1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Strong or weak acid?

  1. Jan 23, 2009 #1
    1. The problem statement, all variables and given/known data

    How can you determine if an acid is strong or weak given the titration curve? I need two reasons.

    2. Relevant equations



    3. The attempt at a solution

    1 Reason that it is weak is that its being titrated by a strong base and the equivalence point is above pH of 7.

    Need a 2nd
     
    Last edited: Jan 23, 2009
  2. jcsd
  3. Jan 23, 2009 #2

    Borek

    User Avatar

    Staff: Mentor

    Take two titration curves and compare them.

    You may try (there is a free trial).
     
  4. Jan 23, 2009 #3
    Not sure what the acid is, but through a tritaion calculation I found the concentration of the acid to be 0.2083M.

    That may not be correct..

    Heres the data:
    NaOH: 0.125M
    25mL
    acid: 15mL
    unknown concentration

    But it is a monoprotic acid
     
  5. Jan 23, 2009 #4

    symbolipoint

    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    Depending on concentrations used, pH will change at a very high rate through pH 7 through the equivalence point. A weak acid will further show fairly small change in pH as the titrant is added over a "long" range of titrant addition until close to the equivalence point. Equivalence point should happen at about pH 8 or greater for a weak acid.
     
  6. Jan 23, 2009 #5
    Yes, it happens at pH = 8.7

    It just occured to me that weak acids do not 100% dissociate

    So could I use the pH of 8.7 to determine the concentration of H+ and compare that number to the concentration of the acid, and if they are different, that means it wasnt 100% dissociated?

    HX ---------> H+ + X-
    0.2M -------> 2x10^-9M

    [H+] = 10^-pH = 2x10^-9
     
  7. Jan 23, 2009 #6

    Borek

    User Avatar

    Staff: Mentor

    At equivalence point there is (almost) no acid, there is (almost) only its salt.
     
  8. Jan 23, 2009 #7
    Well whats another way I could show that this acid is weak?
     
  9. Jan 23, 2009 #8

    symbolipoint

    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    You just did, according to some of your description in post #5.

    Was the original acid available in relatively pure, dry form? Even a pH measurement before titrating might not be strong enough information compared to performing a titration. As long as you can refer to your titration curve, you can easily enough judge whether the acid is weak or strong. Note, that a strong acid will not present pH buffering as you begin and continue to approach the endpoint with titrant additions; and the equivalence point pH should be sudden and extremely close to 7.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Strong or weak acid?
Loading...