Mole Concept

A group of 6.022*10²³ particles ( atoms, molecules or ions) of a substance is called a mole of that substance.

• 1 mole of atoms = 6.022*10²³atoms
• 1 mole of molecules = 6.022*10²³molecules
• Ex-1 mole of oxygen = 6.022*10²³ oxygen atoms.

NOTE: 6.022*10²³ is Avogadro number (L).

1 mole of atoms of an element has a mass equal to gram atomic mass of the element.

MOLAR MASS :

• The molar mass of a substance is the mass of 1 mole of that substance.
• It is equal to the 6.022*10²³ atoms of that element/ substance.

EXAMPLE:

1. Atomic mass of hydrogen(H) is 1u. Its molar mass 1g/mol.
2. Atomic mass of Nitrogen(N) is 14u. So, its molar mass of nitrogen is 14g/mol.
3. Molar mas of S₈ = mass of S*8 = 32*8 = 256g/mol.
4. Molar mass of HCl = mass of H + mass of Cl = 1+ 35.5=36.5g/mol.

IMPORTANT FORMULAE

1. Number of moles(n)= given mass/molar mass=m/M
2. Number of moles(n)= Given number of particles/Avogadro’s number
3. m/M = N/N₀ m=M*N/N₀
4. Percentage of any atom in given compound = Mass of element/mass of compound*100

Example: Calculate the number of moles for the following:-

(1) 52g of He ( Finding mole from mass )

(2) 12.044*10²³ number of He atoms (finding mole from number of particles).

Sol:- No. of moles = n

Given mass = m

Molar mass = M

Given number of particles = N

Avogadro’s number of particles = N₀

• Atomic mass of He = 4u

molar mass of He = 4g

The number of moles = Given mass/Molar mass

n = 52/4 = 13

• 1 mole = 6.022*10²³

number of moles = Given number of particles/ Avogadro number

n = N/N₀ = 12.044*10²³/6.022*10²³ = 2