states that water can be made from hydrogen and oxygen molecules. In addition, it says that two moles of hydrogen and one mole of oxygen are required to produce two moles of water. As an expression of the conservation of atoms, it indicates that for every four atoms of hydrogen and two atoms of oxygen (in and molecules), only two molecules of water can be obtained, which contain the same total of four H and two O atoms.

From the balanced equation, one can obtain information about the relative amounts of reactants and products involved. The molecular weights of , and are 2.02 g, 32.00 g, and 18.02 g, respectively. Hence 2 x 2.02 g of hydrogen react with 1 x 32.00 g of oxygen to form 2 x 18.02 g of water:

2(2.02) g + 32.00 g = 2(18.02) g
36.04 g = 36.04 g

The total weight of reactants before the reaction is the same as the weight of the products after the reaction; mass has been conserved.

Example.
Propane, , is a low-pollution fuel gas that can be burned in existing automobile engines with only minor engine adjustments. How many moles of are required to burn one mole of propane, and how many grams of are used with a kilogram of propane?