Question

Quinine (C20H24N2O2) a white crystalline bitter tasting powder which was originally used as a preventative medication for malaria. It is a weak base with kb=3.3 x 10^-6 M. However, Quinine is now more commonly used in small concentrations in tonic water. Quinine is responsible for the fluorescent properties when tonic water is under ultraviolet light (blacklights). Due to the toxic effects of quinine the United States limits the quinine content to 83 ppm for each 1 liter bottle.

Write a balanced chemical equation of quinine solid dissolving in water including all phases, and indicate which species is an acid, base, conjugate acid and conjugate base.

Calculate the pH of the tonic water.

Answer 1

Quinine is a weak base so will react with water to form the conjugate acid.

The equation is

The equation can be represented as

The concentration of quinine = 83 mg / L

mol wt = 324.47

so moalrity = 83 x 10^-3 / 324.47 = 2.55 X 10^-4M

                    Q +H2O           -->    Q+ +      OH-

Initial     2.55 X 10^-4                0                    0

equilb   2.55 X 10^-4-x             x                    x

so Kb = [Q+][OH-] / [Q]

3.3 X 10^-6 = xXx / 2.55 X 10^-4-x

As Kb is low we can ignore x in denominator

so 3.3 X 10^-6 = x^2 / 2.55 X 10^-4

x^2 = 8.415 X 10^-10

x = 2.9 X 10^-5 = [OH-]

so pOH = -log[OH-] = 4.53

so pH = 9.46