140.The Mole concept(3).

In the previous post, we discussed how to relate a measurable quantity , volume , to a mole. In this post we will discuss how one can equate mole to another measurable quantity – Mass.


One mole of any element =Molar mass / gram atomic weight of that element and
One mole of a compound = Molar mass / gram molecular weight of that compound.

A mole is the amount of a substance that contains as many elementary particles (atoms/molecules/ions) as there are atoms in 12g of carbon -12 isotope.

We have studied the concept of atomic weight in post 41- 

Atomic weight is the weighted average of the masses of all naturally occurring isotopes of an element.”


e.g.- Atomic weight of carbon = 12.0107 amu , which is the weighted average of all isotopes of carbon , namely – C-12 and C-13. How to calculate weighted average? Refer post 41.


An element is composed of atoms and so we term its weight as ‘atomic weight’. For a compound, the elementary particle is a molecule and so its  weight is termed as ‘molecular weight‘. The atomic weights for every element can be found on the periodic table (as shown above) .The molecular weight can be calculated by adding atomic weights.

e.g.–  From the periodic table we know,

Atomic weight of carbon  ≈12 g.
Atomic weight of oxygen ≈ 16 g .
∴ Molecular weight of carbon monoxide (CO) = 12 + 16 = 28g.

Molar mass –

The mass of 1 mole of a substance is called its molar mass. It can be expressed in atomic mass units(amu) or as grams/mole (g/mol). It is also termed gram atomic or gram molecular weight.

1 mole of different substances.

If  weight of 1 mole of a substance gives us the atomic or molecular weight , then we can use this information to calculate either no. of moles or weight of any substance as follows –

Number of moles(n) = Weight of the substance / (At. Wt /Mol.Wt)

Thus, from earlier post and this post we can conclude that –

1 mole of a gas = 1g molecular weight of the gas
= 22.4 dm3 of the gas at NTP
= 6.023 × 10 23 particles of that gas.


We can only understand these concepts better when we solve numericals based on them. The next post will have a lot of numericals on mole concept. Till then ,

Be a perpetual student of life and keep learning…

Good day !

References and Further reading –

1.Precise Chemistry, Higher Secondary Std- XI by Sheth prakashan kendra.
3.Fundarnentals of Analytical Chemistry by Douglas A. Skoog, Donald M. West, F. James Holler.

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