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Quiz: Acid-Base Theory and pH

Test your understanding of acid-base theories, pH calculations, Ka and Kb, and related concepts with these questions.

1. According to Brønsted-Lowry theory, which of the following best describes a base?

  1. A substance that produces \(\ce{OH-}\) ions when dissolved in water
  2. A species that accepts a proton (\(\ce{H+}\)) from another species
  3. A species that donates an electron pair to form a coordinate bond
  4. A substance that produces \(\ce{H+}\) ions when dissolved in water
Show Answer

The correct answer is B. Brønsted-Lowry theory defines a base as any species that accepts a proton (\(\ce{H+}\)), regardless of solvent. Option A is the narrower Arrhenius definition (requires \(\ce{OH-}\) in water). Option C describes a Lewis base (electron-pair donor). Option D describes a Brønsted-Lowry acid (proton donor), not a base.

Concept Tested: Brønsted-Lowry Theory

2. What is the pH of a 0.0050 M \(\ce{HNO3}\) solution?

  1. 2.30
  2. 11.70
  3. 3.30
  4. 1.30
Show Answer

The correct answer is A. \(\ce{HNO3}\) is a strong acid that ionizes completely, so \([\ce{H3O+}] = 0.0050\) M \(= 5.0 \times 10^{-3}\) M. \(\text{pH} = -\log(5.0 \times 10^{-3}) = 3 - \log 5.0 = 3 - 0.699 = 2.30\). Option B is the pOH value. Option C would correspond to a concentration of \(5.0 \times 10^{-4}\) M. Option D corresponds to a concentration of \(0.050\) M.

Concept Tested: Strong Acid pH Calculation

3. Which of the following pairs constitutes a conjugate acid-base pair?

  1. \(\ce{H2SO4}\) and \(\ce{SO4^{2-}}\)
  2. \(\ce{HCl}\) and \(\ce{NaOH}\)
  3. \(\ce{H2O}\) and \(\ce{H2O2}\)
  4. \(\ce{NH3}\) and \(\ce{NH4+}\)
Show Answer

The correct answer is D. \(\ce{NH3}\) and \(\ce{NH4+}\) differ by exactly one proton — \(\ce{NH4+}\) is the conjugate acid of \(\ce{NH3}\). Option A is incorrect because \(\ce{H2SO4}\) loses two protons to reach \(\ce{SO4^{2-}}\) (they differ by 2 protons, not 1). Option B pairs a strong acid with a strong base — they are not related by proton transfer in the same reaction. Option C differs by an oxygen atom, not a proton.

Concept Tested: Conjugate Acid-Base Pairs

4. A weak acid \(\ce{HA}\) has \(K_a = 4.0 \times 10^{-6}\). What is the \(K_b\) of its conjugate base \(\ce{A-}\) at 25°C?

  1. \(4.0 \times 10^{-6}\)
  2. \(4.0 \times 10^{-20}\)
  3. \(2.5 \times 10^{-9}\)
  4. \(2.5 \times 10^{-6}\)
Show Answer

The correct answer is C. For a conjugate acid-base pair, \(K_a \times K_b = K_w = 1.0 \times 10^{-14}\). Therefore \(K_b = K_w / K_a = (1.0 \times 10^{-14}) / (4.0 \times 10^{-6}) = 2.5 \times 10^{-9}\). Option A incorrectly sets \(K_b = K_a\). Option B incorrectly multiplies rather than divides. Option D results from an arithmetic error with the exponent.

Concept Tested: Ka and Kb Relationship

5. Calculate the pH of a 0.100 M solution of acetic acid (\(K_a = 1.8 \times 10^{-5}\)).

  1. 5.00
  2. 2.87
  3. 1.00
  4. 4.74
Show Answer

The correct answer is B. Using an ICE table: \(x^2 \approx K_a \times C_a = (1.8 \times 10^{-5})(0.100) = 1.8 \times 10^{-6}\); \(x = [\ce{H3O+}] = 1.34 \times 10^{-3}\) M; \(\text{pH} = -\log(1.34 \times 10^{-3}) = 2.87\). Option A is far too high for an acid solution. Option C would result from treating acetic acid as a strong acid at full dissociation. Option D is the \(pK_a\), not the solution pH.

Concept Tested: Weak Acid pH Calculation

6. A solution of \(\ce{NH4Cl}\) dissolved in water will produce a solution that is:

  1. Basic, because \(\ce{Cl-}\) hydrolyzes to produce \(\ce{OH-}\)
  2. Neutral, because \(\ce{NH4+}\) and \(\ce{Cl-}\) are both spectator ions
  3. Acidic, because \(\ce{NH4+}\) donates a proton to water
  4. Basic, because \(\ce{NH3}\) is a weak base and donates electrons
Show Answer

The correct answer is C. \(\ce{NH4Cl}\) is formed from a strong acid (\(\ce{HCl}\)) and a weak base (\(\ce{NH3}\)). The \(\ce{NH4+}\) ion is the conjugate acid of the weak base \(\ce{NH3}\) and hydrolyzes: \(\ce{NH4+ + H2O <=> NH3 + H3O+}\), making the solution acidic. Option A is wrong — \(\ce{Cl-}\) (conjugate base of strong \(\ce{HCl}\)) does not hydrolyze. Option B incorrectly labels \(\ce{NH4+}\) as a spectator ion. Option D confuses the base identity.

