1. Determine the Chemical Formula:

* Magnesium (Mg) has a +2 charge (Mg²⁺).

* Chlorine (Cl) has a -1 charge (Cl⁻).

To balance the charges, we need two chloride ions for every magnesium ion. The chemical formula for the compound is MgCl₂ (magnesium chloride).

2. Mole Ratio:

* The formula MgCl₂ tells us that 1 mole of Mg²⁺ ions combines with 2 moles of Cl⁻ ions.

3. Calculate Moles of Cl⁻:

* Since we have 0.25 moles of Mg²⁺ ions, we need twice that amount of Cl⁻ ions.

* Moles of Cl⁻ = 0.25 moles Mg²⁺ * (2 moles Cl⁻ / 1 mole Mg²⁺) = 0.50 moles Cl⁻

Therefore, 0.50 moles of Cl⁻ ions are needed to completely combine with 0.25 moles of Mg²⁺ ions.