So in order to answer this we need the number of moles of oxygen and the right stoichiometry.
The balanced reaction should be 2HgO --> 2Hg + O2, therefore 2 moles of mercury are produced for every one mole of oxygen gas. Since we have the mass of O2 we can calculate the moles as moles = mass/RMM where RMM is the relative molecular mass of O2, which is 32 g/mol. Therefore the number of moles = 125/16 = 3.9 mol.
Since 2 moles of Hg are produced for every one mole of O2, the number of moles of Hg = 3.9*2 = 7.8 mol.
