Yes and no he said that all atoms of a given element are identical in mass and properties.
The last one. A mixture isn’t chemically combined.
Planets revolve around the sun because of gravity. When the planets were made, the sun's gravity was strong enough so that the planets continued to stay around the sun and orbit.
Oxygen gas produced : 0.7 g
<h3>Further explanation</h3>
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 O₂
mass O₂ = 0.7 g