Which one of the following represents the molecular equation for the reaction of NH3 with HI?

• A. NH3(aq) + HI(aq) → NH4I(aq)
• B. NH3(aq) + H+(aq) → NH4+(aq)
• C. NH4+(aq) + I−(aq) → NH3(aq) + HI(aq)
• D. NH3(s) + HI(s) → NH4I(s)

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the nature of the reactants. NH3 (ammonia) is a weak base and HI (hydroiodic acid) is a strong acid, both typically in aqueous solution (aq). When a base reacts with an acid, they form a salt through an acid-base neutralization reaction.
2. Step 2: Write the molecular equation by combining the reactants as they exist in solution. Since both NH3 and HI are aqueous, the molecular equation shows them as NH3(aq) and HI(aq).
3. Step 3: Recognize the product formed. NH3 accepts a proton (H+) from HI, forming the ammonium ion NH4+, and the iodide ion I− remains as the counter ion. Together, they form ammonium iodide, NH4I, which is soluble in water and thus written as (aq).
4. Step 4: Write the molecular equation by combining the reactants and the product: NH3(aq) + HI(aq) → NH4I(aq). This shows the overall reaction without breaking it into ions.
5. Step 5: Confirm that this molecular equation correctly represents the acid-base reaction in aqueous solution, distinguishing it from ionic or solid-state equations.