The painkiller, Advil® contains the active ingredient ibuprofen (IB), which has a pKb of 5.20. If the pH of a solution containing a dissolved tablet of Advil® was found to be
8.20, what was the equilibrium concentration (mol/L) of ibuprofen?

IB + H2O ⇀ IBH+ + OH−

A) 3.7 × 10−4
B) 5.5 × 10−5
C) 2.8 × 10−7
D) 4.0 × 10−7
E) 9.2 × 10−7