Mole Concept

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In chemistry, the term mole represents a pile or mass of atoms, molecules, ions or electrons. Just as common man measures quantity in terms of kilograms or dozens, a chemical scientist deals with a 'mole' of atoms, molecules, ions or electrons.

Mole is defined as the amount of a substance, which contains the same number of chemical units (atoms, molecules, ions or electrons) as there are atoms in exactly 12 grams of pure carbon-12. A mole represents a collection of 6.023 x 10²³ ( Avogadro's number) chemical units. Thus a mole represents the quantity of material, which contains one Avogadro's number of chemical units of any substance. The unit of mole is denoted as 'mol'.

For example

One mole of hydrogen atoms = 6.023 x 10²³ atoms of hydrogen

One mole of hydrogen molecules = 6.023 x 10²³ molecule of hydrogen

One mole of electrons = 6.023 x 10²³ electrons

One mole of sodium ions (Na⁺) = 6.023 x 10²³ Na⁺ ions

It can be thus concluded that,

One mole of atoms = 6.023 x 10²³ atoms = Gram atomic mass of the element.

One mole of molecules = 6.023 x 10²³ molecules = Gram molecular mass.

Molar Volume

The volume occupied by one mole of any substance is called its molar volume. It is denoted by V_m. Molar volume of the substance depends on temperature and pressure. One mole of all gaseous substances at 273 K and 1 atm pressure occupies a volume equal to 22.4 litre or 22,400 mL. The unit of molar volume is litre per mol or millilitre per mol.

The various relationships of mole can be summarized as follows:

Fig: 1.6 - Mole relationships

Problems

10. Calculate the average atomic weight of silicon having 92.2% of Si-28 isotope of relative mass 27.98 amu, 4.7% of Si-29 isotope of relative mass 28.98 amu and 3.1% of Si-30 isotope of relative mass 29.97 amu.

Solution

Data given: 92.20% of silicon of mass 27.98 amu

4.7% of silicon of mass 28.98 amu

3.1 % of silicon of mass 29.97 amu

Average atomic weight

= 25.80 + 1.36 + 0.93

= 28.09 amu.

Average atomic weight of silicon is 28.09 amu.11. One million silver atoms weigh 1.79 x 10¹⁶ g. Calculate the gram atomic mass of silver.

Solution

Number of silver atoms = 1 million = 1 x 10⁶

Mass of one million Ag atoms = 1.79 x 10¹⁶ g

= 107.8 g

Gram atomic mass of silver is equal to the mass of 6.023 x 10²³ atoms of silver. So, the gram atomic mass of silver is 107.8 g

12. Chlorophyll, the green colouring matter of plants responsible for photosynthesis, contains 2.68% of magnesium by weight. Calculate the number of Mg atoms in 2.00 g of chlorophyll.

Solution

100 g of chlorophyll contains 2.68 g of Mg

1 mole of Mg = 24 g of Mg = 6.023 x 10²³ atom of Mg

24 g of Mg contains, 6.023 x 10²³ atom of Mg

=1.34 x 10²¹ Mg atoms

13. A sample of gaseous substance weighing 0.5 g occupies a volume of 1.12 litre under NTP conditions. Calculate the molar mass of the substance.

Solution

1 mole of any gaseous substance at NTP occupies 22.4 L.

1.12 L of gaseous substance = 0.5 g

The molar mass of the substance therefore is 10 g/mol.