This question tests introduction to acids and bases focusing on identifying conjugate acid-base pairs. In the reaction HF(aq) + H₂O(l) ⇌ H₃O⁺(aq) + F⁻(aq), HF donates a proton to become F⁻, making HF/F⁻ a conjugate acid-base pair. Similarly, H₂O accepts a proton to become H₃O⁺, making H₂O/H₃O⁺ another conjugate pair. The question asks for one conjugate pair, and HF/F⁻ is correct because they differ by exactly one proton. Choice D (H₂O/H₃O⁺) is also a conjugate pair but represents the other half of the reaction. Remember that conjugate pairs always appear on opposite sides of the equation and differ by exactly one H⁺.

This question tests introduction to acids and bases using the Brønsted-Lowry definition to identify bases as proton acceptors. In the reaction HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq), HCl donates a proton to water, with water accepting that proton to become H₃O⁺. Since water accepts the proton, it acts as the Brønsted-Lowry base, while HCl is the acid. Choice D (Cl⁻) is incorrect because it's the conjugate base of HCl, not the base in this reaction - it's a product, not a reactant accepting protons. When identifying acids and bases in reactions, focus on which reactant species gains or loses protons during the reaction process.

This question tests introduction to acids and bases using the Brønsted-Lowry definition to identify bases as proton acceptors. In the reaction CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq), the acetate ion CH₃COO⁻ gains a proton from water to become CH₃COOH, while water loses that proton to become OH⁻. Since CH₃COO⁻ accepts a proton, it acts as a Brønsted-Lowry base. Choice C (conjugate acid of CH₃COOH) is incorrect because CH₃COO⁻ is actually the conjugate base of CH₃COOH, not its conjugate acid. When analyzing acid-base reactions, track the proton transfer: the species gaining H⁺ is the base, the species losing H⁺ is the acid.

This question tests introduction to acids and bases focusing on identifying conjugate base relationships. In the reaction H₂PO₄⁻(aq) + H₂O(l) ⇌ HPO₄²⁻(aq) + H₃O⁺(aq), H₂PO₄⁻ donates a proton to water, becoming HPO₄²⁻. The conjugate base of any acid is what remains after the acid loses one proton. Since H₂PO₄⁻ loses H⁺ to form HPO₄²⁻, HPO₄²⁻ is the conjugate base of H₂PO₄⁻. Choice E (PO₄³⁻) is incorrect because it would require losing two protons from H₂PO₄⁻, not just one. Remember that conjugate acid-base pairs always differ by exactly one proton, and the conjugate base has one fewer H⁺ than its conjugate acid.

This question assesses the skill of introduction to acids and bases, focusing on the Brønsted–Lowry acid as the proton donor. In the reaction HNO₂(aq) + OH⁻(aq) → NO₂⁻(aq) + H₂O(l), proton transfer occurs from HNO₂ to OH⁻. HNO₂ loses H⁺ to become NO₂⁻, confirming it as the acid. This determines choice D as correct. A tempting distractor is B, OH⁻, but it accepts the proton, acting as the base, not the acid. Always track H⁺ transfer; the acid is the species that donates the proton in the reaction.

This question assesses the skill of introduction to acids and bases, using the Brønsted–Lowry definition where the base accepts a proton. In the reaction HCl(g) + NH₃(g) → NH₄⁺Cl⁻(s), proton transfer occurs from HCl to NH₃. NH₃ accepts H⁺ to form NH₄⁺, confirming it as the base. This determines choice B as correct. A tempting distractor is C, Cl⁻, but it is the conjugate base produced, not the initial base. Always track H⁺ transfer; the base is the proton acceptor in the reaction.

This question assesses the skill of introduction to acids and bases, focusing on the Brønsted–Lowry concept of amphiprotic species that can both donate and accept protons. To identify amphiprotic behavior, look for species with a proton to donate and the ability to accept one, like H₂PO₄⁻. H₂PO₄⁻ can donate H⁺ to become HPO₄²⁻ or accept H⁺ to form H₃PO₄. This determines C as the correct amphiprotic species. A tempting distractor is B, CO₃²⁻, but it can only accept H⁺ effectively, lacking a proton to donate. Always track H⁺ transfer; amphiprotic species must be capable of both donating and accepting protons in reactions.

This question assesses the skill of introduction to acids and bases, using the Brønsted–Lowry definition of acids as proton donors. In the reaction HSO₄⁻(aq) + H₂O(l) ⇌ SO₄²⁻(aq) + H₃O⁺(aq), proton transfer is from HSO₄⁻ to H₂O. HSO₄⁻ donates H⁺ to become SO₄²⁻, confirming its role as the acid. This matches choice B, describing HSO₄⁻ as a Brønsted–Lowry acid. A tempting distractor is A, suggesting it acts as a base by accepting H⁺, but the reaction shows donation, not acceptance. Always track H⁺ transfer; the donor is the acid, and it forms its conjugate base.

This question assesses the skill of introduction to acids and bases, emphasizing conjugate acid-base pairs that differ by one proton. In the reaction NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq), proton transfer happens from H₂O to NH₃, forming NH₄⁺ and OH⁻. The pair NH₃ and NH₄⁺ differ by H⁺, with NH₄⁺ as the acid and NH₃ as its conjugate base. This identifies D as the correct pair of conjugate partners. A tempting distractor is A, NH₃ and OH⁻, but they are not conjugates since they do not differ by a single H⁺ and belong to different pairs. Always track H⁺ transfer; conjugate pairs are linked by the gain or loss of one proton.

This question assesses the skill of introduction to acids and bases, using the Brønsted–Lowry definition where a base accepts a proton. Among the substances, identify proton transfer potential: NH₃ has a lone pair on nitrogen, allowing it to accept H⁺ from water. This makes NH₃ the Brønsted–Lowry base when dissolved in water. Thus, choice C is correct based on its ability to participate in proton acceptance. A tempting distractor is D, Cl⁻, but it is a very weak base and does not strongly accept H⁺ compared to NH₃. Always track H⁺ transfer; bases are species with lone pairs that can accept protons in aqueous solutions.