(i)  1 mol of Ar = 6.022 x 10²³ atoms.

       52 moles of Ar = 52 x 6.022 x 10²³ atoms

                                = 313.144 x 10²³ atoms

                                = 3.131 x 10²⁵ atoms

(ii)  4 u of He = 1 He atom

$\therefore$    52 u of He $=\frac{52}{4}$ He atoms = 13 He atoms.

(iii)  1 mol atom of He = 4 g = 6.022 x 10²³ atoms.

        52 g of $\text{He}=\frac{52\times 6\text{.}022\times 10^{23}}{4}\text{atoms}$

                           = 78.286 x 10²³ atoms

                           = 7.8286 x 10²⁴ atoms.