how an atom is constructed indicating the location, charge, and number of protons, neutrons, and electrons

Answer is: potassium hydroxide (KOH).

Balanced chemical reaction: MgCl₂ + 2KOH → Mg(OH)₂ + 2KCl.

n(MgCl₂) = 6 mol; amount of magnesium chloride.

n(KOH) = 6 mol; amount of potassium hydroxide.

From balanced chemical reaction: n(MgCl₂) : n(KOH) = 1 : 2.

6 mol : n(KOH) = 1 : 2.

n(KOH) = 6 mol · 2.

n(KOH) = 12 mol; potassium hydroxide is limiting reagent, because we need 12 moles of it for chemical reaction and there is only 6 moles.