How much does a water molecule weigh?
The weight of a molecule is determined by the atomic masses of the atoms that it is built of. The atomic mass of an atom is determined by the addition of the number of protons and neutrons in the nucleus, because the electrons hardly weigh anything. When the atomic masses of the separate atoms are known, one simply has to add them up to find the total atomic mass of a molecule, expressed in grams per mol. A mol is an expression of the molair weight of a molecule, derived from the weight of a hydrogen molecule, which is 1 mol. Hydrogen has a relative atomic mass of 1 g/ mol and oxygen has a relative atomic mass of 16 g/ mol. Water consists of one oxygen atom and two hydrogen atoms. This means that the mass of a water molecule is 1g + 1g + 16g = 18 g/ mol. When the number of moles of water is known, one can calculate how many grams of weight this is, by using the molar weight of water. The molair weight of separate atoms can be found in the periodic table of Mendelejef.
