How does the law of conservation of mass apply to this reaction: mg + hcl → h2 + mgcl2 ?

the law of conservation of mass has already been applied. there is an equal number of each element on both sides of the equation.

only the hydrogen needs to be balanced. there are equal numbers of magnesium and chlorine.

the equation needs to be balanced. there are fewer hydrogen atoms in the equation than magnesium or chlorine.

hydrogen and chlorine need to be balanced. there is an equal amount of magnesium on each side