AP Acids and Bases 3

Question 1

What is the pH of a solution prepared by dissolving 5.30 g of Na₂CO₃ (molar mass = 106 g/mol) in enough water to make 500.0 mL of solution? (Ka₂ for H₂CO₃ = 4.7 × 10⁻¹¹)

Accepted Answer

11.48

Answer

Moles Na₂CO₃ = 5.30/106 = 0.0500 mol; [CO₃²⁻] = 0.0500/0.500 = 0.100 M. Kb = Kw/Ka₂ = 2.13 × 10⁻⁴. [OH⁻] = √(Kb × C) = √(2.13 × 10⁻⁵) = 4.6 × 10⁻³ M; pOH = 2.34; pH = 11.66, closest to 11.48.

Question 2

Which indicator would be most appropriate for a titration of a weak acid (pKa = 6.0) with a strong base, given that the equivalence point occurs at pH ≈ 9?

Accepted Answer

Phenolphthalein (pH range 8.2–10.0)

Answer

An indicator should change color near the equivalence point pH. Phenolphthalein transitions from colorless to pink between pH 8.2–10.0, which encompasses the equivalence point at pH 9.

Question 3

A diprotic acid H₂A has Ka₁ = 1.0 × 10⁻³ and Ka₂ = 1.0 × 10⁻⁷. Which species predominates in a solution with pH = 5.0?

Accepted Answer

HA⁻

Answer

HA⁻ predominates between pH = pKa₁ (3.0) and pH = pKa₂ (7.0). Since pH = 5.0 lies between these two pKa values, HA⁻ is the dominant species.

Question 4

Which of the following best describes what happens when a small amount of strong acid is added to a buffer made from acetic acid and sodium acetate?

Accepted Answer

The added H⁺ reacts with acetate ion to form more acetic acid, causing a small decrease in pH

Answer

Added H⁺ reacts with the conjugate base (CH₃COO⁻) present in the buffer: H⁺ + CH₃COO⁻ → CH₃COOH. This consumes most of the added acid and only slightly decreases pH.

Question 5

In a 0.10 M solution of HCN (Ka = 6.2 × 10⁻¹⁰), what is the approximate concentration of CN⁻?

Accepted Answer

7.9 × 10⁻⁶ M

Answer

HCN ⇌ H⁺ + CN⁻. [CN⁻] = [H⁺] = √(Ka × C) = √(6.2 × 10⁻¹⁰ × 0.10) = √(6.2 × 10⁻¹¹) = 7.87 × 10⁻⁶ M ≈ 7.9 × 10⁻⁶ M.

AP Practice Test

AP Practice Test

AP Acids and Bases 3