The density of Liquid mercury is $13.6 \mathrm{g}/{\mathrm{cm}}^3$. How many moles of mercury are there in 1 litre of the metal? (Atomic mass of Hg = 200)

$13.6\times \frac{1000}{200}=68$

The density of liquid mercury is $13.6 \mathrm{g}/{\mathrm{cm}}^3$.

$1 L (1000 mL)$ of metal corresponds to $13.6\mathrm{g}/{\mathrm{cm}}^3\times 1000{\mathrm{cm}}^3=13600\mathrm{g}$. 

Atomic mass of $\mathrm{Hg} = 200 \mathrm{g}/\mathrm{mol}$.

The number of moles of mercury present in $13600 g$ are $\frac{13600\mathrm{g}}{200\mathrm{g}/\mathrm{mol}}=68\mathrm{mol}$