Answer is: mass of mercury sulfide is 34.9 grams.
Chemical reaction: Hg + S → HgS.
m(Hg) = 30.0 g.
n(Hg) = m(Hg) ÷ M(Hg).
n(Hg) = 30 g ÷ 200.59 g/mol.
n(Hg) = 0.15 mol; amount of substance.
From chemical reaction: n(Hg) : n(HgS) = 1 : 1.
n(HgS) = 0.15 mol.
M(HgS) = 200.59 g/mol + 32.07 g/mol = 232.66 g/mol.
m(HgS) = 0.15 mol · 232.66 g/mol.
m(HgS) = 34.9 g.
