Make transformations!
НCl —> MgCl2

2HCl + Mg —> MgCl2 + H2

Explanation:

For HCl to produce MgCl2, it means that metallic Mg must have reacted with the HCl.

The equation for the reaction of HCl with Mg to produce MgCl2 is given below:

HCl + Mg —> MgCl2 + H2

Now we must balance the equation. This is illustrated below:

There are 2 atoms of Cl on the right side and 1 atom on the left side. It can be balance by putting 2 in front of HCl as shown below:

2HCl + Mg —> MgCl2 + H2

Now the equation is balanced

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