Calculate the number of atoms in each of the following :
(i) 52 moles of He
(ii) 52 amu of He
(iii) 52 grams of He.

Explanation

(i) Helium is monoatomic in nature.
One mole of any substance has `6.022 xx 10^(23)` molecules.
.Number of molecules in 52 moles of He
`=52 xx 6.022 xx 10^(23) = 3.131 xx 10^(25)`
Since, helium is monoatomic, i.e., its one molecule is composed of one atom, the number of atoms present in 52 moles of He = `3.131 xx 10^(25)` . Hence, 52 moles of He contain `3.131 xx 10^(25)` atoms.
(ii) The atomic mass of He is 4.003 amu. This means that the mass of one atom of He is 4.003 amu.
`therefore` Number of atoms contained in 52 amu of He
`=52/(4.003) = 12.99 = 13`
Hence, 52 amu of He contain 13 atoms.
(iii) The gram atomic mass of He = 4.003 g.This means that 4.003 g of He contain `6.022 xx 10^23` atoms.
`therefore` Number of atoms present in 52 g of He
`=(6.022 xx 10^(23))/(4.003) xx 52 = 7.823 xx 10^(24)`
Hence, 52 g of He possesx `7.832 xx 10^(24)` atoms.