CONCEPT OF MOLE
The relative atomic mass of an element, the relative molecular mass of a molecule, the relative formula mass of a substance or ionic mass of ionic specie when expressed in grams is called gram mole or simply mole.
• It is denoted by “n”
• It is abbreviated as “mol”
• It is the basic SI unit of measuring the quantity of substance
Example:
(i) Element Na = 23g = mole
(ii) Molecular compound H2O = 18g = mole
(iii) Ionic compound NaCl = 58.5g = mole
(iv) Ionic specie HCO-3 = 61g = mole

Molar Mass:
The atomic mass, molecular mass, formula mass or ionic mass of a substance expressed in gram is called molar mass of the substance.
Example:
23g of Sodium = molar mass of Sodium
18g of water = molar mass of water
58.5 g of NaCl = molar mass of sodium chloride
61 g of HCO3-1 = molar mass of bicarbonate
One mole for different substances is defined separately as follows:
(i) Gram Atom (mole):
The relative atomic mass of an element when expressed in grams is called gram atom it is also called 1mole.
Example: 1gram atom (1mole) of Na = 23g
1gram atom (1mole) of Cl = 35.5g
Number of gram atoms or moles of an element =
(ii) Gram Molecule (mole):
The relative molecular mass of a molecule when expressed in grams is called gram molecule. It is also called 1mole.
Example:
1gram molecule (1mole) of water = 18 g of H2O
1gram molecule (1mole) of sulphuric acid = 98 g of H2SO4
Number of gram molecules or moles of molecular substance
=
(iii) Gram Formula (mole):
The relative formula mass of a formula unit when expressed in grams, is called gram formula mass. It is also called 1mole.
Example:
1gram formula (1mole) of NaCl = 58.5 g
1gram formula (1mole) of Na2CO3 = 106 g
Number of gram formulas or moles of a substance
=
(iv) Gram Ion (mole):
The ionic mass of an ionic specie when expressed in grams, is called gram ion or one mole of ion. It is also called 1mole.
Example:
1gram ion (1mole) of OH- = 17g
1gram ion (1mole) of = 96 g
Number of gram ions or moles of an ionic specie =
NOTE
The mass of an element in comparison to the 1/12th mass of carbon is called Atomic Mass of that element.
The sum of the relative atomic masses of all the elements present in the molecule of a compound is called Molecular Mass. e.g. H2O = 18 a.m.u., CO2 = 44 a.m.u.
The sum of the relative atomic masses of all the elements present in the formula unit of the compound is called Formula Mass. e.g. NaC =58.5 a.m. u., CaCO3 = 100 a.m.u.
The sum of the relative atomic masses of all the elements present in an ion (ionic specie) is called formula mass of ionic specie or Ionic Mass.
e.g., NH4+=18 a.m.u., = 60 a.m.u., OH- = 17 a.m.u