Concentration of caffeine
is 430 mg/L = 0.43g/L
The molar mass of caffeine is 194.19 g/mol
Therefore the molarity is:
Molarity = (0.43/194.19)
mol/L
Molarity = 0.002214 mol/L
Molarity = 0.002214 M
Given pKb = 10.4:
Kb = 10^-pKb = 10^ -10.4 = 3.981 x 10^ -11
Kb is equivalent to:
Kb = [caffeine H+][OH-] / [caffeine]
3.981 x 10^ -11 = [caffeine H+][OH-] / (0.002214)
[caffeineH+][OH-] = 8.815 x 10^ -14
But since:
[caffeineH+] = [OH-]
Hence,
[OH-]^2 = 8.815 x 10^ -14
[OH-] = 2.969 x 10^ -7
The formula for pH is:
pH = 14 + log [OH-]
pH = 7.47
