Mercury(ii) oxide (hgo) decomposes to form mercury (hg) and oxygen (o2). the balanced chemical equation is shown below.

2hgo > 2hg + o2

the molar mass of hgo is 216.59 g/mol. the molar mass of o2 is 32.00 g/mol. how many moles of hgo are needed to produce 250.0 g of o2?

To answer this item, we determine first the number of moles of O2 produced.
                      n = (250 g O2) / (32 g O2/mol) = 7.8125 moles O2
From the given balanced chemical equation, every one mole of O2 needs 2 moles mercury oxide. 
                     n HgO = (7.8125 moles O2) x (2 moles HgO / 1 mole O2)
                     n HgO = 15.625 moles O2