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

The molar mass of HgS $=200.6+32$
                                           $\Rightarrow 232.6$ g mol^-1 
Mass of Hg in 232.6 g of HgS $=$ 200.6 g
Mass of Hg in 225 g of HgS $=\frac{200.6}{232.6}\times 225$
                                                  $\Rightarrow 194.04$ g 
 

Therefore, 194.04 grams of mercury is present in 225 g of pure HgS.