Number of Moles Formula

Sep 21, 2022, 16:45 IST

How to calculate mole with the use of Avogadro’s constant

One mole is an amount of substance containing Avogadro's number of particles. Avogadro's number is equal to 602,214,199,000,000,000,000,000 or more simply, 6.02214199 × 10²³.

A mole (symbol mol) is defined as the amount of substance that contains as many atoms, molecules, ions, electrons or any other elementary entities as there are carbon atoms in exactly 12 gm of 12C. The number of atoms in 12 gm of 12C is called Avogadro’s number (N_A). For More Chemistry Formulas check out Main page of Physics Wallah.

N_A = 6.022 × 10²³

From mass spectrometer we found that there are 6.022 x 10²³ atoms present in 12 gm of C – 12 isotope. The number of entities in 1 mol is so important that it is given a separate name and symbol known as Avogadro constant denoted by NA. i.e. on the whole we can say that 1 mole is the collection of 6.022 x 10²³ entities. Here entities may represent atoms, ions, molecules or even pens, chairs , paper etc also include in this but as this number (NA) is very large therefore it is used only for very small things.

Mathematically mole can be calculated as = Given Weight / Molecular weight or atomic weight

The atomic mass of an element expressed in grams is called gram atomic mass of that element.
It is also defined as mass of 6.022 x 10²³ atoms.
It is also defined as the mass of one mole atoms.
It is also defined as the mass of 1 gram atom of the element.

For example for oxygen atom:

Atomic mass of ‘O’ atom = mass of one ‘O’ atom = 16 amu

Gram atomic mass = mass of 6.022 × 10²³ ‘O’ atoms

= 16 amu x 6.022 × 10²³

= 16 x 1.66 × 10 ^-24 x 6.022 x 10²³ 16 gm/mole

GRAM MOLECULAR MASS :

The molecular mass of a substance expressed in gram is called the gram-molecular mass of the substance.
It is also defined as mass of 6.022 x 10²³ molecules .
It is also defined as the mass of 1 mole molecules.
It is also defined as the mass of 1 gram molecule.

For examples for ‘O₂’ molecules :

Molecular mass of ‘O₂’ molecule = mass of one ‘O₂’ molecule

= 2 × mass of one ‘O’ atom

= 2 × 16 amu

= 32 amu

Gram molecular mass = mass of 6.022 × 10²³ ‘O₂’ molecules

= 32 amu × 6.022 × 10²³

= 32 x 1.66 × 10^-24 gm 6.022 × 10²³

= 32 gm/mole