Neco Model Questions Vol1 2017 Chemistry Question 20

Practice objective / multiple choice question 20 from the 2017 Neco Model Questions Vol1 Chemistry examination.

Neco Model Questions Vol1 2017 Chemistry Objective / Multiple Choice Hard Difficulty
Question 20 NECO_MODEL_QUESTIONS_VOL1 • 2017 • CHEMISTRY • objective

A 0.100 M solution of a monoprotic weak acid HA (unknown \(pK_a\)) is titrated with 0.100 M NaOH. At the half‑equivalence point the measured pH is 4.75. At the equivalence point the measured pH is 8.20. (a) Determine the \(pK_a\) and the acid dissociation constant \(K_a\) of HA. (b) Assuming the volume change during titration is negligible, calculate the concentration of \(\mathrm{OH^-}\) present at the equivalence point due to hydrolysis of the conjugate base \(\mathrm{A^-}\). (c) A buffer is prepared by mixing 25.0 mL of the 0.100 M HA solution with 25.0 mL of 0.100 M NaOH. Calculate the pH of this buffer.

Answer Options

A. a) pK_a = 3.75, K_a ≈ 1.78 × 10⁻⁴; b) [OH⁻] ≈ 2.0 × 10⁻⁶ M; c) pH of the buffer = 3.75.
Correct Answer B. a) pK_a = 4.75, K_a ≈ 1.78 × 10⁻⁵; b) [OH⁻] ≈ 5.3 × 10⁻⁶ M; c) pH of the buffer = 4.75.
C. a) pK_a = 4.75, K_a ≈ 1.78 × 10⁻⁴; b) [OH⁻] ≈ 5.3 × 10⁻⁷ M; c) pH of the buffer = 4.00.
D. a) pK_a = 5.25, K_a ≈ 5.6 × 10⁻⁶; b) [OH⁻] ≈ 1.2 × 10⁻⁵ M; c) pH of the buffer = 5.25.
Correct Answer
B
Correct Option:
a) pK_a = 4.75, K_a ≈ 1.78 × 10⁻⁵; b) [OH⁻] ≈ 5.3 × 10⁻⁶ M; c) pH of the buffer = 4.75.

About This Question

This is Neco Model Questions Vol1 2017 Chemistry Question 20. It is one of the objective questions from the 2017 Neco Model Questions Vol1 Chemistry examination.

Difficulty level: Hard .

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