MCAT (Chemical and Physical Foundations) 3 — Questions and Answers
Question 1: Which of the following correctly ranks carbon-carbon bond lengths from shortest to longest?
- C≡C < C=C < C–C (Correct answer)
- C=C < C≡C < C–C
- C–C < C=C < C≡C
- C≡C < C–C < C=C
Correct answer: C≡C < C=C < C–C
Triple bonds are shorter than double bonds, which are shorter than single bonds due to increased electron density pulling nuclei closer.
Question 2: A projectile is launched horizontally from a cliff 80 m high at 20 m/s. How far from the base of the cliff does it land? (g = 10 m/s²)
- 40 m
- 60 m
- 80 m (Correct answer)
- 160 m
Correct answer: 80 m
Time to fall: h = ½gt² → t = √(2×80/10) = 4 s; horizontal distance = 20 × 4 = 80 m.
Question 3: Which intermolecular force is responsible for the unusually high surface tension of water?
- London dispersion forces
- Dipole-dipole interactions
- Hydrogen bonding (Correct answer)
- Ion-dipole interactions
Correct answer: Hydrogen bonding
Water's extensive hydrogen bonding network creates strong cohesive forces between molecules, resulting in high surface tension.
Question 4: A reaction has ΔH = –50 kJ/mol and ΔS = –100 J/(mol·K). At what temperature does this reaction become non-spontaneous?
- Below 500 K
- Above 500 K (Correct answer)
- Below 250 K
- Above 250 K
Correct answer: Above 500 K
ΔG = ΔH – TΔS; spontaneous when ΔG < 0. Since both ΔH and ΔS are negative, ΔG becomes positive (non-spontaneous) when T > ΔH/ΔS = 50,000/100 = 500 K.
Question 5: Which of the following best explains why the pH of a strong acid solution with concentration 1×10⁻⁸ M is NOT 8?
- Strong acids only partially dissociate at very low concentrations
- The autoionization of water contributes significantly to [H⁺] at very low acid concentrations (Correct answer)
- The activity coefficient of H⁺ changes at low concentration
- Strong acids form ion pairs at very low concentrations
Correct answer: The autoionization of water contributes significantly to [H⁺] at very low acid concentrations
At 10⁻⁸ M, the H⁺ from water autoionization (10⁻⁷ M) dominates, so the actual [H⁺] ≈ 10⁻⁷ M and pH ≈ 7, not 8.
Question 6: Two point charges of +2 μC and +8 μC are 12 cm apart. At what distance from the +2 μC charge does the electric field equal zero?
- 2 cm
- 4 cm (Correct answer)
- 6 cm
- 8 cm
Correct answer: 4 cm
Setting kQ₁/r² = kQ₂/(d–r)²: 2/r² = 8/(12–r)²; solving gives r = 4 cm from the +2 μC charge.
Question 7: Which statement about enzyme kinetics is correct when substrate concentration is well above Km?
- The reaction rate is proportional to substrate concentration
- The reaction rate is at Vmax and adding more substrate has little effect
- The enzyme follows zero-order kinetics with respect to substrate
- Both B and C are correct (Correct answer)
Correct answer: Both B and C are correct
When [S] >> Km, the enzyme is saturated and operates at Vmax; since rate is constant regardless of [S], this is zero-order kinetics—so both B and C describe the same phenomenon.
MCAT (Chemical and Physical Foundations) 3
Which of the following correctly ranks carbon-carbon bond lengths from shortest to longest?