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!

Each practice test/flash card set has 50 randomly selected questions from a bank of over 500. You'll get a new set of questions each time!

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How is pH calculated for a weak acid?

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

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

To calculate the pH for a weak acid, you would use the Henderson-Hasselbalch equation, which is pH = pKa + log(salt/acid) for a weak acid and its conjugate base. In this equation, pKa represents the negative logarithm of the acid dissociation constant (Ka) of the weak acid. By using this equation, you can determine the pH of a solution containing a weak acid based on the concentrations of the weak acid and its conjugate base. Option A correctly represents this equation, where pKa (the negative logarithm of the acid dissociation constant) is added to the logarithm of the ratio of the concentration of the conjugate base (salt) to the concentration of the weak acid. This formula helps pharmacists and chemists understand and predict how pH changes in a buffer solution composed of a weak acid and its conjugate base, making it an essential concept to grasp for the NAPLEX examination. Therefore, option A is the correct answer because it aligns with the Henderson-Hasselbalch equation for calculating pH for a weak acid.