Calculate the pH of a solution made by dissolving 10.6 g caffeine

[a.a]

1.

Homework Statement

Calculate the pH of a solution made by dissolving 10.6 g caffeine (C8H10N4O2, a weak base, Kb = 4.1 x 10-4) in enough water to make 100. mL of solution.

Homework Equations

pH = -log [H3O+]
kb = kw/ka

The Attempt at a Solution

I first found conc.:
MM= 194.2
m = 10.6
n = 0.05458 mol
conc. = 0.5458

kb = (10^-14)/ (4.1 x 10-4)
= 2.439*10^11

kb= x^2 / 0.5458
... x = 3.64 *10^-6
pOH = -log pOH
pH = 14 - pOH
= 8.56

But this is wrong... where did I go wrong?

 

[Borek]

kb = (10^-14)/ (4.1 x 10-4)

You were already given Kb.