When magnesium ions and chloride ions form a compound the chemical formula would be MgCl2 which means that for every 1 atom of Mg+ there would be 2 atoms of Cl- bonded. This would be proportional to units of moles.
Moles Cl- = 0.25 mol Mg2+ ( 2 mol Cl - / 1 mol Mg2+) = 0.50 mol Cl- would be needed
