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Homework Help: Question about acid and base multiple choice solving

  1. Jan 31, 2016 #1
    1. The problem statement, all variables and given/known data
    I found this problem online for extra practice for my AP Chem class.


    The problem in question is number 4.

    I do not understand how to solve it and why the answer D is correct.

    2. Relevant equations

    Henderson Hasselbalch Equation
    3. The attempt at a solution

    Assuming I have an indicator of the form HIn <------> H+ + In-

    I know that my indicator will have a visible change when the ratio of In- to HIn will be 1/10.

    So I have pH = pKa + log ( [A-] / [HA] ) = -log ( 1.8 * 10 E -5 ) + log (1/10) = 3.744.

    So I put down that my answer was B as my calculated pH falls in that range.

    What am I doing wrong here?

    Thanks in advance for the help!
  2. jcsd
  3. Jan 31, 2016 #2


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    Staff: Mentor

    You are seriously confused, no idea why you speak about ratio of HIn/In yet you plug pKa for acetic acid into HH equation.

    First things first: you need an indicator that changes color as close to the equivalence point as possible, What is the equivalence point for the acetic acid titration?
  4. Jan 31, 2016 #3
    I was told that if I have the following:

    HIn <---------> H+ + In-

    and I am titrating an acid with a base, then the first visible change of color would occur with a ratio of [In-]/[HIn] = 1/10.

    Vice versa with the titration of a base with an acid with the ratio of [In-]/[HIn] being 10/1.

    Hmm... I see how you are saying how I am confused. Seems that I am.

    All I know about the equivalence point is that it is going to be greater than 7 as the equivalence point will result in a basic solution.

    The problem does not specify anything more, and I do not know what to do.

    Thanks for your help.
  5. Jan 31, 2016 #4


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    Staff: Mentor

    Problem specifies everything you need. First step is to find the equivalence point pH (easy to calculate from the pKa value for acetic acid), the to choose indicator that changes color as close to the equivalence point as possible.

    1/10 ratio doesn't matter here (although you are right it matters in some cases).
  6. Jan 31, 2016 #5
    How would I calculate the equivalence point pH?

    I know that a reaction of a weak acid + strong base would yield an equivalence point of basic pH.

    The amount of CH3COO- remaining at the equivalence point would impact the basicity of the solution as it would react with H2O in hydrolisis and form OH-.

    I do not have concentrations of the substances and I do not know how much of CH3COOH and NaOH I would need respectively to reach equivalence point.

    Thus, I do not understand how I am supposed to reach equivalence point pH.

    If you could please help, if would be greately appreciated!

  7. Jan 31, 2016 #6


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    Staff: Mentor

    You don't need a very precise value - typical concentration of the acid during titration is around 0.1-0.01 M, titrating more diluted samples becomes difficult. You can safely assume titrant to be 0.1 M NaOH, as this is typical value. That's enough information.
  8. Jan 31, 2016 #7
    I assumed the volumes to be equal and their molarity to be the same.

    I gave both volumes and molarities value of 0.1.

    I calculated the pH at equivalence to be then 8.72 [within range], which gives me D.

    Is that the right approach?
  9. Jan 31, 2016 #8


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    Staff: Mentor

    Sounds OK to me.
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