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Chemistry 20

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Acids and bases

Ten questions of mixed difficulty, covering Acids and bases. Print it, or work through it on screen — the answer key starts on its own page.

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Acids and bases

Chemistry 20 · maddyhelps.com

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Circle the best answer for each question. Show your work in the space provided.

  1. Acids turn

    1. a) red litmus blue
    2. b) blue litmus red
    3. c) litmus colourless
    4. d) litmus green

  2. A solution in which [H₃O⁺] = 1.0 × 10⁻⁵ mol/L has a pH of

    1. a) 0.00001
    2. b) 9
    3. c) −5
    4. d) 5

  3. A solution with pH 9 has a pOH of

    1. a) 5
    2. b) −5
    3. c) 9
    4. d) 14

  4. A 25.0 mL sample of HCl is exactly neutralized by 20.0 mL of 0.100 mol/L NaOH. The concentration of the acid is

    1. a) 0.0500 mol/L
    2. b) 0.100 mol/L
    3. c) 0.125 mol/L
    4. d) 0.0800 mol/L

  5. Of these, the weak acid is

    1. a) H₂SO₄
    2. b) HNO₃
    3. c) CH₃COOH
    4. d) HCl

  6. Under the Brønsted–Lowry definition, an acid is a substance that

    1. a) donates a proton
    2. b) accepts a proton
    3. c) dissolves in water
    4. d) releases hydroxide ions

  7. An Arrhenius acid is a substance that

    1. a) produces hydroxide ions in water
    2. b) produces hydrogen ions in water
    3. c) turns red litmus blue
    4. d) accepts a proton

  8. A strong acid is one that

    1. a) reacts violently with metals
    2. b) has a pH below 1
    3. c) is very concentrated
    4. d) ionizes completely in water

  9. The pH of 0.010 mol/L NaOH is

    1. a) 2
    2. b) 1.0 × 10⁻¹²
    3. c) 12
    4. d) 10

  10. A solution with pH 3 is

    1. a) basic
    2. b) impossible
    3. c) acidic
    4. d) neutral

Answer key · Acids and bases

Chemistry 20 · maddyhelps.com

  1. b) blue litmus red — The rhyme worth keeping is that acids turn litmus red. Bases do the reverse. Litmus only tells you which side of neutral you are on, not how far.
  2. d) 5 — pH = −log[H₃O⁺] = −log(10⁻⁵) = 5. The minus sign in the definition is what keeps everyday pH values positive.
  3. a) 5 — At 25 °C, pH + pOH = 14, so pOH = 14 − 9 = 5. That relationship comes from the ion product of water, K𝓌 = 1.0 × 10⁻¹⁴.
  4. d) 0.0800 mol/L — The base supplies n = (0.100)(0.0200) = 2.00 × 10⁻³ mol. HCl and NaOH react 1:1, so the acid sample contained the same amount, and c = 2.00 × 10⁻³/0.0250 = 0.0800 mol/L. The mole ratio has to be checked — it is not 1:1 for every acid.
  5. c) CH₃COOH — Acetic acid only partly ionizes — most of it stays as whole molecules in solution, which is why vinegar is safe to put on food. The other three are strong acids and ionize essentially completely.
  6. a) donates a proton — Brønsted–Lowry moves the focus from water to proton transfer, so it also covers reactions in other solvents and reactions with no water at all. Every Brønsted acid has a conjugate base left behind.
  7. b) produces hydrogen ions in water — Arrhenius defined an acid by what it releases in water: H⁺, which immediately attaches to a water molecule to form H₃O⁺. A base under the same definition releases OH⁻.
  8. d) ionizes completely in water — Strong is about how completely it ionizes, not how much of it is present. A dilute solution of a strong acid and a concentrated solution of a weak acid are entirely different things, and this distinction is tested every year.
  9. c) 12 — Sodium hydroxide is a strong base, so [OH⁻] = 0.010 mol/L and pOH = 2. Then pH = 14 − 2 = 12. Taking the logarithm of the concentration and stopping there gives 2, which is the pOH, not the pH.
  10. c) acidic — Below 7 is acidic, 7 is neutral and above 7 is basic at 25 °C. Lower pH means more hydronium ions, so the scale runs backwards from the way most measurements do.