Caffeine (c8h10n4o2) is a weak base with a pkb of 10.4. part a calculate the ph of a solution containing a caffeine concentration of 430 mg/l .

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-]