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Chemistry 30

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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 30 · maddyhelps.com

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

  1. At 25 °C, pH + pOH =

    1. a) 7.00
    2. b) 14.00
    3. c) 10.00
    4. d) 1.00

  2. A solution has [H₃O⁺] = 1.0 × 10⁻³ mol/L. Its pH is

    1. a) 0.001
    2. b) 11.00
    3. c) −3.00
    4. d) 3.00

  3. A solution has [OH⁻] = 1.0 × 10⁻⁵ mol/L. Its pH at 25 °C is

    1. a) 9.00
    2. b) 14.00
    3. c) 5.00
    4. d) −5.00

  4. When a weak acid is titrated with a strong base, the pH at the equivalence point is

    1. a) above 7, because excess base has been added
    2. b) below 7, because some acid always remains
    3. c) exactly 7, because the acid is neutralized
    4. d) above 7, because the salt formed is basic

  5. In a solution of a weak acid at equilibrium, the predominant species is

    1. a) the hydroxide ion
    2. b) the conjugate base
    3. c) the un-ionized acid molecule
    4. d) the hydronium ion

  6. The conjugate base of H₂CO₃ is

    1. a) HCO₃⁻
    2. b) CO₃²⁻
    3. c) H₂CO₃
    4. d) H₃CO₃⁺

  7. At 25 °C, a solution with a pH of 3.0 is

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

  8. The reaction of an acid with a base produces

    1. a) hydrogen only
    2. b) an acid and a base
    3. c) a gas and water
    4. d) a salt and water

  9. 25.0 mL of hydrochloric acid is neutralized by 18.4 mL of 0.100 mol/L NaOH. The concentration of the acid is

    1. a) 0.136 mol/L
    2. b) 0.0184 mol/L
    3. c) 0.0736 mol/L
    4. d) 0.100 mol/L

  10. According to the Arrhenius definition, an acid is a substance that

    1. a) produces hydroxide ions in water
    2. b) donates an electron pair
    3. c) produces hydrogen ions in water
    4. d) accepts a proton

Answer key · Acids and bases

Chemistry 30 · maddyhelps.com

  1. b) 14.00 — Water's ion product gives [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C, which becomes pH + pOH = 14.00 in logarithmic form. It is the quickest route between the two scales.
  2. d) 3.00 — pH = −log[H₃O⁺] = −log(1.0 × 10⁻³) = 3.00. The minus sign in the definition is what makes an acidic pH a positive number.
  3. a) 9.00 — pOH = −log(1.0 × 10⁻⁵) = 5.00, so pH = 14.00 − 5.00 = 9.00. Answering 5.00 is forgetting to convert pOH to pH — and a hydroxide concentration that high should give a basic pH.
  4. d) above 7, because the salt formed is basic — At equivalence, exactly enough base has been added — none is in excess. What remains is the conjugate base of the weak acid, which reacts with water to produce hydroxide ions, so the solution is basic.
  5. c) the un-ionized acid molecule — A weak acid ionizes only slightly, so most of it stays as intact molecules — which is exactly why its Ka is small and its pH is higher than a strong acid of the same concentration.
  6. a) HCO₃⁻ — A conjugate base is what remains after the acid donates exactly one proton: H₂CO₃ − H⁺ = HCO₃⁻. Removing two protons gives the carbonate ion, which is the conjugate base of HCO₃⁻ instead.
  7. b) acidic — Below 7 is acidic at 25 °C, 7 is neutral and above 7 is basic. Each whole number is a factor of ten in hydrogen ion concentration, so pH 3 is a hundred times more acidic than pH 5.
  8. d) a salt and water — Neutralization pairs H⁺ with OH⁻ to make water, leaving the remaining ions as a salt. HCl + NaOH gives NaCl and H₂O.
  9. c) 0.0736 mol/L — Moles of NaOH = (0.0184 L)(0.100 mol/L) = 1.84 × 10⁻³ mol. HCl and NaOH react 1:1, so the acid contains the same number of moles: 1.84 × 10⁻³ ÷ 0.0250 L = 0.0736 mol/L.
  10. c) produces hydrogen ions in water — Arrhenius defined acids and bases by what they release in water: H⁺ for an acid, OH⁻ for a base. Bronsted-Lowry broadens this to proton donors and acceptors, which covers reactions outside water.