Final answer:
At the halfway point of a weak acid-strong base titration, the pH equals the pKa of the weak acid. The pKa of HNO2 is around 3.34, hence none of the provided options are correct.
Step-by-step explanation:
The subject of this question is a titration of a weak acid (HNO2) with a strong base (NaOH). To find the pH after the addition of 25.0 ml of 0.125 M NaOH to 50.0 mL of 0.125 M HNO2, we should consider that this is the halfway point of the titration, where equal amounts of acid and base have reacted.
According to the titration of weak acids with strong bases, at the halfway point, the pH is equal to the pKa of the weak acid. For nitrous acid (HNO2) with Ka = 4.6 x 10⁻⁴, the pKa is -log(4.6 x 10⁻⁴), which is approximately 3.34. However, since we are looking for the pH at the halfway point, this equals the pKa of HNO2. So, the correct answer is not listed in the options provided.
Based on the given information, a new option would be necessary to accurately reflect the pH at this point in the titration. Should a student encounter this issue on an exam, it would be advisable to select the closest correct option or bring the discrepancy to the attention of the instructor.