CBSE Class 9 Answered

Mole is basically a unit. It is the SI unit of amount of substance.

It is used for the measurement of very small, microscopic particles such as atoms and molecules.

Broadly speaking, it is used to measure the number of particles –whether atoms or molecules present in a given amount of substance.

One mole is the amount of substance that contains as many particles as there are atoms present in exactly 12g of ¹²C isotope off Carbon. It has been found that there are 6.023 X 10²³ atoms present in 12 g of ¹²C isotope of Carbon.

1. One mole of any substance will always contain 6.023 X 10²³ particles, no matter what the substance is. Therefore 1 mole of Na atoms contains 6.023 X 10²³ atoms of Na, 1 mole of Hydrogen atom contains 6.023 X 10²³ atoms of Hydrogen atom, 1 mole of Hydrogen molecule will contain 6.023 X 10²³ molecules of Hydrogen molecule (H₂), 1 mole of CO₂ will contain 6.023 X 10²³ molecules of H₂O.
2. One mole of any substance is always equal to its molar mass in grams. We know that the molar mass of a substance in grams is always equal to the atomic mass (for atoms) or molecular mass (for molecules).
   Therefore 1 mole of Na atoms = molar mass of Na atoms = 23 g.
   1 mole of H₂O molecules = molar mass of H₂O molecules = 18.02 g