[Boxxxed]

Coffee contains caffeine (C8H10N4O2, MW= 194 g/mol) which is a weak base with a Kb = 2.5 x10-4.  A 250 ml cup of coffee contains about 100 mg of caffeine.  What is the pH of a 250 ml aqueous solution containing 100 mg of caffeine?  (20 points)

C8H10N4O2(aq)  + H2O(l) ↔  C8H11N4O2+   +  OH-

100 mg = 0.1 g base = 5.15x10^-4 moles

5.15x10^-4 / 0.250 L = 2.06x10^-3 M

kb= x² / 2.06x10^-3-x

pOH---->pH

Is this correct? If so not sure why it's worth 20 points.

[rabolisk]

Looks right to me. What did you get for the answer?