Question

Phenylephrine is a nasal decongestant and is the active ingredient in Sudafed, which contains phenylephrine hydrochloride (PEHCL). The conjugate acid (PEH+) of phenylphrine has a pKa = 5.5. At a physiological pH of 7.4. what is the ratio of concentrations, [PE]/[PEH+]?

Answer 1

The ratio of concentrations [PE]/[PEH+] is practically equal to the concentration of PE.

Step-by-step explanation:

The question asks for the ratio of concentrations of phenylephrine (PE) and its conjugate acid (PEH+) at a physiological pH of 7.4. In order to calculate this ratio, we need to consider the dissociation of phenylephrine hydrochloride (PEHCL), which is the salt form of phenylephrine. The dissociation of PEHCL in water can be shownas:

PEHCL ⇔ PE + HCL

At a pH of 7.4, which is higher than the pKa of phenylephrine (5.5), we can assume that most of the PEH+ has dissociated into PE and H+. This means that the concentration of PEH+ is negligible compared to the concentration of PE. Therefore, the ratio of concentrations [PE]/[PEH+] is practically equal to the concentration of PE, which is much higher than the concentration of PEH+.

Answer 2

The ratio of concentrations is 79.4

Step-by-step explanation:

Using Henderson-Hasselbalch equation we can solve the [PE]/[PEH+] ratio:

pH = pKa + log [A-] / [HA]

Where pH is 7.4

pKa of the phenylphrine is 5.5

[A-] is the weak base = [PE]

And [HA] is the conjugate acid [PEH+]

Replacing:

7.4 = 5.5 + log [PE] / [PEH+]

1.9 = log [PE] / [PEH+]

79.4 = [PE] / [PEH+]

The ratio of concentrations is 79.4