AP - Advanced Placement Acids and Bases 1

Question 1

A 0.150 M solution of methylamine (CH₃NH₂) is prepared at 25°C. Given that Kb = 4.4 × 10⁻⁴ for methylamine, what is the pH of this solution?

Accepted Answer

11.90

Answer

Setting up the Kb expression: x² / (0.150 – x) = 4.4 × 10⁻⁴. Solving (or using the quadratic) gives x = [OH⁻] ≈ 7.91 × 10⁻³ M. pOH = –log(7.91 × 10⁻³) ≈ 2.10, so pH = 14.00 – 2.10 = 11.90.

Question 2

Which of the following species is amphiprotic?

Accepted Answer

HCO₃⁻

Answer

HCO₃⁻ is amphiprotic because it can donate a proton (acting as an acid: HCO₃⁻ → H⁺ + CO₃²⁻) or accept a proton (acting as a base: HCO₃⁻ + H⁺ → H₂CO₃). SO₄²⁻ and CO₃²⁻ are only bases; NH₄⁺ is only an acid.

Question 3

A 0.050 M solution of a weak acid HA has a measured pH of 3.17 at 25°C. What is the percent dissociation of HA?

Accepted Answer

1.35%

Answer

[H⁺] = 10⁻³·¹⁷ = 6.76 × 10⁻⁴ M. Percent dissociation = ([H⁺] / [HA]₀) × 100 = (6.76 × 10⁻⁴ / 0.050) × 100 = 1.35%.

Question 4

For carbonic acid, Ka1 = 4.3 × 10⁻⁷ and Ka2 = 4.7 × 10⁻¹¹. Which statement best explains why Ka1 is approximately 10,000 times larger than Ka2?

Accepted Answer

The second proton must be removed from HCO₃⁻, which is already negatively charged, requiring greater energy to overcome the electrostatic attraction

Answer

Removing the first proton from the neutral H₂CO₃ molecule is easier. The second proton must be pulled away from the already-negative HCO₃⁻ ion, and the increased charge–charge repulsion (and stronger electrostatic attraction holding that proton) makes the second dissociation far less favorable.

Question 5

A buffer solution has maximum resistance to pH change (maximum buffer capacity) when which condition is met?

Accepted Answer

The molar concentrations of weak acid and conjugate base are equal

Answer

According to the Henderson–Hasselbalch equation, pH = pKa + log([A⁻]/[HA]). When [A⁻] = [HA], pH = pKa and the log term is zero. At this ratio the buffer can absorb equal amounts of added acid or base before the ratio shifts dramatically, giving maximum buffer capacity.

AP Practice Test

AP Practice Test

AP - Advanced Placement Acids and Bases 1