Chemistry Question?

[匿名]

2016-05-10 4:49 am

2. Codeine (C18H21NO3), a cough suppressant extracted from crude opium, is a weak base with a Kb value of 1.6 x 10-6. You have a solution of codeine with a concentration of 0.020M. Determine the pH and the equilibrium concentrations of all of the species in solution. (10 marks)

C18H21NO3(aq) + H2O(l) C18H21NO3H+(aq) + OH-(aq)

回答 (1)

fooks

2016-05-10 10:06 am

✔ 最佳答案

C₁₈H₂₁NO₃(aq) + H₂O(l) ⇌ C₁₈H₂₁NO₃H⁺(aq) + OH⁻(aq) ... Kb = 1.6 × 10⁻⁶

Initial concentrations :
[C₁₈H₂₁NO₃]ₒ = 0.020 M
[C₁₈H₂₁NO₃H⁺]ₒ = [OH⁻]ₒ = 0 M

At equilibrium :
Assume that y M of C₁₈H₂₁NO₃ dissociates at equilibrium.
[C₁₈H₂₁NO₃] = (0.020 - y) M ≈ 0.020 M, by assuming that 0.020 ≫ y
[C₁₈H₂₁NO₃H⁺] = [OH⁻] = y M

Kb = [C₁₈H₂₁NO₃H⁺] [OH⁻] / [C₁₈H₂₁NO₃]
1.6 × 10⁻⁶ = y² / 0.02
y² = 3.2 × 10⁻⁸
y = 1.79 × 10⁻⁴ (This fulfills the assumption that 0.020 ≫ y

pH = 14.0 - pOH = 14.0 - (-log(1.79 × 10⁻⁴)) = 10.3

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