Answer:
Mass of mercury is 798.35 g.
Explanation:
Balanced chemical equation.
2HgO(s)  →  2Hg(l) + O₂(g)
Given data:
Mass of mercury oxide = 3.98 g
Mass of pure mercury = ?
solution:
First of all we will calculate the moles of mercury oxide.
Number of moles = mass / molar mass
Number of moles = 3.98 g/ 216.59 g/mol
Number of moles = 0.0184 mol
Now we will compare the moles of mercury and mercury oxide from balanced chemical equation.
            HgO  :   Hg
             2    :    2
            3.98  :   3.98
Mass of mercury:
Mass =  number of moles × molar mass
Mass = 3.98 mol × 200.59 g/mol
Mass = 798.35 g