NAPLEX (North American Pharmacist Licensure Examination) Practice Exam

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Study for the NAPLEX exam with a comprehensive quiz featuring multiple-choice questions and in-depth explanations. Build your pharmacological knowledge and increase your chances of passing the pharmacist licensure examination!

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How is pH calculated for a weak base?

pKa + log (base/salt)
pKa - log (base/salt)
pKa + log (salt/base)
pKa - log (salt/base)

The correct answer is: pKa + log (base/salt)

For a weak base, the pH is calculated using the equation: pOH = pKb + log ([A-]/[HA]), where pOH is the negative logarithm of the hydroxide ion concentration, pKb is the negative logarithm of the base dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak base. In choice A, pKa is mentioned, which is actually the negative logarithm of the acid dissociation constant (Ka), not the base dissociation constant (Kb). However, the formula in choice A (pKa + log (base/salt)) is the closest approximation to the correct formula for calculating the pH of a weak base solution, where in this case, the base refers to the weak base and the salt refers to its conjugate acid. Choices B, C, and D all involve incorrect equations for calculating pH based on the pKa value and the relationship between the base and its conjugate acid, which is not applicable when working with a weak base solution.