The Mole Weighing and Counting Molecules

The laws of chemical combination assert that chemical reactions occur in such a way that the number of atoms of a given type are conserved in every chemical reaction, except nuclear reactions. How do we weigh out a sample containing exactly the number of atoms or molecules needed for a particular chemical reaction What is the mass of an atom or a molecule These questions must be answered indirectly, because atoms and molecules are far too small to be weighed individually. 

Relation between Atomic and Macroscopic Masses Avogadro s Number 

The mass of a single atom is then found by dividing exactly 12 g carbon (C) by Na  

This is truly a small mass, reflecting the large number of atoms in a 12-g sample of carbon. 

Avogadro s number is defined relative to the atom because that isotope has been chosen by international agreement to form the basis for the modern scale of relative atomic masses. We can apply it to other substances as well in a particularly simple fashion. Consider sodium, which has a relative atomic mass of 22.98977. A sodium atom is 22.98977/12 times as heavy as a atom. If the mass of Na atoms of is 12 g, then the mass of Na atoms of sodium must be 

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