Phenylephrine (PE, See the Structure Below) Is a Nasal Decongestant

Multiple Choice

Phenylephrine (PE, see the structure below) is a nasal decongestant and is the active ingredient in Sudafed, which contains phenylephrine hydrochloride (PEHCl) . This conjugate acid of phenylephrine (PEH^) has a pK_a  5.5. At a physiological pH of 7.4, what is the ratio of concentrations, [PE]/[PEH^]?

A) 6.7
B) 0.01
C) 0.14
D) 79
E) 21