Ncert Exercises & Solutions

1.33 Calculate the number of atoms in each of the following

(i) 52 moles of Ar (ii) 52 u of He (iii) 52 g of He.

(i) 1 mole of Ar = 6.022 × $10^{23}$ atoms of Ar
$∴$ 52 mol of Ar = 52 × 6.022 × $10^{23}$ atoms of Ar
= 3.131 × $10^{25}$ atoms of Ar

(ii) 1 atom of He = 4 u of He
Or,
4 u of He = 1 atom of He
1 u of He = 1/4 atom of He
52u of He = 52/4 atom of He
= 13 atoms of He

(iii) 4 g of He = 6.022 × 1023 atoms of He
$∴$ 52 g of He = $\frac{6.0222 x 10^{21} x 52}{4}$ atoms of He
= 7.8286 × $10^{24}$ atoms of He