CBSE Class 9 Answered

The mole is a unit of measurement used  to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 grams of pure carbon-12 (¹²C), the isotope of carbon with atomic weight 12. This corresponds to a value of 6.02214179(30)×10²³ elementary entities of the substance. It is one of the base units in the International System of Units, and has the unit symbol mol. The number of molecules in a mole (known as Avogadro's number) is defined so that the mass of one mole of a substance, expressed in grams, is exactly equal to the substance's mean molecular weight.

Avogadro constant (N_A) is defined as the ratio of the number of constituent particles  N (usually atoms or molecules) in a sample to the amount of substance n (unit mole) through the relationship N_A = N/n.  For example, the mean molecular weight of natural water is about 18.015, so one mole of water is about 18.015 grams.