Question Detail
Question
Stomach juices have a pH of 1.0. Aspirin is a monobasic acid represented by HA (Ka = 10⁻³ mol dm⁻³) which dissociates into H⁺ and A⁻. What are the relative concentrations of H⁺, A⁻ and HA when aspirin from a tablet enters the stomach?
Correct Answer
Option A is the correct answer.
Detailed Explanation
pH = 1.0 means [H⁺] = 0.1 M. Ka = [H⁺][A⁻]/[HA] = 10⁻³. [A⁻]/[HA] = Ka/[H⁺] = 10⁻³/0.1 = 0.01. So [HA] is 100 times larger than [A⁻]. [H⁺] from stomach acid (0.1 M) is much larger than both, so [H⁺] > [HA] > [A⁻].
Hint
Calculate the ratio [A⁻]/[HA] using the Henderson-Hasselbalch equation or Ka expression.