Answer: 
Explanation:
According to the law of conservation of mass, mass can neither be created nor be destroyed. Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.
The balanced equation will be:

Answer:
1 gram
Explanation:
Half life = 25 years
Starting mass = 16 grams
Time = 100 years
Number of half lives = Time / Duration of Half life = 100 / 25 = 4
After first Half life;
Remaining mass = 16 / 2 = 8 g
After Second Half life;
Remaining mass = 8 / 2 = 4 g
After Third Half life;
Remaining mass = 4 / 2 = 2 g
After Fourth Half life;
Remaining mass = 2 / 2 = 1 g