Which statement correctly uses conservation of mass?

Correct answer: In a closed system total reactant mass equals total product mass

Direct answer: Option A, in a closed system the total mass of reactants equals the total mass of products. Conservation of mass means that matter is neither created nor destroyed during a chemical reaction. Atoms only rearrange, so the total mass before and after reaction remains equal when nothing can enter or leave. Step by step: add the masses of all reactants; add the masses of all products; the two totals are equal. Option A is correct because it uses total mass and states the important closed-system condition. Option B is wrong because a mixture can have variable composition; its components need not be in a fixed ratio. Option C is wrong because gases have mass, even though they may be less dense and sometimes seem weightless. Option D is wrong because one particular product does not have to equal one particular reactant; conservation applies to the total mass, not to matching individual substances. Exam cue: write ‘total mass before = total mass after.’