Answer:
Well, the mass of the products is equal to the mass of the reactants.
Step-by-step explanation:
The law of conservation of mass states that,
For any system closed to all transfers of matter and energy, the mass of the system must remain constant over time, as system's mass cannot change, so quantity cannot be added nor removed.
Taken from here.
Suppose we have the balanced equation of combustion of hydrogen to form water.
The equation will be:
2H2(g)+O2(g)→2H2O(g)
Now, we look at the total mass of the reactants and compare it with the mass of the products.
H2 has a mass of 1⋅2 amu, and so 2H2 will have a mass of 2⋅2=4 amu.
O2 has a mass of 16⋅2=32 amu.
So, the total mass of the reactants (2H2+O2) is equal to 4+32=36 amu.
Now, let's take a look at the mass of the products.
Water has a chemical formula of H2O.
In here, there are two hydrogen atoms and one oxygen atom, and so its mass will be 2+16=18 amu.
There are two water molecules formed, and that means the total mass of the products is 18⋅2=36 amu.
As you can see now, the mass of the products is equal to the total mass of the reactants together, and the chemical equation obeys the law of conservation of mass.
Note that this applies to every balanced chemical equation.