When mercury (II) oxide is heated, it decomposes to form liquid mercury and oxygen gas according to the following balanced chemical equation:
2HgO(s) ? 2Hg(l) + O2(g)
Suppose in a certain experiment, a quantity of mercury (II) oxide decomposes completely and produces 72.3 mL of oxygen gas, measured at 692.8 torr and 43.4oC. What mass of HgO must have been present initially? (R = 0.08206 L atm K-1 mol-1)