Acids, Bases and pH
Master one of chemistry's most important topics with this comprehensive acids, bases, and pH flashcard deck. Understanding acid-base chemistry is essential for biology, medicine, environmental science, and industrial chemistry — and it is one of the most heavily tested topics in AP Chemistry, A-Level, GCSE, and university general chemistry courses. This deck covers the three major definitions of acids and bases (Arrhenius, Brønsted-Lowry, and Lewis), the pH scale, strong and weak acids and bases, neutralization reactions, buffer solutions, titrations, Ka and Kb values, and common acid-base indicators. Includes worked examples of pH calculations.
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Question
What is the Arrhenius definition of acids and bases?
Answer
Acid: Produces H⁺ ions in water. Base: Produces OH⁻ ions in water. Example: HCl → H⁺ + Cl⁻. NaOH → Na⁺ + OH⁻. Simplest definition but limited to aqueous solutions.
Question
What is the Brønsted-Lowry definition of acids and bases?
Answer
Acid: A proton (H⁺) donor. Base: A proton (H⁺) acceptor. More general than Arrhenius — applies to non-aqueous solutions too. Example: HCl donates H⁺ to NH₃ (base).
Question
What is the Lewis definition of acids and bases?
Answer
Lewis acid: An electron pair acceptor. Lewis base: An electron pair donor. Broadest definition — explains reactions with no proton transfer. Example: BF₃ (Lewis acid) + NH₃ (Lewis base).
Question
What is the pH scale and what does it measure?
Answer
pH measures the concentration of H⁺ ions in solution. Scale runs from 0–14. pH < 7 = acidic. pH = 7 = neutral. pH > 7 = basic/alkaline. Each unit is a 10-fold change in H⁺ concentration.
Question
What is the formula for pH?
Answer
pH = −log[H⁺] (where [H⁺] is the molar concentration of hydrogen ions). Example: [H⁺] = 0.01 mol/L → pH = −log(0.01) = 2.
Question
What is the formula for pOH and its relationship to pH?
Answer
pOH = −log[OH⁻]. At 25°C: pH + pOH = 14. Example: If pH = 3, then pOH = 11.
Question
What are strong acids and give 6 examples?
Answer
Strong acids completely dissociate in water. The 6 common strong acids: HCl (hydrochloric), HBr (hydrobromic), HI (hydroiodic), HNO₃ (nitric), H₂SO₄ (sulfuric), HClO₄ (perchloric).
Question
What are weak acids and give examples?
Answer
Weak acids partially dissociate in water — establishing an equilibrium. Examples: CH₃COOH (acetic/ethanoic acid), H₂CO₃ (carbonic acid), HF (hydrofluoric acid), H₃PO₄ (phosphoric acid).
Question
What are strong bases and give examples?
Answer
Strong bases completely dissociate in water. Examples: NaOH (sodium hydroxide), KOH (potassium hydroxide), Ca(OH)₂ (calcium hydroxide), LiOH (lithium hydroxide).
Question
What are weak bases and give examples?
Answer
Weak bases partially ionize in water. Examples: NH₃ (ammonia), CH₃NH₂ (methylamine), Na₂CO₃ (sodium carbonate). Form equilibrium with their conjugate acid.
Question
What is a conjugate acid-base pair?
Answer
An acid and base that differ by a single proton (H⁺). When an acid donates H⁺, it becomes its conjugate base. Example: HCl/Cl⁻ and H₂O/OH⁻ are conjugate pairs.
Question
What is a neutralization reaction?
Answer
The reaction between an acid and a base to form salt + water. Example: HCl + NaOH → NaCl + H₂O. In all neutralizations: H⁺ + OH⁻ → H₂O.
Question
What is the Ka (acid dissociation constant)?
Answer
A measure of the strength of a weak acid. Larger Ka = stronger acid (more dissociated). Ka = [H⁺][A⁻] / [HA]. For strong acids, Ka is very large (essentially complete dissociation).
Question
What is a buffer solution?
Answer
A solution that resists changes in pH when small amounts of acid or base are added. Contains a weak acid and its conjugate base (or weak base and conjugate acid). Example: CH₃COOH / CH₃COO⁻.
Question
What is the Henderson-Hasselbalch equation?
Answer
pH = pKa + log([A⁻]/[HA]). Used to calculate the pH of a buffer solution. pH equals pKa when [A⁻] = [HA] (equal concentrations of acid and conjugate base).
Question
What is a titration?
Answer
A technique to determine the concentration of an unknown acid or base by slowly adding a solution of known concentration (titrant) until the reaction is complete (equivalence point).
Question
What is the equivalence point in a titration?
Answer
The point at which the moles of acid exactly equal the moles of base — the reaction is complete. Detected using an indicator or pH meter.
Question
What is the half-equivalence point?
Answer
The point in a weak acid titration where exactly half the acid has been neutralized. At this point, pH = pKa — the buffer capacity is greatest.
Question
What are common acid-base indicators?
Answer
Litmus: Red in acid, blue in base. Phenolphthalein: Colorless in acid, pink/red in base (changes at pH 8.2–10). Methyl orange: Red in acid, yellow in base (changes at pH 3.1–4.4).
Question
What is autoionization of water?
Answer
Water can act as both an acid and a base: H₂O ⇌ H⁺ + OH⁻. At 25°C, Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴. In pure water, [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol/L → pH = 7.
Question
What is an amphoteric substance?
Answer
A substance that can act as both an acid and a base. Water is the most common example. Amino acids and bicarbonate (HCO₃⁻) are also amphoteric.
Question
What is acid rain and what causes it?
Answer
Rain with pH < 5.6 caused by SO₂ and NO₂ (from burning fossil fuels) dissolving in rainwater to form H₂SO₄ (sulfuric acid) and HNO₃ (nitric acid). Damages ecosystems and stone structures.
Question
What is the difference between dilute and concentrated acids?
Answer
Dilute: Low concentration of acid in water (e.g., 0.1 M HCl). Concentrated: High concentration of acid (e.g., 12 M HCl). NOTE: Dilute vs concentrated is independent of strong vs weak.
Question
What is hydrolysis of salts?
Answer
The reaction of salt ions with water to produce an acidic or basic solution. Salts from strong acid + weak base = acidic solution. Weak acid + strong base = basic solution. Strong acid + strong base = neutral.
Question
What is pKa and what does a low pKa indicate?
Answer
pKa = −log(Ka). A lower pKa means a stronger acid (more dissociated). Example: HCl pKa ≈ −7 (very strong). Acetic acid pKa = 4.76 (weak acid).