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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+]?

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Answer:

The ratio of concentrations is 79.4

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

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