Question:
Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury are present in $225 \mathrm{~g}$ of pure HgS?
Solution:
Molar mass of cinnabar $(\mathrm{HgS})=(200 \cdot 6+32)=232 \cdot 6 \mathrm{~g} \mathrm{~mol}^{-1}$.
Mass of mercury in $232.6 \mathrm{~g}$ of $\mathrm{HgS}=200.6 \mathrm{~g}$
Mass of mercury in $225 \mathrm{~g}$ of $\mathrm{HgS}=\frac{(200 \cdot 6 \mathrm{~g})}{(232 \cdot 6 \mathrm{~g})} \times(225 \mathrm{~g})=194 \cdot 04 \mathrm{~g}$.