbon dioxide came from, because we remember that during the combustion reaction precipitates come out: gas and water)
2) we find the number of islands:
formula: n = m: Mr
n = m (C3H8) 100g: Mr (O2) 32 = 3.125 mol
Answer: n (O2) -3.125 mol