Concept Tested: Salt Hydrolysis — Acidic Salts

7. For binary acids going down Group 17 (halogens), which trend in acid strength is observed, and what is the primary reason?

  1. Acid strength decreases because electronegativity decreases, weakening the H—X bond polarity
  2. Acid strength decreases because larger atoms hold electrons more tightly
  3. Acid strength increases because greater electronegativity of the halogen stabilizes the conjugate base anion
  4. Acid strength increases because the H—X bond becomes longer and weaker, facilitating proton release
Show Answer

The correct answer is D. Going down Group 17, the central halogen atom grows larger, making the H—X bond longer and weaker (bond energy decreases: H—F > H—Cl > H—Br > H—I). The weaker bond makes it easier to release the proton, so acid strength increases: \(\ce{HF} < \ce{HCl} < \ce{HBr} < \ce{HI}\). Option A describes the electronegativity trend correctly but draws the wrong conclusion — bond strength dominates over electronegativity here. Option B states the wrong trend direction — larger atoms have weaker bonds, not tighter electron hold. Option C describes an electronegativity argument that actually decreases going down the group, contradicting the observed trend.

Concept Tested: Binary Acid Strength Trends

8. For the diprotic acid \(\ce{H2CO3}\) (carbonic acid), which statement accurately describes its two ionization steps?

  1. \(K_{a1} \approx K_{a2}\) because both protons are on the same molecule
  2. \(K_{a1} \ll K_{a2}\) because removing the second proton is easier once the molecule is charged
  3. \(K_{a1} \gg K_{a2}\) because removing a proton from a negatively charged ion requires overcoming electrostatic attraction
  4. The two ionizations occur simultaneously and cannot be separated experimentally
Show Answer

The correct answer is C. For \(\ce{H2CO3}\), \(K_{a1} = 4.3 \times 10^{-7}\) while \(K_{a2} = 4.7 \times 10^{-11}\) — a difference of nearly 10,000-fold. After the first ionization, the resulting \(\ce{HCO3-}\) ion carries a negative charge, which strongly resists releasing another proton (like-charge repulsion and electrostatic attraction hold the proton more tightly). Options A and B are incorrect about the relative magnitudes. Option D is false — the two steps can be studied individually.

Concept Tested: Polyprotic Acids / Diprotic Acid Equilibria

9. At 25°C, \([\ce{OH-}] = 2.5 \times 10^{-3}\) M in a solution. What are the pH and pOH of this solution?

  1. pOH = 2.60, pH = 11.40
  2. pOH = 11.40, pH = 2.60
  3. pOH = 2.60, pH = 2.60
  4. pOH = 3.40, pH = 10.60
Show Answer

The correct answer is A. \(\text{pOH} = -\log(2.5 \times 10^{-3}) = 3 - \log 2.5 = 3 - 0.40 = 2.60\). Then \(\text{pH} = 14.00 - 2.60 = 11.40\). Option B swaps pH and pOH. Option C incorrectly sets pH = pOH. Option D applies an arithmetic error (\(-\log(2.5 \times 10^{-3}) \neq 3.40\)).

Concept Tested: pOH and pH-pOH Relationship

10. A student has a 0.10 M solution of acetic acid and a 0.10 M solution of hydrochloric acid. Which of the following comparisons is correct at 25°C?

  1. Both solutions have the same pH because they have the same initial concentration
  2. Both solutions have the same \([\ce{H3O+}]\) at equilibrium because \(K_w\) is constant
  3. The acetic acid solution has a lower pH because the equilibrium favors the products
  4. The \(\ce{HCl}\) solution has a lower pH because it is fully dissociated while acetic acid is only partially ionized
Show Answer

The correct answer is D. \(\ce{HCl}\) is a strong acid and dissociates 100%, giving \([\ce{H3O+}] = 0.10\) M and pH = 1.00. Acetic acid is a weak acid (\(K_a = 1.8 \times 10^{-5}\)) with only ~1.3% ionization, giving \([\ce{H3O+}] \approx 1.34 \times 10^{-3}\) M and pH \(\approx\) 2.87. The strong acid produces a much more acidic solution. Options A and B ignore the difference between strong and weak acids. Option C is backwards — weak acid equilibrium strongly favors the reactants, producing far less \(\ce{H3O+}\) than the strong acid.

Concept Tested: Strong vs. Weak Acids — Degree of Ionization