Mistake Master
Introduction to acids and bases
Strip acid-base chemistry to its core and it is one proton changing hands. Even pure water plays the game, quietly splitting a few of its molecules into ions.
§1
Proton donors and acceptors.
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In the Brønsted-Lowry model, an acid donates a proton (H⁺) and a base accepts one. After the transfer, the acid becomes its conjugate base and the base becomes its conjugate acid.
A conjugate pair differs by exactly one proton (HA and A⁻; H₂O and OH⁻). The same substance can act as an acid or a base depending on its partner — water is amphoteric.
Water autoionizes: a tiny fraction of water molecules transfer a proton, giving equal H⁺ and OH⁻. At 25 °C, Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴. This sets the neutral baseline for the pH scale.
§2
Following the proton.
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Track the H⁺ to name every role.
- Find the proton donor. The species that loses an H⁺ is the acid.
- Find the proton acceptor. The species that gains an H⁺ is the base.
- Name the conjugates. Acid minus H⁺ is its conjugate base; base plus H⁺ is its conjugate acid.
- Remember autoionization. Water self-ionizes to equal H⁺ and OH⁻, with Kw = 1.0 × 10⁻¹⁴ at 25 °C.
§3
The pieces you'll meet.
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Proton transfer, plus water's own equilibrium.
§4
Worked example: ammonia in water.
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Reaction. NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Base. NH₃ accepts a proton, so it is the base; it becomes NH₄⁺, its conjugate acid.
Acid. H₂O donates a proton here, so it acts as the acid; it becomes OH⁻, its conjugate base.
Autoionization. Separately, pure water always has a small equal amount of H⁺ and OH⁻ from self-ionization, fixing Kw = 1.0 × 10⁻¹⁴ — the reference for a neutral solution.
§5
Mistakes that cost real points.
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"A base must contain OH."
A Brønsted base is any proton acceptor, not only hydroxide-containing species. Ammonia is a base because it accepts a proton, despite having no OH. Defining a base by OH misses many bases.
Fix. Identify a base as a proton acceptor; check whether it gains an H⁺, not whether it contains OH.
"Pure water contains no ions."
Water autoionizes, so even pure water contains a small, equal amount of H⁺ and OH⁻ (each 1.0 × 10⁻⁷ M at 25 °C). It is not completely un-ionized; that autoionization is what makes water neutral, not ion-free.
Fix. Remember pure water has a tiny equal concentration of H⁺ and OH⁻ from autoionization (Kw = 1.0 × 10⁻¹⁴).
"A substance is fixed as either an acid or a base."
Roles depend on the partner. Water donates a proton (acts as an acid) to ammonia but accepts one (acts as a base) from HCl. Amphoteric species switch roles, so you cannot fix a label in advance.
Fix. Determine acid or base per reaction by following the proton, not by a fixed label.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.