Applying the law of conservation of mass

[Christiano]

How do you do this?

I have been presented with this question, with no idea what to do:

Priestly converted mercury(II) oxide(HgO) to mercury and oxygen. If there were the only 3 substances present, apply the law to calculate:

a)the mass of mercury oxide that would produce 2.00g of liquid mercury and 0.16g of oxygen gas.
b)the mass of mercury produced when 4.6g of mercury oxide is completely decomposed to produce 0.37g of oxygen

NO TEACHER/BOOK EXPLANATION FTW!

[appianway]

The mass of the products is the mass of the reactants, as you're not going to be losing matter in the experiment.

a) Final mass = 2.00 + 0.16 g
Therefore the initial mass must be 2.16 g, as the only three substances involved are mercury, oxygen and mercury oxide, and mercury and oxygen are only present once the reaction commences.

b) 4.6 g = x + 0.37 g (as the mass is the same before and after)
x = 4.23 g

[Christiano]

Thanks!