Which one is the best procedure to prepare 100 mL of buffer solution?哪种方法最适合制备100毫升缓冲液？

Question

BlackTom AI · Accepted Answer

A buffer should be prepared by dissolving the solute in less than the final volume, adjusting the pH while there is room to add titrant, then bringing the solution to the final volume — therefore option A is correct.

1) Relevant concepts
- A buffer’s pH is set by the ratio of acid/base species; adding small amounts of strong acid/base to adjust pH changes both pH and solution volume.  
- Best laboratory practice: dissolve salts in a portion of the final volume, measure and adjust pH, then dilute to the exact final volume to ensure correct concentrations.  
- If you prepare the full final volume first, any pH adjustment will change concentration or require re‑measuring after dilution.

2) Step‑by‑step reasoning for option A
- Step (1): Dissolve the correct amount of salts in 30 mL deionised water. Using a reduced volume (here 30 mL for a 100 mL target) gives ample headroom to add acid or base during pH adjustment without reaching the final volume prematurely.  
- Step (2): Test pH. Measure the pH of this concentrated solution so you know how much titrant is needed.  
- Step (3): Adjust to target pH. Add small aliquots of acid or base while monitoring pH. Because the solution is concentrated and in a smaller volume, pH responds reliably and you can add titrant accurately; any added volume remains part of the later final volume calculation.  
- Step (4): Add deionised water to reach the final 100 mL. This brings the buffer to the intended concentration and locks in the adjusted pH (provided additions were small and accounted for).

3) Why A is correct
- It follows standard procedure: dissolve in less than final volume, adjust pH, then make up to final volume. The 30 mL starting volume provides clear room for titration and accurate final concentration control.

4) Why the other options are incorrect
- B (start in 80 mL): leaves little headroom for titration; adding titrant may require significant volume changes and makes precise final concentration and pH control harder.  
- C (dissolve in 100 mL then adjust pH): adjusting pH after reaching final volume changes concentration and requires re‑measuring and re‑diluting, giving less control.  
- D (no pH testing): unacceptable because a buffer must be adjusted and verified to the target pH.

Conclusion: Option A matches correct lab practice and is the best procedure; final answer = A.

Which one is the best procedure to prepare 100 mL of buffer solution?哪种方法最适合制备100毫升缓冲液？单项选择题

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