Chemistry - Mole concepts and molar mass

Molecular Mass : 

Molecular mass of a substance is the sum of average atomic masses of the atoms of an element which constitute the molecule. Molecular mass of a substance is the mass of one molecule of that substance relative to the mass of one carbon-12 atom.
It is obtained by multiplying average atomic
mass of each element by the number of its
atoms and adding them together.

Formula Mass :

Some substances such as sodium chloride
do not contain discrete molecules as the
constituent units. In such compounds, cationic
(sodium) and anionic (chloride) entities are
arranged in a three dimensional structure.

In sodium chloride crystal, one Na⊕ ion
is surrounded by six Cl ions, all at the same
distance from it and vice versa. Therefore,
NaCl is just the formula used to represent
sodium chloride, though it is not a molecule.
Similarly, a term 'formula mass' is used for
such ionic compounds, instead of molecular
mass. The formula mass of a substance is 
the sum of atomic masses of the atoms 
present in the formula.

Mole concept and molar mass :

Some substances such as sodium chloride
do not contain discrete molecules as the
constituent units. In such compounds, cationic
(sodium) and anionic (chloride) entities are
arranged in a three dimensional structure.

Mole : 

Expressing large count of objects is made easy by using quantitative adjectives such as dozen, gross. You know that even a small amount of any substance contains very large number of atoms or molecules. We use a quantitative adjective 'mole' to express the large number of submicroscopic entities like atoms, ions, electrons, etc. present in a substance.

Definiton : One mole is the amount of a 
substance that contains as many entities or 
particles as there are atoms in exactly 12 g 
(or 0.012 kg) of the carbon -12 isotope.

Thus one mole is the amount of a substance
that contains 6.0221367 × 1023 particles/
entities (such as atoms, molecules or ions).
Note that the name of the unit is mole and
the symbol for the unit is mol.

The number 6.0221367 × 1023 is known as 
'Avogadro's Constant' in the honour of 
Amedo Avogadro.
In SI system, mole (Symbol mol) was 
introduced as seventh base quantity for the 

Example :
1 mole of oxygen atoms = 6.0221367 × 1023
atoms of oxygen
1 mole of water molecules = 6.0221367 × 1023
molecules of water
1 mole of sodium chloride = 6.0221367 × 1023
formula units of NaCl

Molar Mass : 

The mass of one mole of a substance (element/compound) in grams is called its molar mass. The molar mass of any element in grams is numerically equal to atomic mass of that element in u.

Simillary molar mass of any substance,
existing as polyatomic molecule, in grams is
numerically equal to its molecular mass or
formula mass in u.

Moles and gases : 

Many substances exist as gases. If we want to find the number of moles of gas, we can do this more conveniently by measuring the volume rather than mass of the gas. Chemists have deduced from Avogardro law that ''One mole of any gas occupies a volume of 22.4 dm3
 at standard temperature and pressure (1 atm) (STP). The volume of 22.4 dm3 at STP is known as molar volume of a gas.

Number of molecules = number of moles ×
6.022 × 1023 molecules mol-1
(Note : IUPAC has recently changed the
standard pressure to 1 bar. Under these new
STP conditions the molar volume of a gas is
22.71 L mol-1)