Question

When solid magnesium burns, it reacts with oxygen in the air to form a powder called magnesium oxide. A chemist performed this reaction in a lab and found that the mass of the magnesium oxide was greater than the mass of the magnesium. What is the reason behind this increase in mass?
A. The increase in the mass of the magnesium oxide was due to oxygen atoms in the air.
B. The increase in the mass of the magnesium oxide happened because oxygen is heavier than magnesium.
C. The increase in the mass of the magnesium oxide happened because magnesium is heavier than oxygen.
D. The increase in the mass of the magnesium oxide was due to magnesium atoms in the air.

Answer 1

Answer:

A. The increase in the mass of the magnesium oxide was due to oxygen atoms in the air.

Explanation:

Burning occurs in the presence of oxygen. A chemical combination occurs between Mg and O in that the atom of magnesium attracts one another.

          Mg + O₂ → MgO

Starting with the magnesium, on reacting with oxygen a new compound forms. This is why there is mass increase in the MgO compared to the starting material.

Answer 2

Answer: A. The increase in the mass of the magnesium oxide was due to oxygen atoms in the air.

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 chemical equation for burning of magnesium in air is:

$Mg(s)+O_2(g)\rightarrow 2MgO(s)$

When magnesium combines with oxygen, the mass of magnesium oxide formed will be same as the sum of masses of magnesium and oxygen.

Thus the increase in the mass of the magnesium oxide was due to oxygen atoms in the air.