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vera_ewing

  • one year ago

Given that the pKa of acetic acid is 4.8, what would be the likely result of adding 1 mL of 0.1 M HCl to a 0.1 M acetate buffer at a pH of 5.0? A. The pH would drop slightly. B. The pH would increase significantly. C. The pH would drop significantly. D. The pH would increase slightly.

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  1. cuanchi
    • one year ago
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    any idea?

  2. vera_ewing
    • one year ago
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    Well I feel like it might D maybe?

  3. cuanchi
    • one year ago
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    why did you choose that?

  4. vera_ewing
    • one year ago
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    Since the pKa is 4.8

  5. vera_ewing
    • one year ago
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    Actually, I think it's A

  6. cuanchi
    • one year ago
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    OK now a little bit closer, If you add an acid to a buffer always the pH is going to drop. If you add a base the pH will increase. How do you know if the pH is dropping slightly or significantly?

  7. vera_ewing
    • one year ago
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    It would drop significantly, since the pKa is 4.8?

  8. cuanchi
    • one year ago
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    The pKa is not going to tell you if the pH is going to drop slightly or significantly. The buffer capacity and the concentration of the buffer and the amount of acid that you add is going to tell you if the pH change slightly or significantly

  9. vera_ewing
    • one year ago
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    Ohh okay! So the answer is C?

  10. Rushwr
    • one year ago
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    Shouldnt it be A? @Cuanchi Can u explain it a little more?

  11. cuanchi
    • one year ago
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    the problem is giving you two very similar options A and C, how can you figure out which one is the correct answer?

  12. vera_ewing
    • one year ago
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    The pH?

  13. cuanchi
    • one year ago
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    you have the Henderson-Hasselbalch Equation pH = pKa + log ([base]/[acid]) when the problem said that the concentration of the buffer is 0.1M that means that the concentration of the base/acid = 1 (both are 0.1M) an then the log 1 = 0 then the pH of the buffer is equal to the pKa. when you add HCl to the buffer, the HCl is reacting with the sodium acetate , and produce more acetic acid and decrease the concentration of sodium acetate in the buffer. At this point the relationship [base]/[acid] is <1 then the log of a number <1 is negative and in the equation the pH is going to be lower than the pKa

  14. Rushwr
    • one year ago
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    I thought it in this way: since its an acid there should be a slight drop in pH as its concentration is small

  15. vera_ewing
    • one year ago
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    @Cuanchi Is it C then?

  16. cuanchi
    • one year ago
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    Your idea @Rushwr is correct the only thing you have to be sure that the amount of strong acid (moles) is lower than the amount of salt that you have in your buffer. If the amount of strong acid is bigger than the amount of salt you don't have buffer anymore and you have just acid and NaCl

  17. cuanchi
    • one year ago
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    I will go for A because the problem doesn't mention the volume of the buffer that you have

  18. Rushwr
    • one year ago
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    yey my answer is correct ! @Cuanchi Thank you

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