Question

Calculate the number of atoms in each of the following

(i) 52 moles of Ar

(ii) 52 u of He

(iii) 53 g of He

Answer 1

(i) 1 mole of Ar $=6.022\times {10}^{23}$ atoms of Ar

52 mol of Ar$=52\times 6.022\times {10}^{23}$ atoms of Ar

$=3.131\times {10}^{25}$ atoms of Ar

(ii) Atomic mass of He = 4 amu

Therefore, number of He atoms in 52 u $=\frac{52}{4}=13$ atoms of He

(iii) Gram atomic mass of He = 4 g

Or 4 g of He contains $=6.022\times {10}^{23}$ atoms

Therefore, 52 g of He contains $=\frac{6.022\times {10}^{23}}{4\times 52}=7.83\times {10}^{24}$ atoms