Rocket engines can be fueled by liquid hydrogen. The liquid hydrogen reacts with liquid oxygen, producing water vapor and releasing an enormous amount of energy. Consider the following reaction:
2 molecules of hydrogen (H2) react with 1 molecule of oxygen (O2) to produce some amount of water (H2O).

To find the amount of water produced, first determine the number of each type of atom in the products.

Hint: During a chemical reaction, atoms are not created, destroyed, or changed from one chemical element to another.