Answer
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Work Step by Step
As per Dalton's atomic theory, in a reaction mass remains conserved because atoms remains conserved. In a chemical reaction the atoms are only exchanged between molecules no new atoms forms or old atoms are destroyed so the total mass must remain conserved.
In the given reaction initially there were two atoms of oxygen and 4 atoms of hydrogen and after the reaction also there are two atoms of oxygen and 4 atoms of hydrogen so the mass remains conserved.
