We buy things either by number or by mass. For example, we generally buy oranges, eggs, bananas, etc., by number, viz., by dozen (1 dozen=12 pieces). We buy most of our vegetables by mass, viz., gram or kilogram. The unit were choose to express the quantity is just a matter of convince. Thus, a mole is a concept of quantity in terms of number and mass. This concept relates the mass of any material to the number of atoms, ions or molecules in it.
The mole, as per definition accepted internationally, is defined as follows.
“The mole is the amount of a substance which contains the same number of chemical units (atoms, molecules or ions) as there are atoms in exactly 12 gram of pure carbon-12”.
12g of carbon-12 is found to contain 6.023 × 1023 atoms of carbon-12. Thus, a mole represents a collection of 6.023 × 1023 chemical units (atoms, molecules or ion).
The number 6.023 × 1023 is called the Avogadro’s number. The Avogadro’s number is commonly denoted by NA.
Thus, a mole is the quantity of any material which contains one Avogadro’s number (6.023 × 1023) of chemical units (atoms, molecules or ion).
Mole concept can be applied to both molecular and ionic substances. For example,
One mole of CO2
= are mole of CO2
= 6.023 × 1023 molecules of CO2
= mass of 6.023 × 1023 molecules of CO2
=44g of CO2
One mole of NaCl
=1 mol of Na+Cl–
= 6.023 × 1023 Na+ ions + 6.023 × 1023 Cl– ions
= mass of 6.023 × 1023 Na+ Cl– ion pairs
= (23.3 + 35.5) g of NaCl = 58.5 g of NaCl
Symbol of the mole unit: Each unit has been given a standard symbol. For example, gram is denoted by g, a kilogram is denoted by kg, a metre by m , and a Newton by N(Newton is the unit of force). The unit of mole is given by a symbol mol. So, if you want to express one mole, you may write it as 1 mol.
Historically, the terms gram-atomic mass (or gram-atom), gram molecular mass (or gram-mole), and gram-ionic mass (or gram-ion), were used to represent the mass of one Avogadro’s number of atom, molecule or ion respectively. These terms are no more used. The term which is used for representing one Avogadro’s number of chemical units, now-a-days is called a mole. Thus, for any substance,
1 gram-atomic mass = 1 gram-atom = Mass of 6.023 × 1023 atoms of the substance
1 gram-molecular mass = 1 gram-mole = Mass of 6.023 × 1023 molecules of substance
1 gram-ion = Mass of 6.023 × 1023 ions