Question

How do you balance this equation? C5H12+ O2-> H2O + CO2

Answer 1

We can solve this in a couple of ways. The way I like, is to first say there is only one mole of fuel, then balance the other species. Let me explain. First, rewrite the equation as such\[\rm C_5H_{12} + x~O_2 \rightarrow y~H_2O + z~CO_2\]Now, lets balance the carbon, hydrogen, and oxygen.
Carbon: 5 = x
Hydrogen: 12 = 2y
Oxygen: 2x = y + 2z

We can solve these three equations for x, y, and z. Always balance in that order: carbon, hydrogen, and oxygen.

If we end up with a decimal coefficient, we can adjust the number of moles of fuel. Note that it is common practice to balance combustion equations using one mole of fuel, because we are usually interested in how much air and how much products we get on a per mole fuel basis. This is important for engine design, where we need to know how much air to breathe to have a stoich combustion and how to manage our emissions.