Question #d02c8

1 Answer
Feb 6, 2017

5 moles of oxygen are needed for one mole, so 5x4=20 moles for 4.0 moles of propane

Explanation:

C_3H_8+5O_2 ->3CO_2+4H_2O

Assuming complete combustion as in the balanced equation above, ie all carbon is oxidised to CO_2 and none to C or CO, the 3 carbons will require 3 moles of oxygen (remember oxygen is O_2, not just O), and the 8 hydrogens will require four oxygens, i.e 2 moles of oxygen each. Adding those two up you get 5 moles of oxygen ie 5xO_2). This is per mole of propane. If we have 4 moles of propane, we will need 4x5=20 moles of oxygen in total.

A quick shortcut to balance these types of combustion reactions (ie combustion of hydrocarbons) is to add the number of carbon atoms in the hydrocarbon to the number of hydrogens divided by four e.g pentane is C_5H_12, so it requires 5+(12/4)=5+3=8 moles of oxygen for complete combustion. The number of carbons equals the number of carbon dioxides formed and the number of hydrogens divided by two equals the number of waters.