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Homework Help: Why is one a buffer solution but not the other?

  1. Dec 3, 2017 #1
    1. The problem statement, all variables and given/known data
    A student prepares two solutions:
    Solution A is prepared by mixing 50cm3 of 0.100mol dm-3 CH3COOH(aq) with 25cm3 of 0.100mol dm-3 NaOH(aq)
    Solution B is prepared by mixing 25cm3 of 0.200mol dm-3 CH3COOH(aq) with 50cm3 of 0.100mol dm-3 NaOH(aq)

    Explain why solution A is a buffer solution, but solution B is not.

    2. Relevant equations
    None

    3. The attempt at a solution
    I believe it is because of the reaction
    CH3COOH(aq) + NaOH(aq) ↔ CH3COONa(aq) + H2O(aq)

    In solution A, the reacants are in the same proportion (concentration wise) as the stoichiometric constants ij the equation, but in solution B the concentrations are in the ratuo 2:1.
     
  2. jcsd
  3. Dec 3, 2017 #2

    Borek

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    What do you know about buffers? What is the definition of the buffer solution?
     
  4. Dec 3, 2017 #3
    A buffer solution is a solution that is made up of a weak acid and conjugate base, which minimises a change in pH when lsmall amounts of acid or base are added. In the question, both solutions are made up of the same weak acid and base, the only differences are volumes used and concentratioj.
     
  5. Dec 3, 2017 #4

    Borek

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    Have you calculated amounts of acid and conjugate base in each case? Just by following stoichiometry?
     
  6. Dec 3, 2017 #5
    You mean by n=C*v?
    n - number of moles (mol)
    C - concentration (mol dm-3)
    v - volume (dm3)
     
  7. Dec 3, 2017 #6

    Borek

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    There were reactions taking place - what is present in the solutions afterwards?
     
  8. Dec 3, 2017 #7
    H2O, NaOH, CH3COOH and CH3COONa?
     
  9. Dec 3, 2017 #8

    Borek

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    Check their concentrations. Use simple stoichiometry (hint: check what is the limiting reagent).
     
  10. Dec 3, 2017 #9
    Well, the concentrations in solution A in the ratio of CH3COOH:NaOH is 1:1, however in solution B it is 2:1, the stoichiometry ratio is 1:1.

    Also, what is the limiting reagent? I don't remember doing it, I don't think it's part of our course.
     
  11. Dec 3, 2017 #10

    Borek

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    If you are expected to do buffers, you are expected to know how to do stoichiometry calculations, limiting reagents are part of that.

    No, you are looking at concentrations of reactants, not at concentrations of products of the reaction.

    Write reaction equation first.
     
  12. Dec 3, 2017 #11

    epenguin

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    :confused: I am not seeing that this: "solution A is a buffer solution, but solution B is not." is correct.
     
  13. Dec 3, 2017 #12
    With buffers I know we only need to know what a buffer is, how to calculate its pH and find the ratio of the two compounds to get the pH, amongst a few other things.

    I'm OK with the definition and a few other things, but not good with the maths and equations for buffers.

    The question I asked in the OP was an example question

    CH3COOH(aq)+NaOH(aq)↔CH3COONa(aq)+H2(l)O

    Products are still 1:1.

    In our second year textbook I don't find limiting reagent in the index, maybe it could be in the first year textbook.
     
  14. Dec 3, 2017 #13
    Let me post the image.
     
  15. Dec 3, 2017 #14
    IMG_20171203_204335.jpg

    Not the best of quality. NaOH is partially cut off from the left hand side.
     
  16. Dec 3, 2017 #15
    All I know is that the mole ratio of CH3COOH(aq):NaOH(aq) in solution A is 2:1 and in solution B it is 1:1, while the stoichiometric ratio is 1:1.
     
  17. Dec 3, 2017 #16

    epenguin

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    Well maybe a speck of dust has surrounded my brain, that is one hypothesis.

    However I am still seeing the statement you are required to explain as wrong. It requires a lot of ingenuity the student to give the justification for a wrong statement. If I give you a counter authority to not have to justify this statement, but just to work out what would be the right answer, e.g. which of these solutions is a better buffer, then maybe that is easier for you to do.

    I think your conclusions in #9 are wrong but maybe you were trying to force them to conform to what you’re being told,So have another look. What are the ratio of moles in the two cases?

    Edit: I see you have corrected in #15
     
  18. Dec 3, 2017 #17
    Yeah, they have made mistakes before. Once, in the first year physics textbook, they quoted the density of seawater as being 10 times less than what it is meant to be. It was for an exam style question.

    So, what do I do with the tree moe ratios?
     
  19. Dec 3, 2017 #18

    epenguin

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    Okay to get to the answer, at a given acid-base ratio do you understand whether increasing the total concentration makes the Buffer better or worse.?

    And at given concentration What is the acid base ratio that makes the best buffering?
     
  20. Dec 3, 2017 #19

    Borek

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    OK, for both cases calculate how much acid and how much conjugate base is present after the reaction.
     
  21. Dec 3, 2017 #20
    Based on what I remember reading from the textbook, changing the concentrations of the acid and/or base changes the pH of the buffer solution.
     
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