Law of conservation of mass:

Law of conservation of mass states that    “mass can neither be created nor destroyed in a chemical reaction and the total mass of the reactant is always equal to the total mass of the product in a chemical reaction.”

Law of conservation of mass is also known as principle of mass conservation.

A chemical reaction is just separation and reunion of atoms i.e. no new atom or atoms are formed in a chemical reaction. Therefore, mass remains same during a reaction.

Law of conservation of mass Examples:

e.g. 2H_2(g)      +    O_2(g) → 2H₂O_(g)

(2×2) =4g (16X2)=32g 2(18)=36g

36g 36g

(Total mass of Reactant) (Total mass of Product)

Similarly:

N_2(g)      +    3H_2(g) to form 2NH_3(g)

Mass before reaction               (14×2) = 28g   +   3(1×2) = 6g                          0 g                    = 34 g

Mass after reaction                  0 g       +          0 g                                   2(14+1×3) = 34       = 34 g

Total moles of atoms before reaction = 2 mole of Nitrogen atoms   +   6 mole of Hydrogen atoms = 6 moles of atoms

Total moles of atoms after reaction = 2 mole of Nitrogen atoms   +   6 mole of Hydrogen atoms = 6 moles of atoms

As number of moles of atoms remains same in the reaction so mass remains same.

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