Download PDF – Molecular Mass and Mole Concept

The summation of the atomic masses of all the atoms in a molecule of the substance is known as the molecular mass of that substance. It is expressed in atomic mass units (AMU) or (u).

For example – the molecular mass of H ₂ O will be calculated as :
The atomic mass of hydrogen = 1 u,
Atomic mass of oxygen = 16 u
Thus,
The molecular mass of water which contains two atoms of hydrogen and one atom of Oxygen is

=  ( 2 × 1 ) + (1 × 16)
= 18 u

Formula Unit Mass

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

It can be calculated in the same way as we do to calculate the molecular mass.

The difference between calculating the molecular mass and calculating the formula unit mass is just that we use the word formula unit for those substances whose constituent particles are ions.

For example – the formula unit mass of (CaCL ₂) can be calculated as –

= Atomic mass of Ca + (2 × atomic mass of Cl)
= 40 + 2 × 35.5
= 40 + 71
= 111 u

Mole concept :

Interpreting chemical equation :

For instance –

2H ₂  + O ₂  –  2H ₂ O ( reaction of hydrogen and water to form water )

1.  We can say that two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water, or

2.  We can also say that 4u of hydrogen molecules combine with 32u of oxygen molecules to form 36u of water molecules.

 Here, we have concluded that a substance can be characterized by its mass or the number of molecules.

Therefore, we can find conclusion from the above reaction and concerning procedures that it is more convenient to refer to the quantity of a substance in terms of the number of its molecules or atoms, rather than their masses.

This interpretation results in a new unit “mole”

Mole :

It can be defined as the quantity in numbers having a mass equal to its atomic or molecular mass in grams of any species (atoms, molecules, ions, or particles).

The number of particles (atoms, molecules, and ions) present in 1 mole of any substance is fixed, having a value of  6.022 × 10 ²³ .

The number 6.022 × 10 ²³  is an experimentally obtained value which is also known as Avogadro constant or Avogadro number, represented by N_o.

This value is named in honor of the  Italian scientist, Amedeo Avogadro.

Mass of 1 mole of a substance is known as its molar mass. Everything will be just the same as that of atomic or molecular mass, we just need to change the unit from ‘u’ to ‘g’.

For example –

The atomic mass of sulphur is 32 u but the gram Atomic mass is 32 g.

The molar mass of atoms is also known as gram atomic mass.

Gram Atomic Mass – the atomic mass of a substance when expressed in grams is known as its gram atomic mass.

For example –

The atomic mass of hydrogen = 1 u

So, the gram atomic mass of hydrogen = 1 g

Gram Molecular Mass – the molecular mass of a substance when expressed in grams is known as its gram molecular mass.

For example –

1 u hydrogen = 1 atom of hydrogen

1 g hydrogen = 1 mole atom, i.e.,

6.022 × 10 ²³ atoms of hydrogen.

In the case of any chemical equation, the mole is used as the measuring unit.

Wilhelm Ostwald introduced the word ‘mole’ in 1896 from the Latin word moles meaning a ‘pile’ or ‘heap’.

The unit “mole” was accepted in 1967.

No. of moles ( = { given mass over molar mass } )

No. of moles   ( = {  given number   of  particles over  Avogadro   number } )   

Mass = molar mass × number of moles

The number of atoms  ( = { given mass over molar mass } ) × Avogadro number

The number of Molecules  ( = { given mass over molar mass } ) × Avogadro number