How many moles of cl - ions are needed to completely combine with 0.25 moles of mg 2 ions?

So our equation is:

2Cl + Mg - - > MgCl2

As you can see we have a 2:1 ratio of Chlorine to Magnesium atoms therefore, we need to double the amount of moles in the Mg.

Making the answer 0.5 mol