Answer : The mass of
decomposed will be, 104 grams
Explanation :
Law of conservation of mass : It states that mass can neither be created nor be destroyed but it can only be transformed from one form to another form.
This also means that total mass on the reactant side must be equal to the total mass on the product side.
The balanced chemical reaction will be,

According to the law of conservation of mass,
Total mass of reactant side = Total mass of product side
Total mass of
= Total mass of

As we are given :
Total mass of
= 2(72) + 64 = 208 grams
So,
Total mass of
= 208 grams
Total mass of
=

Therefore, the mass of
decomposed will be, 104 grams