Oxygen gas produced : 0.7 g
Given
10.0 grams HgO
9.3 grams Hg
Required
Oxygen gas produced
Solution
Reaction⇒Decomposition
2HgO(s)⇒2Hg(l)+O₂(g)
Conservation of mass applies to a closed system, where the masses before and after the reaction are the same
mass of reactants = mass of products
mass  HgO = mass Hg + mass O₂
10 g = 9.3 g + mass Oâ‚‚
mass Oâ‚‚ = 0.7 g