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Homework Help: Titration question on unkown pH

  1. May 27, 2012 #1
    1. The problem statement, all variables and given/known data
    24 ml of 1.5 M solution of unknown acid is titrated with 54 ml 2 M NaOH. The unknown acid is triprotic. If the unknown acid is strong, what is the pH of this acid before titration


    2. Relevant equations

    Mvn= Mvn to find that it was tri-protic.

    3. The attempt at a solution
    literally no idea where to begin
     
  2. jcsd
  3. May 27, 2012 #2
    What does it mean for an acid to be "strong"?
     
  4. May 27, 2012 #3
    that it dissociates completely in solution
     
  5. May 27, 2012 #4
    OK, how many moles of acid do you have?
     
  6. May 27, 2012 #5
    .036 moles
     
  7. May 27, 2012 #6
    And so how many moles of protons, when it dissociates?
     
  8. May 27, 2012 #7
    i dont know... do you?
     
  9. May 27, 2012 #8
    What does triprotic mean?
     
  10. May 27, 2012 #9
    that the formula is H3___ (3 being a subscript) so its got 3 hydrogen
     
  11. May 27, 2012 #10
    OK, so if there are 0.036 moles of acid, how many hydrogens?
     
  12. May 27, 2012 #11
    .012 or .108??
     
  13. May 27, 2012 #12
    If a cake needs 3 eggs, and I have 4 cakes, how many eggs do I need?
     
  14. May 27, 2012 #13
    12? so .012?
     
  15. May 27, 2012 #14
    How did you get 12 to my question?

    Consider that you have 0.036 cakes, each containing 3 eggs...
     
  16. May 27, 2012 #15
    so .108 i did .012 because i thought you were saying 12 egga for 4 cakes
     
  17. May 27, 2012 #16
    ah, sorry, probably a bad choice of numbers on my part then :)

    OK, 0.108 moles of protons. Can you define pH in terms of concentration?
     
  18. May 27, 2012 #17
    ya - [log concentration]
     
  19. May 27, 2012 #18

    Borek

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    Staff: Mentor

    And what is the concentration? (given that you know number of moles and volume)
     
  20. May 27, 2012 #19
    1.38, but thats too high the negative log of that is negative itself
     
  21. May 27, 2012 #20
    How have you got 1.38? Also, negative pH is not a problem either.
     
    Last edited: May 27, 2012
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