CBSE Class 9 Answered

First of all the need for mole. In very simple terms as you use litre and mL as unit for measuring the volume , Kelvin or degree Celsius is taken as the unit for measuring temperature, similarly for measuring the amount of the substance (element  or  compound) we use the term mole.

The idea of mole to count entities at the microscopic level (i.e. atoms/molecules/ particles, electrons, ions, etc).

The definition of mole is as:

One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g (or 0.012 kg) of the 12 C isotope.

When you are using mole concept for solving numericals, we use it differently for an atom and a molecule.

For an atom,

1 mole of an atom will contain 6.022 x 10²³ atoms and the mass of 1 mole of an atom will be equal to its gram atomic mass.

So, we can write the simple equation as:

1 mole = 6.022 x 10²³ atoms = Gram atomic mass

In a question relationship between two quantities will be used. So, take the two quantities and leave the third one. For example - if you are asked to calculate the mass of 2 moles of oxygen atom, then take the relationship between moles and mass and apply unitary method.

For a molecule,

1 mole of an molecule will contain 6.022 x 10²³ molecules and the mass of 1 mole of molecule will be equal to its gram molecular mass.

So, we can write the simple equation as:

1 mole = 6.022 x 10²³ molecules = Gram molecular mass

In a question relationship between two quantities will be used. So, take the two quantities and leave the third one. For example - if you are asked to calculate the the number of molecules present in 20g of  of oxygen molecule, then take the relationship between number of moleculesand mass and apply unitary method.

Now there can be third type of problems also in which you are asked to use the relationship like calculate the mass of sulphur atoms in 3 moles of Sulphur molecules. For such kind of questions, use the relationship as:

1 S₈ molecules contains 8 S atoms

So, we can write the relationship as:

1 mole of S₈ molecules =  8 moles of S atoms

Convert the number of moles to respective quantities you need

1 mole of S₈ molecules =  8 x 32 g  of S atoms

3 moles of S₈ molecules =  8 x 32 x 3  g  of S atoms

For stoichiometric numericals also use the balanced chemical equation and convert the equation into relationship of moles as:

2H₂ + O₂ → 2H₂O

We can write the relationship as:

2H₂       +     O₂    →    2H₂O

2 moles    1 mole      2 moles

This was the explanation of the mole concept. If you have any specific query on solving a particular numerical, do post a question.