Amount of substance and its units of measure.

A third definition of the Avogadro number.

 

by Cesare Curti

 

 

 

Definition of natural unit of amount of substance.

Definition of mole

Avogadro number like the equivalent, in natural units of substance amount, of the mole

 

 

 

                                      Used symbols

 

 

 

Avogadro number ……………………………………..

N

dalton ............................................................................

kilodalton .......................................................................

u

Ku

gram ..............................................................................

kilogram .........................................................................

g

Kg

carbon 12 ……………………………………………..

mole  …………………………………………………..

kilomole ……………………………………………….

natural unit (or elementary entity) of amount of substance

1000 ee ……………………………………………….                                 

12C

mol

Kmol

ee

Kee

 

 

 

 

The substances, at least all those that have a chemical composition very defined and constant, are constituted of a great number, but integer one, of atoms or atom aggregates, characteristic of the same substance.

 

Here some examples:

 

1)                A given helium amount contains an integer of He atoms and can only increase or diminish of an integer of He

2)                A given water amount contains an integer of  H2O molecules and can only vary of an integer of H2O

3)                A given amount of sodium chloride contains an integer of aggregates NaCl and can vary, without to modify its electrical charge, only of an integer of the same aggregates

4)                A given aluminium fluoride amount contains an integer of aggregates AlF3 and can vary, without to modify its electrical charge, only of an integer of same aggregates

 

Atoms, molecules or other aggregates of atoms or ions, are therefore the “natural units” or “elementary entities” of amount of substance. A given amount of substance is constituted of an integer of such units.

Submultiples of these natural units do not exist, but they can be defined their multiples.  The multiple commonly used is the mole that can be defined in the following way:

the mole is the amount of substance, which contains a number of natural units (or elementary entities) equal to the Avogadro number.  The type of natural unit or elementary entity:  atom, molecule, aggregate of atoms or ions, must be specified.

The previous definition is substantially equal to that one of International System [1] that places the mol between the base units.

 

Avogadro number N, maintaining its character of pure number, assumes therefore also the following meant:

 

the Avogadro number is the equivalent, in natural units of substance amount, of the mole

 

Its inverse 1/N is the equivalent, in mole, of the natural unit of amount of substance

 

Also with reference to the definitions of N given in Two equivalent definitions of Avogadro number. A possible definition of the kilogram the following equalities can be written:

 

N ee/mol = N Kee/Kmol = N u/g = N Ku/Kg

 

 

 

Questions and answers

 

Question:  twenty-five moles to how many natural units correspond?

Answer:   twenty-five moles correspond to  25 mol  N ee mol-1 = 25  6.022 1023 ee = 1.506 1025 ee

 

Question:  one million of natural units to how many moles correspond?

Answer:  one million of natural units correspond to  106 ee  N-1 ee-1 mol = 106  6.022-1 10-23 mol =  1.661 10-18 mol

 

Question:  which is the amount of substance in moles of 1 atom of 12C?

Answer: the amount of substance in moles of a 12C atom is  1 ee  N-1 ee-1 mol  =  6.022-1 10-23 mol =  1.661 10-24 mol

 

Bologna, lunedμ 26 marzo 2001

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