The amount of grams in a mole varies depending on what substance you are measuring. A mole equals an atom or molecule's atomic mass in grams. For example, carbon has an atomic mass of 12, so 1 mole of carbon weighs 12 grams.
The work of Amedeo Avogadro led to the development of the mole as a way to quantify the atomic particles in a given substance. This is needed because chemical reactions depend on the number of reactant particles, not the mass of the reactants. The mole gives chemists a way to easily predict and understand reaction outcomes. One mole of any given substance contains 6.022 x 10 ^23 particles.