Final answer:
The correct net ionic equation for the reaction of ammonium fluoride with magnesium chloride in water is 2 F− (aq) + Mg2+ (aq) → MgF2 (s), which represents the formation of solid magnesium fluoride, while the other ions remain as spectators.
Step-by-step explanation:
When mixing ammonium fluoride (NH4F) and magnesium chloride (MgCl2) to form magnesium fluoride (MgF2) and ammonium chloride (NH4Cl), the net ionic equation represents the formation of the solid precipitate if any. According to solubility rules, both ammonium (NH4+) and chloride (Cl−) ions are soluble in water, as are most compounds containing these ions. Magnesium fluoride, on the other hand, is poorly soluble in water and hence can form a precipitate under the right conditions.
Consulting the given information and options, the correct net ionic equation for this reaction occurring in water is option 3:
2 F− (aq) + Mg2+ (aq) → MgF2 (s)
This equation reflects that magnesium ions (Mg2+) and fluoride ions (F−) combine to form solid magnesium fluoride, which is the precipitate. The ammonium (NH4+) and chloride (Cl−) ions remain in the solution as spectator ions and do not appear in the net ionic equation.