___ C8H18 + ___ O2 --> ___ CO2 +  ___ H2O

This is a balanced chemical equation for the combustion of octane (C8H18). Here's how you balance it:

1. Write down the unbalanced equation: C8H18 + O2 --> CO2 + H2O

2. Balance the carbon (C) atoms first. There are 8 carbon atoms in C8H18, so you need 8 CO2 molecules to balance the carbon atoms: C8H18 + O2 --> 8CO2 + H2O

3. Next, balance the hydrogen (H) atoms. There are 18 hydrogen atoms in C8H18, so you need 9 H2O molecules to balance the hydrogen atoms: C8H18 + O2 --> 8CO2 + 9H2O

4. Finally, balance the oxygen (O) atoms. There are 25 oxygen atoms on the right side (16 from CO2 and 9 from H2O), so you need 12.5 O2 molecules to balance the oxygen atoms: C8H18 + 12.5O2 --> 8CO2 + 9H2O

5. However, we can't have half molecules in a balanced chemical equation, so we multiply everything by 2 to get rid of the fraction: 2C8H18 + 25O2 --> 16CO2 + 18H2O

So, the balanced chemical equation is: 2C8H18 + 25O2 --> 16CO2 + 18H2O