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!

Acid-Base Titration Problems

  1. Apr 8, 2017 #1
    1. The problem statement, all variables and given/known data
    a) Determine the concentration of NaCH3COO. The volume of HCl used to reach the equivalence point is 25.05 mL, the concentration of HCl is 0.10 M, and the volume of NaCH3COO solution used is 10.0 mL.

    b) Using the initial concentrations of NaCH3COO and HCl, and a reliable literature value for the pKb for NaCH3COO or PKa for CH3COOH, predict the theoretical pH of the equivalence point.

    2. Relevant equations


    3. The attempt at a solution
    Here is my reasoning and calculations:

    a)

    The equivalence point is the point in the titration when the number of moles of a standard solution (titrant) is equal to the number of moles of a solution of unknown concentration (analyte).

    Therefore, at the equivalence point:

    ntitrant = nanalyte

    nHCl = nNaCH3COO

    moles of HCl = (concentration)(volume)

    moles of HCl = (0.10 M)(0.02505 L)

    moles of HCl = 2.5 x 10-3 mol

    moles of HCl = moles of NaCH3COO = 2.5 x 10-3 mol


    Concentration of NaCH3COO = moles / total volume

    Concentration of NaCH3COO = 2.5 x 10-3 mol / (0.01 L + 0.02505 L)

    Concentration of NaCH3COO = 0.07 M

    b)

    A reliable literature value for the pKa for acetic acid is 4.756.

    pKa = -log Ka

    Ka = 10-pKa

    Ka = 10-4.756

    Ka = 1.754 x 10-5

    At the equivalence point of the titration of a weak base with a strong acid, the general reaction H3O+ (aq) + B (aq) ---> BH+ (aq) + H2O (l) has gone approximately to completion. Therefore, approximately all sodium acetate (NaCH3COO) will be converted to its conjugate acid, acetic acid (CH3COOH), in a 1:1 molar ratio.

    Ka = [CH3COO-][H3O+] / [CH3COOH]
    1.754 x 10-5 = x2 / (0.07 M – x)

    Ka is very small. Therefore, the x value in the denominator is negligible and can be omitted.

    1.754 x 10-5 = x2 / 0.07 M

    x2 = (1.754 x 10-5 )(0.07 M)

    x2 = 1.228 x 10-6 M

    x = 1.108 x 10-3 M

    [H3O+] = x = 1.108 x 10-3 M

    Validity check:
    (1.108 x 10-3 M / 0.07 M )(100%) = 1.58%

    Theoretical pH = -log[H3O+]
    Theoretical pH = -log[1.108 x 10-3]
    Theoretical pH = 2.96

    Could anyone verify my reasoning and calculations? Thanks. All help is very much appreciated.
     
  2. jcsd
  3. Apr 8, 2017 #2

    Borek

    User Avatar

    Staff: Mentor

    There is no CH3COONa at the endpoint so your calculation of concentration (using total volume at the endpoint) doesn't make much sense. Question most likely asks for the initial CH3COONa concentration.

    b looks reasonalby accurate.

    Question is a bit idiotic to be honest. Good luck detecting the end point in this titration (not your fault though).

    acetate_titrated_with_hydrochloric_acid.png
     
  4. Apr 8, 2017 #3
    Thank you for your response. Assuming that the question is asking for the initial concentration of CH3COONa, does the question provide enough information to solve for this? Would I simply use the volume of CH3COONa used instead of the total volume?

    Concentration of NaCH3COO = 2.5 x 10-3 mol / 0.01 L = 0.25 M?
     
  5. Apr 8, 2017 #4

    Borek

    User Avatar

    Staff: Mentor

    Looks OK.
     
  6. Apr 9, 2017 #5

    epenguin

    User Avatar
    Homework Helper
    Gold Member

    I concur with BoreK that the question does not make a lot of sense. Are you sure you have copied it and any background out exactly? One titrates an acid with a base or vice versa;, one does not titrate a salt which is what sodium acetate is.

    Your calculation in #1 is just giving you the pH of 0.07 M acetic acid.

    Maybe that was not the question but until we know exactly what it was it is not really worth further thought
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Acid-Base Titration Problems
  1. Acid/Base Titration (Replies: 5)

Loading...