Everyone is very familiar with water. We observe it as rain and snow and can see it in the oceans, lakes, rivers, and streams. Although the water in our bodies is not as apparent, recognize that most of our weight is made up of water. In fact, the normal adult is made up of approximately 60% water. Thus, water is essential for life.
Water is made up of hydrogen ions (H+) linked to hydroxyl ions (OH-) to form H2O. The molecular formula for water is H2O. From this formula and the atomic weights for hydrogen and oxygen you can calculate that the molecular weight of water is approximately 18 grams.