How do you balance #C_3H_6 + O_2 -> CO + H_2O#?

In order to maintain balance, we add molecules to make sure that each element has the same number of atoms on both sides of the equation.

Since carbon monoxide formed instead of carbon dioxide, which would have formed if there was enough oxygen to ensure complete combustion, this equation represents the incomplete combustion of propane.

We must equalize the three carbons on the reagents' side and the one on the products' side in order to achieve equilibrium.

We must equalize the six hydrogens on the reagents' side and the two on the products' side in order to achieve equilibrium.

Since there are two oxygens on the reagents' side and six on the products' side, we must equalize them in order to balance the two. Since the reagents' side only contains one oxygen-containing substance, this will be easier to do.

Note that in the event that you had equalized the first time by

It would already be balanced, but having integer coefficients in equations is generally regarded as good practice. This could have been resolved by multiplying everything by three.

To balance the equation C3H6 + O2 -> CO + H2O, you need to adjust the coefficients. The balanced equation is:

C3H6 + 4O2 -> 3CO + 3H2O