Answer: -
8.00 g
The law of conservation of mass
Explanation: -
When magnesium burns in air it combines with oxygen to form magnesium oxide.
The mass of the product is the mass of the magnesium oxide.
The mass of the reactant is the mass of the magnesium + mass of the oxygen.
Since matter cannot be created or destroyed according to the law of conservation of mass,
Mass of Magnesium oxide formed = mass of magnesium reacted + mass of oxygen reacted
= 3.00 g + 5.00 g
= 8.00 g