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What is the composition of water?

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.

Note: The atomic weight of hydrogen (H) is 1 gram and the atomic weight of oxygen (O) is 16 grams.

18 grams of water can also be referred to as being 1 mole of water.  A mole of a substance (e.g. water), contains a particular number of molecules.  That number is 6.02 x 1023 and is often referred to as Avogadro's number; named after Amedeo Avogadro, an Italian physicist.

Recognize that 6.02 x 1023 is in scientific notation and represents a huge number: 602 billion trillion.  Written in standard form, this number is: 602,000,000,000,000,000,000,000.  Thus, a mole of water which weighs 18 grams contains a huge number of water molecules. 

18 grams or 1 mole of water occupies a volume of 18 milliliters.  Therefore, 1000 milliliters (1 liter) of water contains 55.6 moles of water (1000 milliliters / 18 milliliters per mole).

Water molecules exist in the form of H2O; hydrogen ions (H+) linked to hydroxyl ions (OH-).   A few of these water molecules split apart to create free H+ and OH- ions.  Pure, deionized water contains the same number of H+ ions and OH- ions.  One liter of pure, deionized water contains 1 x 10-7 moles of H+ and 1 x 10-7 moles of OH- ions.  This is still a very large number of free hydrogen ions, namely: 6.02 x 1016 or 60,200,000,000,000,000.  

For an explanation of scientific notation, click here.

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