What is a mole? What is the unit of mole? How many molecules are there in a certain mass of a substance?

What is a mole? What is the unit of mole? How many molecules are there in a certain mass of a substance?

Answer:
A mole is the amount of a substance which contains the same number of chemical units (atoms, molecules or ions) as there are atoms in exactly 12 g of carbon-12. The unit of mole is given by the symbol ‘mol’.