Question

How does the law of conservation of mass apply to this reaction: Mg + HCl → H2 + MgCl2?

Answer 1

Only the hydrogen needs to be balanced. There are equal numbers of magnesium and chlorine.

Answer 2

Answer: Option (d) is the correct answer.

Explanation:

According to law of conservation of mass, mass can neither be created nor it can be destroyed but it can be converted into one form to another.

For example, $Mg + 2HCl \rightarrow H_{2} + MgCl_{2}$

As for a chemical reaction to follow law of conservation of mass it must have same number of participating atoms on both reactant and product side.

So, in the given reaction mass of reactants and products is as as follows.

Mass of reactants: 24 g/mol + $(2 \times 36 g/mol)$ = 96 g/mol

Mass of products: 2 g/mol + $(24 g/mol + [2 \times 35 g/mol])$ = 96 g/mol

Thus, we can conclude that the equation needs to be balanced because hydrogen and chlorine need to be balanced. There is an equal amount of magnesium on each side.