# Pentane gas (C5H12) combusts with oxygen gas (O2) to form water (H2O) and carbon dioxide (CO2). Write a balanced chemical equation for this reaction.

enotechris | College Teacher | (Level 2) Senior Educator

Posted on

Specifying the equation:

C5H12 + O2  --> H2O + CO2

So let's count up the number of atoms on each side:

5 carbons, 12 hydrogens, 2 oxygens for reactants, and

1 carbon, 2 hydrogens, 3 oxygens for products.

Now the number of atoms on each side must be equal, because atoms are neither created or destroyed in the reaction. If you have 5 carbons as reactants, you have to have 5 carbons as products, so:

C5H12 + O2  |  H2O + 5 CO2

There are 12 hydrogens as reactants, so you need 12 as products:

C5H12 + O2  |  6 H2O + 5 CO2

But 6 water molecules have 6 oxygen atoms on the product side, along with the 10 that are already there from the carbon dioxide, for a total of 16 oxygens.  To get 16 oxygens as products, you need 8 oxygen molecules as reactants:

C5H12 + 8 O2  -->  6 H2O + 5 CO2