0.1 mole of chromium sulphide will contain (valency of Cr = 3, valency of sulphide = 2)

$1{\text{\hspace{0.17em}mole\hspace{0.17em}of\hspace{0.17em}Cr}}_{\text{2}}{\text{S}}_{\text{3}}\text{\hspace{0.17em}contains}\to 2{\text{g\hspace{0.17em}atoms\hspace{0.17em}of\hspace{0.17em}Cr}}^{+3}$

$\Rightarrow 0.1{\text{\hspace{0.17em}mole\hspace{0.17em}of\hspace{0.17em}Cr}}_{\text{2}}{\text{S}}_{\text{3}}\text{contains}\to 0.1\times 2=0.2{\text{\hspace{0.17em}g\hspace{0.17em}atoms of Cr}}^{+3}$

$1{\text{\hspace{0.17em}mole\hspace{0.17em}of\hspace{0.17em}Cr}}_{\text{2}}{\text{S}}_{\text{3}}\text{\hspace{0.17em}contains}\to 3{\text{g\hspace{0.17em}atoms\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}\hspace{0.17em}S}}^{-2}$

$\Rightarrow 0.1{\text{\hspace{0.17em}mole\hspace{0.17em}of\hspace{0.17em}Cr}}_{\text{2}}{\text{S}}_{\text{3}}\text{contains}\to 0.1\times 3=0.3{\text{\hspace{0.17em}g\hspace{0.17em}atoms of S}}^{-2}$

.