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 C9.  COMPARING CHEMICAL AMOUNTS OF DIFFERENT SUBSTANCES 

Equal chemical amounts of similar substances substances – as far as can easily be specified below – contain the same number of so-called elementary entities (aka particles), e.g., molecules, ions, atoms, etc.  Some examples follow.

Equal chemical amounts of the following pairs of elements

0.25 mol of He  &  0.25 mol of Ne

​0.50 mol of Mg  &  0.50 mol of Cu

contain the same number of atoms.

contain the same number of atoms.

Equal chemical amounts of discrete molecular covalent substances

0.75 mol of Cl-Cl

0.75 mol of H-F

0.75 mol of O=C=O

0.75 mol of H-O-H

0.75 mol of H-S-H

0.75 mol of O=O

0.75 mol of Br-Cl

0.75 mol of CO

0.75 mol of H-O-O-H

0.75 mol of S=C=S

ALL contain the same number of molecules.

Equal chemical amounts of ionic compounds of similar formula contain the same numbers of ions:

1.5 mol of AgF

1.5 mol of NaCℓ

1.5 mol of KBr

1.5 mol of MgO

1.5 mol of CaS

1.5 mol of AlN

Using chemical amounts of substances enables us conveniently to keep track – on a macroscopic scale – of the exceedingly large numbers of ions, molecules or atoms that are reacting in various chemical processes.

It is important to appreciate that the physical quantity amount of substance is NOT a number - otherwise it would not have an SI unit - but rather the amount of substance of a system is a measure of the number of specified elementary entities.

While amount of substance is directly proportional to the number of elementary entities, this is not something that we need to concern ourselves with unduly at this early stage.

In chemistry, combinations are based on molecule-molecule, ion-ion, atom-atom interactions, i.e., whole number relationships exist between the various elementary chemical entities involved, meaning that atoms and molecules do not react together in simple mass-based relationships like g-g, mg-mg, kg-kg, etc, but rather on an amount of substance basis.  This recognized by the importance of the mole as the SI unit for the base physical quantity amount of substance.

Not only do atoms of different elements possess different masses, but also their bonding capacity or valence depends upon their electronic structures, governing the formulae observed in compounds.  These have
a direct bearing on stoichiometric relationships.

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