Atomic Mass: It is deļ¬ned as the average relative mass of an atom of an element as compared to the mass of an atom of carbon 12 taken as 12.  Atomic mass is represented by u (uniļ¬ed mass).
Atomic mass unit (amu): 1/12th the mass of a C12 atom is called atomic mass unit (amu).  
i.e. 1 amu = 1/12 × mass of a C12 atom      
           = 1.66 × 10-24 g      
           = 1.66 ×10-27 kg
Today, ‘amu’ has been replaced by ‘u’ which is known as unified mass.

Gram Atomic Mass: It may be defined as the mass of 1 mole atoms of an element. For eg. Mass of one oxygen atom = 16 amu = 16/NA gms
Mass of NA Oxygen atom = 16/NA×NA = 16 gms

Average atomic mass: All most all the elements have isotopes. So, we can calculate an average atomic mass of an element by considering the atomic mass of the isotopes and their relative abundance. For e.g. Chlorine has two isotopes 35Cl and 37Cl in the ratio 3:1. So the average atomic mass Cl = (3×35 + 1×37)/4 = 35.5

Molecular mass: It is the sum of the atomic mass of the elements present in the molecule. For example: Molecular mass of CH4 = (1 × 12) + (4 × 1) = 16 u
Molecular mass of H2SO4 is calculated as: 2 × 1 + 32 + 4 × 16 = 98 u.

 Que. Find the molecular mass of  HNO3, C12H22O11 And C6H12O6