Concept:The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12, Its symbol is ‚¬Å“mol‚¬.No. of particles (atoms, molecules, ions) present in one mole = 6.022140857 À” 1023It is also known as Avogadro Constant?Explanation:? As 1 amu = 1.66054 À” 10-24 g? 4.002 g of He contains 1 mole or 6.022140857 À” 1023 of He.? 20 amu = 20 À” 1.66054 À” 10-24 = 3.3210 À” 10-23 g? Hence in 4.002 g, there is 1 mol of He therefore in 3.3210 À” 10-23 g there will be ? mol of He in 20 amu = \(\frac{3.3210À” 10^{-23}}{4.002}\) = 8.298 À” 10-24 mol