APCE Acids & Bases 1 — Questions and Answers
Question 1: What defines an acid according to the Brønsted-Lowry theory?
- An acid accepts a proton.
- An acid donates a proton. (Correct answer)
- An acid accepts an electron.
- An acid donates an electron.
Correct answer: An acid donates a proton.
According to the Brønsted-Lowry theory, an acid is defined as a proton (H⁺) donor. When an acid reacts, it gives up a hydrogen ion to another substance, which acts as a base. This definition focuses on the transfer of protons in acid-base reactions.
Question 2: What is the pH of a neutral solution?
- 0.
- 7. (Correct answer)
- 14.
- 3.
Correct answer: 7.
A neutral solution has a pH of 7 at 25°C. This specific pH value indicates that the concentration of hydrogen ions (H⁺) is equal to the concentration of hydroxide ions (OH⁻) in the solution. Pure water is a classic example of a neutral solution.
Question 3: What is the relationship between pH and hydrogen ion concentration?
- As the concentration of H⁺ increases, pH increases.
- As the concentration of H⁺ increases, pH decreases. (Correct answer)
- As the concentration of H⁺ increases, pH remains the same.
- As the concentration of H⁺ increases, pH increases slowly.
Correct answer: As the concentration of H⁺ increases, pH decreases.
pH is defined as the negative logarithm of the hydrogen ion concentration (pH = -log[H⁺]). Therefore, as the concentration of H⁺ ions increases, the pH value decreases, indicating a more acidic solution. Conversely, a lower H⁺ concentration leads to a higher pH.
Question 4: Which of the following is a strong acid?
- HCl (Hydrochloric acid). (Correct answer)
- HF (Hydrofluoric acid).
- CH₃COOH (Acetic acid).
- H₂CO₃ (Carbonic acid).
Correct answer: HCl (Hydrochloric acid).
Hydrochloric acid (HCl) is classified as a strong acid because it completely dissociates into its ions (H⁺ and Cl⁻) when dissolved in water. This complete ionization means that virtually all HCl molecules donate their protons, leading to a high concentration of H⁺ ions in the solution. In contrast, weak acids only partially dissociate.
Question 5: What is the conjugate base of HCl?
- Cl⁻. (Correct answer)
- H₂O.
- H₃O⁺.
- HCl₂.
Correct answer: Cl⁻.
When an acid donates a proton (H⁺), the remaining species is its conjugate base. For HCl, when it loses a proton, it forms the chloride ion (Cl⁻). Therefore, Cl⁻ is the conjugate base of the strong acid HCl.
Question 6: What is a buffer solution?
- A solution that completely dissociates in water.
- A solution that resists changes in pH when acid or base is added. (Correct answer)
- A solution with a very high pH.
- A solution that contains only acids.
Correct answer: A solution that resists changes in pH when acid or base is added.
A buffer solution is specifically designed to resist significant changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base, or a weak base and its conjugate acid. These components work together to neutralize added H⁺ or OH⁻ ions, thereby maintaining a relatively stable pH.
Question 7: What is the conjugate acid of NH₃?
- NH₄⁺. (Correct answer)
- NH₂⁻.
- NH₃.
- OH⁻.
Correct answer: NH₄⁺.
A conjugate acid is formed when a base accepts a proton (H⁺). Ammonia (NH₃) acts as a base by accepting a proton. When NH₃ gains an H⁺, it forms the ammonium ion (NH₄⁺), which is its conjugate acid.
Question 8: What is the pH of a 0.001 M NaOH solution?
- 4.
- 11. (Correct answer)
- 7.
- 1.
Correct answer: 11.
Sodium hydroxide (NaOH) is a strong base, meaning it completely dissociates in water to produce 0.001 M OH⁻ ions. The pOH is -log[OH⁻] = -log(0.001) = 3. Since pH + pOH = 14, the pH of the solution is 14 - 3 = 11, indicating a basic solution.
Question 9: What is the primary difference between strong and weak acids?
- Strong acids dissociate completely, while weak acids only partially dissociate. (Correct answer)
- Strong acids have a higher pH.
- Weak acids are always stronger than strong acids.
- There is no difference.
Correct answer: Strong acids dissociate completely, while weak acids only partially dissociate.
The primary distinction between strong and weak acids lies in their extent of dissociation in water. Strong acids, like HCl, completely ionize, releasing all their protons into the solution. Weak acids, such as acetic acid (CH₃COOH), only partially dissociate, establishing an equilibrium between the undissociated acid and its ions.
APCE Acids & Bases 1
What defines an acid according to the Brønsted-Lowry theory?