In any chemical reaction, the total mass of the reactants must be equal to the total mass of the products. So, if you have
This is as straightforward as it gets. If however you need to determine how much of each product is produced, then the balanced equation is a must.
Usually, gasoline is expressed as octane in combustion reactions, which means that the balanced equation for its combustion is
But, like I've said, for the combined mass of the products all you really need to know is the combined mass of the reactants.
Let's say you want to know exactly how much of each compound is formed. In this case, you must check for a limiting reagent. The number of moles of octane is
The number of moles of oxygen is
According to the mole ratio between these two, each 2 moles of octane need 25 moles of oxygen, so
This means that the combined mass of the produced water and carbond dioxide is actually