Question at position 2 (related to question 1) The standard addition does not tell you the ammonia concentration, but you can figure this out with the measurement described above and a pH reading. What is the concentration of ammonia in the fish tank water, whose concentration of ammonium was 0.44 mM and whose pH was 8.80? The pKa of ammonia is 9.30. Report your answer in mM to two decimal places.Answer(related to question 1) The standard addition does not tell you the ammonia concentration, but you can figure this out with the measurement described above and a pH reading. What is the concentration of ammonia in the fish tank water, whose concentration of ammonium was 0.44 mM and whose pH was 8.80? The pKa of ammonia is 9.30. Report your answer in mM to two decimal places.[input]简答题
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Question

Question at position 2 (related to question 1) The standard addition does not tell you the ammonia concentration, but you can figure this out with the measurement described above and a pH reading. What is the concentration of ammonia in the fish tank water, whose concentration of ammonium was 0.44 mM and whose pH was 8.80? The pKa of ammonia is 9.30. Report your answer in mM to two decimal places.Answer(related to question 1) The standard addition does not tell you the ammonia concentration, but you can figure this out with the measurement described above and a pH reading. What is the concentration of ammonia in the fish tank water, whose concentration of ammonium was 0.44 mM and whose pH was 8.80? The pKa of ammonia is 9.30. Report your answer in mM to two decimal places.[input]简答题

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BlackTom AI · Accepted Answer

Henderson–Hasselbalch relates the pH to the ratio of the unprotonated base (NH3) and protonated acid (NH4+): pH = pKa + log([NH3]/[NH4+]). Rearranging gives [NH3]/[NH4+] = 10^(pH − pKa).

Step-by-step:
1. Write the ratio: [NH3]/[NH4+] = 10^(8.80 − 9.30) = 10^(−0.50).
2. Evaluate the power: 10^(−0.50) = 0.31622777... (≈ 0.3162).
3. Multiply by the known [NH4+]: [NH3] = 0.31622777 × 0.44 mM.
4. Compute: 0.31622777 × 0.44 = 0.13914022 mM.
5. Round to two decimal places: 0.14 mM.

Why this is correct: the Henderson–Hasselbalch equation directly gives the ratio of unprotonated (NH3) to protonated (NH4+) species at a given pH for the NH4+/NH3 equilibrium. Given [NH4+] = 0.44 mM and the calculated ratio 0.3162, the concentration of NH3 follows by simple multiplication.

Common incorrect approaches:
- Using the inverse ratio ([NH4+]/[NH3]) or swapping pH and pKa sign would produce a wrong, larger value.
- Using pKa as 14 − pKb (unnecessary here) or forgetting to convert units would also mislead.
- Treating 0.44 mM as total ammonia (NH3 + NH4+) rather than specifically [NH4+] would require a different calculation.

Final answer (to two decimal places): 0.14 mM, which matches the provided correct answer.

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