0123475 12 Time (minutes A. 0.25 ?.0.5 c. 1.0 D. 2.5 is the concentration of com
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0123475 12 Time (minutes A. 0.25 ?.0.5 c. 1.0 D. 2.5 is the concentration of compound B at 11 minutes. A. OmM B. 5 mM C. 10mM D. 20 mM 3. You need to prepare a buffer at pH-4. A buffer based on which molecule below is most appropriate? Buffer choices pK, A. propanoic acid 1.3 x 10 M 4.89 B. benzoic acid 6.4x 10M 4.19 C. chloroacetic acid 1.35 x 103M 2.87 D. hypochlorous 3.5x 10 M 7.46 4. A solution has a pH of 6.24. What is the concentration of protons (hydronium ions) in solution, in units of m A. 0.000000575 B. 0.000229 C. 0.795 O. 1737800Explanation / Answer
3) The closer is the pKa of the weak acid to the desired pH, the more effective is the buffer. Infact, a weak acid can best show its buffering action best when (pKa – 1) < pH < (pKa + 1). The desired pH is 4.00. Both propanoic acid and benzoic acid satisfy the above criterion; however, benzoic acid is a better choice since the pKa is closer to the desired pH. Therefore, (B) is the correct answer.
4) We define pH = -log [H+]; therefore,
[H+] = antilog (-pH) = antilog (-6.24) = 5.75*10-7 M = 0.000000575 M.
(A) is the correct answer.
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