pH of the Solution
The given solution has a pH of 9.95, which indicates that it is basic in nature. In a basic solution, the concentration of hydroxide ions (OH-) is higher than the concentration of hydrogen ions (H+). To calculate the ionization constant and pKb, we need to consider the ionization of codeine.

Ionization of Codeine
Codeine is a weak base and can ionize in water according to the following equilibrium reaction:
C18H21NO3 + H2O ⇌ C18H21NO3OH- + H3O+

The equilibrium constant for this reaction is given by the expression:
Kw = [C18H21NO3OH-][H3O+]/[C18H21NO3]

Since the concentration of water remains constant in an aqueous solution, we can define the ionization constant (Kb) for codeine as:
Kb = [C18H21NO3OH-][H3O+]/[C18H21NO3]

Calculating Ionization Constant (Kb)
To determine the ionization constant, we need to convert the pH to the concentration of hydrogen ions (H+). Since pH is defined as the negative logarithm of the hydrogen ion concentration, we have:
pH = -log[H3O+]

By rearranging the equation, we can find the concentration of H+:
[H3O+] = 10^(-pH)

Given that the pH is 9.95, we can calculate the concentration of H+:
[H3O+] = 10^(-9.95) = 1.07 x 10^(-10) M

Since codeine is a weak base, the concentration of OH- ions is equal to the concentration of H3O+ ions. Therefore, [OH-] = 1.07 x 10^(-10) M.

Now, let's define the concentration of codeine as [C18H21NO3] = 0.005 M.

Substituting these values into the expression for Kb:
Kb = [C18H21NO3OH-][H3O+]/[C18H21NO3]
= (1.07 x 10^(-10))(1.07 x 10^(-10))/(0.005)
= 1.15 x 10^(-19)

Calculating pKb
The pKb is defined as the negative logarithm of the ionization constant (Kb):
pKb = -log(Kb)

Substituting the value of Kb calculated above:
pKb = -log(1.15 x 10^(-19))
= 19.94

Therefore, the ionization constant (Kb) of the codeine solution is 1.15 x 10^(-19) and the pKb is 19.94.