Let's break down the problem step by step: ### **Step 1: Identify the species** - **A:** \(\mathrm{H-F}\) (hydrofluoric acid), pKa = 3.17 - **B:** \(\mathrm{NH_3}\) (ammonia) - **C:** \(\mathrm{F^-}\) (fluoride ion) - **D:** \(\mathrm{NH_4^+}\) (ammonium ion), pKa = 9.2 ### **Step 2: Recall Bronsted Base Definition** A Bronsted base is a species that **accepts a proton (H⁺)**. ### **Step 3: Strongest Base?** The **strongest Bronsted base** is the species most willing to accept a proton. This is the **conjugate base of the weakest acid** (highest pKa value). - The conjugate bases here are: - From A: \(\mathrm{F^-}\) (C) - From D: \(\mathrm{NH_3}\) (B) ### **Step 4: Compare Conjugate Bases** - The acid with the **higher pKa** is the **weaker acid**, and its conjugate base is the **stronger base**. - \(\mathrm{NH_4^+}\) (pKa = 9.2) → conjugate base: \(\mathrm{NH_3}\) (B) - \(\mathrm{HF}\) (pKa = 3.17) → conjugate base: \(\mathrm{F^-}\) (C) - Since \(\mathrm{NH_4^+}\) has a higher pKa (9.2 vs 3.17), \(\mathrm{NH_3}\) (B) is the **stronger base** than \(\mathrm{F^-}\) (C). ### **Final Answer** **The strongest Bronsted base in this reaction is:** \[ \boxed{\text{b) B}} \] That is, **NH₃ (ammonia)**.

The concept used in answering this question is based on the fundamental principles of Bronsted acid-base theory, which states that a Bronsted base is a species capable of accepting a proton (H⁺). To determine the strongest base among the given species, we analyze their conjugate acid-base relationships and compare the strengths of the conjugate acids. The strength of an acid is quantified by its pKa value; a higher pKa indicates a weaker acid. The conjugate base of a weaker acid is inherently stronger because it has a greater tendency to accept protons. In this context, comparing the pKa values of the conjugate acids—ammonium (\(\mathrm{NH_4^+}\), pKa = 9.2) and hydrofluoric acid (\(\mathrm{HF}\), pKa = 3.17)—reveals that ammonium is a weaker acid, and thus its conjugate base, ammonia (\(\mathrm{NH_3}\)), is a stronger Bronsted base than fluoride ion (\(\mathrm{F^-}\)). Therefore, the key concept involves understanding conjugate acid-base relationships and how pKa values determine the relative basic strengths of species.