Question

Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury are present in 225 g of pure HgS? Molar mass of Hg and S are 200.6 g mol^-1 and 32 g mol^-1 respectively.

Answer 1

Given data:

Molar mass of $\mathrm{Hg}$= 200.6 g mol^-1

Molar mass of $\mathrm{S}$= 32 g mol^-1

Calculating molecular mass of $\mathrm{HgS}$:

Molar Mass of $\mathrm{HgS}$ = Molar Mass of $\mathrm{Hg}$ + Molar mass of $\mathrm{S}$

= (200.6 + 32) g mol^-1

= 232.6 g mol^-1

Apply Unitary Method:

1 molecule of $\mathrm{HgS}$ contains 1 atom of $\mathrm{Hg}$

or, 232.6 grams of $\mathrm{HgS}$ contains 200.6 grams of $\mathrm{Hg}$

1 grams of $\mathrm{HgS}$ contains = $\frac{200.6}{232.6}$ grams of $\mathrm{Hg}$

So, 225 grams of $\mathrm{HgS}$ contains = $\frac{200.6}{232.6}\times 225$ grams of $\mathrm{Hg}$ = $194.04$ grams of $\mathrm{Hg}$

$\therefore$194.4 grams of Mercury($\mathrm{Hg}$) are present in 225 grams of pure Cinnabar($\mathrm{HgS}$).