# What is a mole of substance?

The amount of substancesometimes also called a chemical quantity, is defined as a fundamental unit proportional to the number of elementary entities present in a sample of pure substance.

Said proportionality between quantity of substance and elementary entities is established through a constant that can be arbitrary, but once this constant is chosen, the proportionality is the same for any chemical substance and any elementary entity (atoms, molecules, ions, electrons, etc. ).

In the International system (SI), this fundamental unit that measures the amount of substance is the mole and the constant that relates the mole to the number of elementary entities is the Avogadro's number.

## Definition of mole

One mole of substance is defined as the amount of substance it contains exactly 6.022140857(74)×1023 elemental entities. This quantity is called Avogadro's number, symbol N.A and generally rounded to 6.022×1023.

Elementary entities are understood as atoms, molecules, ions, electrons or any other elementary particle that forms the substance in question.

For example, if a substance is made up of molecules, say water, 1 mole of that substance will contain 6,022×1023 molecules, but if it is made up of atoms, for example most noble gases are monatomic, then 1 mol 6.022×1023 atoms.

## Relationship to mass

Previously, a mole was defined through mass, since it was defined as the amount of substance that contained the same number of elementary entities as there are in 12g carbon-12 (12C), the isotope of carbon that has an atomic weight of 12.

That is, by definition, 1 mol of 12Pure C has a mass of exactly 12 g.

Similarly, the mass of 1 mole of any other substance can be obtained through the atomic weight and molecular weight.

For example, water is made up of H molecules.twoO. The atomic weight of hydrogen is 1.00797 and the atomic weight of oxygen is 15.9994, so the molecular weight or mass of water is 18.01528. Thus, 18.01528 g of water contains 1 mole of substance, that is, 18 g of water contains 6,022×1023 H moleculestwoEITHER.

Despite this relationship between mole and mass, the definition of mass does not really correspond to the amount of matter in a body, but to the inertia of said body. Therefore, in the theory of relativity, mass changes with temperature, speed or gravity, because changing these parameters changes the inertia of matter, while the amount of substance remains constant.

That is, the measure of mass changes by changing certain physical parameters, but the number of moles or number of elementary units does not. This is why the mole, and not the mass, is the unit to measure the amount of substance par excellence in chemical sciences.

For practical purposes and under normal laboratory conditions, this effect of relativity on mass is negligible, so the relationship between mass, moles and atomic weight is still used, but it must be clear that to determine the amount of substance that corresponds to 1 mole exactly, experimental methods must be used and not its calculation through the atomic mass or the molecular mass.

## Equivalences and related units

• 1 mole contains 6.022140857(74) × 1023 elementary units (the parentheses is the degree of uncertainty).
• The molar mass (mr) of a substance is the mass of that substance that contains one mole and is equal to the atomic or molecular mass expressed in grams. For example, if oxygen has an atomic mass of 16, the oxygen gas in the atmosphere (Otwo) has a molecular mass of 16+16 = 32 g.
• The number of moles (n) of a mass sample m is equal to m/mr. That is, the number of moles can be obtained by dividing the mass by the molar mass.
• 1 mole of an ideal gas, whatever it may be, always occupies 22.4 L at 0 ºC and 1 atmosphere of pressure.
• The molar concentration or molarity is equal to mol/m3.
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