When the following equation is balanced, what is the coefficient for Mg? #Mg(s) + HCl(aq) -> MgCl_2(aq) + H_2(g)#
1 is the coefficient. See below.
So, to balance a chemical equation we make the number of each element the same on both sides.
To make Cl and H have 2 on both sides, we place a 2 in front of the HCl
Mg=1, H=2, Cl=2
Mg=1, H=1, Cl=2
Thus the equation is balanced and the coefficient of Mg is 1, which is understood so it is not written in the final equation.