How do you balance #Mg + HCl -> MgCl_2 + H_2#?
Garbage in must equal garbage out.......
For a simple redox reaction like this we could simply write:
Is this balanced stochiometrically? It must be if it is to represent chemical reality.
We could do it from first principles, i.e. the hydronium ion is a strong enuff oxidant to oxidize magnesium metal:
And of course the protium ion, the acidum or hydrogen ion, is reduced:
And we cross multiply to give
The chloride ions are along for the ride, and do not participate in the redox transfer.