Which of the following indicator(s) would be most suitable for the titration of acetic acid (pKa = 4.74) with NaOH?

Question

BlackTom AI · Accepted Answer

To determine the best indicator for titrating acetic acid with NaOH, we need an indicator that changes color around the pH at the equivalence point of a weak acid–strong base titration. For acetic acid (pKa = 4.74), the equivalence point in this titration is above 7, typically around pH 8.7, due to the hydrolysis of the acetate ion.
Option 1: methyl red with a transition range of about pH 4.0 to 6.3. This range lies well below the equivalence point pH, so the color change would occur before the end point, making it unsuitable for accurately signaling the equivalence point.
Option 2: phenolphthalein with a transition range of roughly pH 8.3 to 11. This range encompasses the typical equivalence point region (around 8.7) for a weak acid–strong base titration, providing a sharp color change right at or near the endpoint, which is desirable.
Option 3: bromothymol blue, cycling from ~pH 6.0 to 8.0. Its transition is mostly below and around neutral to slightly basic; it would begin changing color before the true equivalence point and may not offer a clear, decisive endpoint for this titration.
Option 4: all of these choices. Given the mismatch of options 1 and 3 with the required endpoint region, this cannot be correct since not all listed indicators are suitable.
Option 5: none of these choices. This would only be true if none of the indicators could signal the endpoint well, which isn’t the case here since phenolphthalein is appropriate.
Overall, the critical point is that the equivalence point for a weak acid with a strong base lies in the basic pH range, so an indicator with a basic-transition window is most appropriate. Methyl red and bromothymol blue have transition ranges that do not align well with that endpoint, making them poor choices for this titration. Phenolphthalein, by contrast, offers a clear and well-timed color change around the endpoint, which is why it is the suitable indicator here.

CHM-1020-LD01.2025SU Quiz 5-Buffers

Which of the following indicator(s) would be most suitable for the titration of acetic acid (pKa = 4.74) with NaOH?

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