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

  the   balanced   equation   for   reaction of   ca metal   with   o2 to   produce   cao   is as below

2 ca+ o2→ 2 cao2

  explanation

the reaction is   balanced     since the   number of atoms   in reactant side is   equal to   number of atoms   in   product side.   for   example   they   are 2   atom of   ca in   reactant   side   and   2   atoms of ca   in   the product side.

  the   process   is a   meet the   requirement   of   redox   reaction because :     calcium   is   oxidized   from   oxidation state 0   to 2+   while   oxygen   is reduced from   oxidation   state   0   to 2-