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PH and buffers

  1. Nov 25, 2011 #1
    1. The problem statement, all variables and given/known data
    An buffer is made from a 0.1M acid and a 0.25 M salt.
    According to the equation
    pH = pKa - log[acid]/[base]
    the ratio acid:base is 1:2

    What volumes are required to make a 1 dm3 solution?

    2. Relevant equations



    3. The attempt at a solution
    According to the 1:2 ratio we would need
    333.3 cm3 of acid and 666.6 cm3 of base
    However as the initial concentration of acid to base is 0.1:0.25 we instead say
    volume acid = 333.3cm3
    volume base = 666.6 cm3/2.5 = 266.6 cm3
    volume water = 400 cm3

    (all of these volumes are rounded to the nearest 0.1 cm3)

    is this correct?
     
    Last edited: Nov 25, 2011
  2. jcsd
  3. Nov 25, 2011 #2

    Borek

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    Staff: Mentor

    Question doesn't say anything about using additional water for dilution, so I don't think that's the correct answer.
     
  4. Nov 26, 2011 #3
    yes but as I need to make up a litre the rest of the solution must be water
     
  5. Nov 26, 2011 #4

    Borek

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    Staff: Mentor

    No, you don't have to add water. If you mix correct volumes of both solutions you will have a liter of solution with a correct ratio of acid and conjugate base. Try to write two equations describing both ratio and volume - that will give you two equations in two unknowns, easy to solve.
     
  6. Nov 26, 2011 #5
    666.67 cm3 acid
    and
    333.333 cm3[/base]
     
  7. Nov 26, 2011 #6

    Borek

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    Staff: Mentor

    Have you checked if the ratio is correct if you use these volumes?
     
  8. Nov 26, 2011 #7
    ratio acid:base assuming 1M:1M initial ratio gives 1:2
    BUT initial molar ratio is 1:2.5
    therefore new acid:base ratio from pH = pKa - log[acid]/[base] is 1:(2/2.5) = 1:0.8
    ...oh
    acid: 555.6 cm3
    base: 444.4 cm3

    right?
     
  9. Nov 26, 2011 #8

    Borek

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    Staff: Mentor

    Sorry, we are not checking homework, we are helping solve it.
     
  10. Nov 26, 2011 #9
    This is not homework
    this is a revision question I can't find an answer to
    am i right?
     
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