MCAT (Chemical and Physical Foundations) 3

Question 1

Which of the following correctly ranks carbon-carbon bond lengths from shortest to longest?

Accepted Answer

C≡C < C=C < C–C

Answer

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)

Accepted Answer

80 m

Answer

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?

Accepted Answer

Hydrogen bonding

Answer

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?

Accepted Answer

Above 500 K

Answer

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?

Accepted Answer

The autoionization of water contributes significantly to [H⁺] at very low acid concentrations

Answer

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.

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