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1 mole of a substance consists of Avogadro number (6.023 * 10²³ ) of molecules in it.    There are many properties of substances that are based on moles.
   A mole of any idea gas is supposed to occupy a fixed volume at a standard pressure and temperature.
                          P V = n  R T

         n = number of moles of a gas   T = absolute temperature
                      P = pressure ,  V = volume,   R = gas constant   

     In a chemical reaction like :
                Na OH + H Cl ==>  H2 O + Na Cl         

     One mole of sodium hydroxide and one mole of Hydrochloric acid react to form one mole of water and 1 mole of salt.