Answer:
5 is the coefficient in front of O2
Step-by-step explanation:
You need to balance the chemical equation:
C3H8 + O2 → CO2 + H2O
To balance the equation, you can see the amount of carbons: 3, so you need to put in the other side of equation the number 3 in fron of the CO2.
Then the number of Hidrogen must be the same in both side of ecuation: 8, you need put an 4 in front of the H20, because you multiply 4 to 2.
Now you addit the number of oxigen in the right side of ecuation and divide to 2 (because you have O2) and put this result (in this case 5) in front of O2.
C3H8 + 5 O2 → 3 CO2 + 4 H2O