Mercury(II) oxide HgO decomposes to form mercury Hg and oxygen open paren cap o sub 2 close paren The balanced chemical equation is shown. 2 HgO rightarrow 2 Hg plus cap o sub 2 The molar mass of cap o sub 2 is 32 point 0 0 g over mol How many moles of HgO are needed to produce 250 point 0 g of cap o sub 2 ?