Which of the following represents the law of conservation of mass (a balanced chemical equation)? Hg + O2 → HgO

Hg + O2 → HgO

2NaN3 → 2Na + 3N2

2NaN3 → 2Na + 3N2

C+O → CO2

C+O → CO2

CH4 + O2 → CO2 +H2O

The reaction represents the law of conservation of mass (a balanced chemical equation :

2NaN₃ → 2Na + 3N₂

Conservation of mass applies to a closed system, where the masses before and after the reaction are the same, so balanced chemical equations show that mass is conserved in chemical reactions.

So we just have to look at the amount of each type of element present in the reactants and products must be the same

1. Hg + O₂ → HgO

not balanced, because element O not equal(2 in left, 1 in right)

2. 2NaN₃ → 2Na + 3N₂

balanced, because  element in reactant and product are the same

Na, left = 2, right=2

N, left =2x3=6, right=3x2=6

3. C+O → CO₂

not balanced, because element O not equal(1 in left, 2 in right)

4. CH₄ + O₂ → CO₂ +H₂O

not balanced, because element O and H not equal