1. One mole of a substance is equal to the amount with Avogadro number of particles (atoms or molecules) in it. Avogadro number is N = 6.022 * 10^23. One mole of a substance weighs as much as the moelcular weight in grams.
2. One mole of carbon (atomic mass = 12 amu) isotope contains N number of atoms. Its mass is 12 grams. One mole of Oxygen (O2) with a molecular mass = 32 amu, has a mass of 32 grams. One mole of H2O thus weighs 18 grams.
3. Further one mole of a gas occupies 22.4 litres volume at STP.
4. P V = n R T , the ideal gas equation involves the number n of moles.
Pressure, volume, Temperature of an ideal gas are linked through number of moles.
5. In a chemical reaction, we balance mass in terms of multiples of moles of substances.
Na OH + H Cl ==> H2 O + Na Cl
One mole each of sodium hydroxide and Hydrochloric acid react with one mole each of water and Salt.