Answer is: pKb for codeine is 5,8.
Chemical reaction: C₁₈H₂₁NO₃ + H₂O ⇄ C₁₈H₂₁NO₃H⁺ + OH⁻.
c(C₁₈H₂₁NO₃) = 5·10⁻³ M = 0,005 mol/L.
pH = 9,95.
pOH = 14 - 9,95 = 4,05.
[OH⁻] = 10∧(-4,05) = 0,000089 mol/L.
[OH⁻] = [C₁₈H₂₁NO₃H⁺] = 8,9·10⁻⁵ mol/L.
[C₁₈H₂₁NO₃] = 0,005 mol/L - 0,000089 mol/L = 0,004911 mol/L.
Kb = [OH⁻] · [C₁₈H₂₁NO₃H⁺] / [C₁₈H₂₁NO₃].
Kb = (0,000089 mol/L)² / 0,004911 mol/L = 1,613·10⁻⁶.
pKb = -log(1,613·10⁻⁶) = 5,8.
