The number of sulphur atoms present in 0.2 mole of sodium thiosulphate is $\left(\text{N}=\text{Avogadro number}\right)$   

The formula of sodium thiosulphate, ${\text{Na}}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_3$

${\text{1\hspace{0.17em}mol\hspace{0.17em}of Na}}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_3\to 2\text{\hspace{0.17em}mol\hspace{0.17em}'S' atoms}$

0.2 mol of ${\text{Na}}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_3$ contains $0.2\times 2=0.4$mol ‘S’ atoms.