Molecular Mass: 

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. E.g- Molecular Mass of water(H₂O).

Atomic mass of Hydrogen = 1 u 

Atomic mass of  Oxygen = 16 u 

There are two atoms of hydrogen and one atom of oxygen in a water molecule.  

∴ Molecular mass of water = 2 x 1 + 16 = 18 u 

It is therefore the relative mass of a molecule expressed in atomic mass units (u).

Formula Unit Mass: 

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. 

Example - 

NaCl, CaCl₂, Al₂O₃, MgCl₂ etc.  

Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent particles are ions.

• The amount of substances can be calculated by its mass or by the number of atoms.

Mole: 

Mole is the amount of substance in which the number of particles of any species (atom, molecules, ions or particles) are equal to the atoms present in carbon-12 in 12 gram. This amount of species is known as mole.  

Mole is a heap of many atoms. In which numbers of atoms, molecules or ions of any element is equal to 6.022 x 10²³​. 

Avogadro Constant or Avogadro Number: The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 x 10²³​. This is an experimentally obtained value. This number is called the Avogadro Constant or Avogadro Number (represented by N0).

Example:  

See the table and try to understand the following things: 

- There is 1 mole in 1 g of hydrogen atoms and 6.022 x 10²³ atoms in that of 1 mole.   

-- There is 1 mole in 12 g of Carbon atoms and 6.022 x 10²³ atoms in that of 1 mole. 

-- There is 1 mole in 16 g of Oxygen atoms and 6.022 x 10²³​  atoms in that of 1 mole.  

Similarily you can find the mole of every element and you can also find the number of atoms in this mole. 

Now understand this: 

-- There is 1 atom in 1 u of hydrogen. 

-- There is 1 atom in 12 u of Carbon. 

-- There is 1 atom in 14 u of Nitrogen.

-- There is 1 atom in 16 u of Oxygen.  

Similarly, you can find atom of all elements.

Molar mass : The Mass of one mole of atoms of that element is known as  molar mass.  Molar mass of atoms is also known as gram atomic mass.

For example:

Atomic mass of hydrogen=1u.

So, gram atomic mass of hydrogen = 1 g. and 1 g of hydrogen has 1 mole. 1 mole of hydrogen has  6.022 x 10²³​ atoms.  

Similarly; The molar mass of carbon is 12 g.

Molar mass of 1 mole of oxygen is 16 g. 

Molar mass of 1 mole of sodium is 23 g. 

Similarily: 

In 1 u of Hydrogen has only 1 Hydrogen atom.  

In 4 u of Helium has only 1 Helium atom. 

In 12 u of Carbon has only 1 Carbon atom.