An atom is the basic unit of matter.
The moon surface is
visible to an observer on earth due to the gravitational force that attracts the
moon to the earth. The more massive the body is, the more the smaller body is
attracted to the larger body. We can only see the side of the moon due to the
speed at which the moon rotates.
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.
