AP Chemistry – Unit 8 Practice: Acids & Bases
Practice key concepts for acids and bases.
Choose the best answer for each question, then check the answer below.
Question 1
Which of the following is a strong acid?
A. HF
B. HCl
C. CH₃COOH
D. NH₃
... Correct Answer: B Why: HCl is a strong acid because it fully dissociates in water. Common Mistake: Many students confuse HF as strong acid, but it is actually weak. Fix It: Memorize strong acids: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄.
Question 2
Which of the following is a strong base?
A. NH₃
B. NaOH
C. H₂O
D. CH₃COOH
... Correct Answer: B Why: NaOH is a strong base that completely dissociates to produce OH⁻ ions. Common Mistake: NH₃ is a weak base because it only partially reacts with water. Fix It: Group 1 hydroxides (NaOH, KOH) are strong bases.
Question 3
What happens to pH when [H⁺] increases?
A. increases
B. decreases
C. stays same
D. unknown
... Correct Answer: B Why: pH = -log[H⁺], so when [H⁺] increases, pH decreases. Common Mistake: Students confuse direction: more H⁺ = more acidic = LOWER pH. Fix It: Remember: High H⁺ → Low pH
Question 4
Which is the conjugate base of HCl?
A. Cl⁻
B. H⁺
C. OH⁻
D. H₂O
... Correct Answer: A Why: The conjugate base is formed when an acid loses H⁺. HCl → H⁺ + Cl⁻, so Cl⁻ is the conjugate base. Common Mistake: Choosing H⁺, but that is the proton lost, not the conjugate base. Fix It: Acid loses H⁺ → what's left = conjugate base
Question 5
Which solution is most acidic?
A. pH 2
B. pH 5
C. pH 7
D. pH 10
... Correct Answer: A Why: Lower pH means higher acidity. pH 2 is more acidic than pH 5, 7, or 10. Common Mistake: Thinking higher number = stronger (this is wrong for pH). Fix It: pH scale: lower = more acidic
Question 6
Which is a weak acid?
A. HCl
B. HNO₃
C. CH₃COOH
D. HBr
... Correct Answer: C Why: CH₃COOH (acetic acid) is a weak acid that only partially dissociates. Common Mistake: Confusing all acids as strong. Fix It: Organic acids (like CH₃COOH) are usually weak.
Question 7
If pH = 7, the solution is:
A. acidic
B. basic
C. neutral
D. strong base
... Correct Answer: C Why: pH = 7 indicates a neutral solution (pure water at 25°C). Common Mistake: Thinking pH 7 is slightly acidic or basic. Fix It: pH 7 = basic
Question 8
As pH increases, solution becomes:
A. more acidic
B. more basic
C. neutral
D. unstable
... Correct Answer: B Why: As pH increases, the solution becomes more basic (lower [H⁺]). Common Mistake: Confusing pH direction again. Fix It: Higher pH → less H⁺ → more basic
Question 9
Which ion determines acidity?
A. OH⁻
B. H⁺
C. Na⁺
D. Cl⁻
... Correct Answer: B Why: Acidity depends on the concentration of hydrogen ions (H⁺). Common Mistake: Choosing OH⁻, which determines basicity instead. Fix It: H⁺ → acidity OH⁻ → basicity
Question 10
Diluting an acid will:
A. decrease pH
B. increase pH
C. no change
D. neutralize
... Correct Answer: B Why: Diluting an acid decreases [H⁺], which increases pH. Common Mistake: Thinking dilution makes solution more acidic. Fix It: Dilution = less concentrated = less acidic
Question 11
What is the pH of 1×10⁻⁴ M HCl?
A. 2
B. 3
C. 4
D. 5
... Correct Answer: C Why: pH = -log(10⁻⁴) = 4 Common Mistake: Mixing exponent incorrectly. Fix It: Exponent = pH
Question 12
Which has the highest pH?
A. 0.1 M HCl
B. 0.01 M HCl
C. 0.001 M HCl
D. 1 M HCl
... Correct Answer: C Why: Lower concentration → fewer H⁺ → higher pH. Common Mistake: Thinking stronger acid = higher pH. Fix It: More dilute → higher pH
Question 13
Which is a weak base?
A. NaOH
B. KOH
C. NH₃
D. Ca(OH)₂
... Correct Answer: C Why: NH₃ partially reacts with water → weak base. Common Mistake: Assuming all bases are strong. Fix It: Non-metal bases → usually weak
Question 14
When a base is added to an acid:
A. pH decreases
B. pH increases
C. no change
D. becomes neutral immediately
... Correct Answer: B Why: Base reduces [H⁺], increasing pH. Common Mistake: Thinking neutral instantly. Fix It: Neutralization depends on amount
Question 15
Which species is amphoteric?
A. HCl
B. NH₃
C. H₂O
D. NaOH
... Correct Answer: C Why: Water can act as acid or base. Common Mistake: Forgetting water role. Fix It: H₂O = amphoteric
Question 16
pOH + pH =
A. 7
B. 10
C. 14
D. 1
... Correct Answer: C Why: pH + pOH = 14 at 25°C. Common Mistake: Forgetting this formula. Fix It: Always: pH + pOH = 14
Question 17
If pH = 3, what is [H⁺]?
A. 10⁻³
B. 10⁻⁷
C. 10³
D. 3
... Correct Answer: A Why: [H⁺] = 10⁻pH = 10⁻³ Common Mistake: Confusing log relationship. Fix It: pH ↔ exponent
Question 18
Which solution has highest [OH⁻]?
A. pH 2
B. pH 5
C. pH 7
D. pH 12
... Correct Answer: D Why: Higher pH → higher OH⁻ concentration. Common Mistake: Mixing H⁺ and OH⁻. Fix It: High pH = basic = high OH⁻
Question 19
Which best describes a buffer?
A. strong acid
B. strong base
C. resists pH change
D. neutral solution
... Correct Answer: C Why: Buffers resist changes in pH when acid/base added. Common Mistake: Thinking buffer = neutral. Fix It: Buffer ≠ neutral, buffer = stable pH
Question 20
Adding acid to a buffer will:
A. drastically decrease pH
B. slightly decrease pH
C. increase pH
D. no change
... Correct Answer: B Why: Buffer absorbs added acid, minimizing pH change. Common Mistake: Thinking strong change occurs. Fix It: Buffer = small change
FRQ
Explain how dilution affects the pH of an acid.
... [H⁺] decreases concentration decreases pH increases