An acidic drug has pKa value 4.5. Percentage of ionized drug at pH 3.5-
**Core Concept**
The ionization state of a weak acid or base in the body is determined by its pKa value and the pH of the surrounding environment. The Henderson-Hasselbalch equation is used to calculate the ionization state: pH = pKa + log10([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the undissociated acid.
**Why the Correct Answer is Right**
To calculate the percentage of ionized drug at pH 3.5, we need to use the Henderson-Hasselbalch equation. Given that the pKa value of the acidic drug is 4.5, the equation becomes: 3.5 = 4.5 + log10([A-]/[HA]). Rearranging the equation to solve for [A-]/[HA], we get: log10([A-]/[HA]) = -1. The antilog of -1 is 0.1, so [A-]/[HA] = 0.1. This ratio indicates that 10% of the drug is in the ionized state and 90% is in the undissociated state.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so it cannot be evaluated.
**Option B:** This option is also not provided, so it cannot be evaluated.
**Option C:** This option is not provided, so it cannot be evaluated.
**Option D:** This option is also not provided, so it cannot be evaluated.
**Clinical Pearl / High-Yield Fact**
The Henderson-Hasselbalch equation is a useful tool for calculating the ionization state of weak acids and bases in different pH environments. It is essential to understand this concept when designing medications that are weak acids or bases.
**Correct Answer:** C.