Unit 8: Acids & Bases

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.

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.

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

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

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

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.

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 = neutral

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

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

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

Correct answer: C

Why: pH = -log(10⁻⁴) = 4

Common mistake: Mixing exponent incorrectly.

Fix it: Exponent = pH

Correct answer: C

Why: Lower concentration → fewer H⁺ → higher pH.

Common mistake: Thinking stronger acid = higher pH.

Fix it: More dilute → higher pH

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

Correct answer: B

Why: Base reduces [H⁺], increasing pH.

Common mistake: Thinking neutral instantly.

Fix it: Neutralization depends on amount

Correct answer: C

Why: Water can act as acid or base.

Common mistake: Forgetting water role.

Fix it: H₂O = amphoteric

Correct answer: C

Why: pH + pOH = 14 at 25°C.

Common mistake: Forgetting this formula.

Fix it: Always: pH + pOH = 14

Correct answer: A

Why: [H⁺] = 10⁻pH = 10⁻³

Common mistake: Confusing log relationship.

Fix it: pH ↔ exponent

Correct answer: D

Why: Higher pH → higher OH⁻ concentration.

Common mistake: Mixing H⁺ and OH⁻.

Fix it: High pH = basic = high OH⁻

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

Correct answer: B

Why: Buffer absorbs added acid, minimizing pH change.

Common mistake: Thinking strong change occurs.

Fix it: Buffer = small change

[H⁺] decreases
concentration decreases
pH increases