Question

Calculate how many grams of oxygen form when each quantity of reactant completely reacts.

2HgO----->2Hg+O2

Answer 1

since you dont have amount of any reactant/product lets take you have 100 g of HgO and go from there:

n(O2)/n(HgO) = 1/2 --> n(O2)= 1/2 * n(HgO)

n(O2) = 1/2 * m(HgO)/M(HgO)
m(O2)= 1/2 * m(HgO)/M(HgO) * M(O2)