Avogadro’s Number and the "Mole"
When you write a reaction such as 2H2 + O2 = 2H2O this indicates that two hydrogen molecules will combine with one oxygen molecule to form two water molecules. But of course atoms and diatomic gas molecules are so tiny that there is no way to carry out such a scheme. But we can measure weights and volumes of pure substances and carry out reactions on a scale that involves huge numbers of atoms and molecules. If we know the relative masses of the molecules we can determine the number of molecules we have and carry out the reaction as written by combining a certain number of oxygen molecules with twice that many hydrogen molecules. To do this we work with the gram-molecular-weight or "mole" for short. The gram molecular weight is the molecular weight in grams. The molecular weight is the sum of the atomics weight of all the atoms in the molecule. The atomic weight of water is calculated by taking twice the atomic weight of hydrogen and adding the atomic weight of oxygen, 2 x 1.00797 + 15.9994 = 18.0153. A mole of water is 18.0153g of water. A mole of any substance contains the same number of atoms or molecules. This is a very large number known as Avogadro’s number. Avogadro’s number is 6.0229 x 1023. A mole of hydrogen gas contains 6.0229 x 1023 H2 molecules. A mole of oxygen contains 6.0229 x 1023 O2 molecules. Two moles of hydrogen will combine with one mole of oxygen to form two moles of water. So we can substitute moles for molecules in the above reaction and still have the same ratios. We can describe the molecular weight as grams per mole (g/mol). Avogadro’s number is often written as NA or N0.
When dealing with ionic compounds such as potassium chloride which doesn’t form molecules we use the chemical formula in the same way we would deal with a molecule, that is we use the gram-formula-weight. The formula weight of KCl is found by adding the atomic weights of potassium and chlorine, 39.098 + 35.453 = 74.551. Thus 74.551g of KCl is one mole of KCl and contains 6.0229 x 1023 potassium ions and 6.0229 x 1023 chloride ions.