An equivalent is defined as the product between the number of moles of a substance and its valence.
When it comes to ions, you can think of an equivalent as being a measure of that ion's capacity to combine with opposite-charge ions.
In your case, magnesium chloride,
#"MgCl"_text(2(aq]) -> "Mg"_text((aq])^(2+) + color(red)(2)"Cl"_text((aq])^(-)#
Notice that every mole of magnesium chloride produces
This tells you that in a magnesium chloride solution, every magnesium cation can combine with
The concentration of magnesium cations will be equal to that of the solution, since it forms in a
This means that every liter of this solution will contain
#10 color(red)(cancel(color(black)("mM"))) * "2 mEq Mg"^(2+)/(1color(red)(cancel(color(black)("mM")))) = color(green)("20 mEq Mg"^(2+)#
This is what the product of the number of moles and the valence of the ion is all about.
Now, in any solution, the number of equivalents of cations must be equal to the number of equivalents of anions.
In this case, the solution will contain
Chlorine's valence is equal to
#20 color(red)(cancel(color(black)("mM"))) * "1 mEq Cl"^(-)/(1color(red)(cancel(color(black)("mM")))) = color(green)("20 mEq Cl"^(-)#
Here you get one equivalent per mole of chloride anions, but you have twice as many present in solution.