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

Law of conservation of mass do not apply to this equation.

Explanation:

The law of conservation of mass states that the mass of objects/element before any reaction must be equal to the mass of objects/element after the reaction.  

Here in this case –  

Mg+HCL->H2+MgCl2

Mass of Mg before reaction = 1

Mass of H before reaction = 1

Mass of Cl before reaction = 1

Mass of Mg after reaction = 1

Mass of H after reaction = 2

Mass of Cl after reaction = 2

Thus it is clear that mass of H and Cl before and after the reaction is not equal and thus law of conservation of mass do not apply to this equation.