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Homework Help: Acid&Base question.

  1. Nov 25, 2006 #1
    Hypochlorous acid, HOCl, is a weak acid in water. The Ka expression for HOCl is shown below:
    [H3O+][OCl-]/ [HOCl]= 3.2 X 10^-8

    1) write a chemical equation showing how HOCl behaves as an acid in water.
    i got: HOCl + H2O = H3O + OCl

    2) calculate the PH of a 0.175 M solution of HOCl.
    i got:
    HOCl + H2O = H3O + OCl
    Initial: 0.175 0 0 0
    Change: -x -x +x +x
    Equilibrium: 0.175-x -x +x +x

    am i doing it right? how do u calculate the PH from x?


    3) write the net ionic equation for the reaction between the weak acid HOCl and the strong base NaOH.
    I got: HOCl + NaOH = H3O +Na +OH
    is it right?


    4) In an experiment, 20 mL of 0.175 M of HOCl is placed in a flask and titrated with 6.55 mL of 0.435 M NaOH.
    A)Calculate the number of moles of NaOH added.
    I did: (6.55/1000) x (0.435 M) = 0.0025

    B) Calculate [H3O+] in the flask after the NaOH has been added.
    where do i start? i dont get what the question is asking.

    C) Calculate [OH-] in the flask after the NaOH has been added.
    where do i start? i dont get what the question is asking.

    thank you.
     
  2. jcsd
  3. Nov 27, 2006 #2
    bump, anyone?
     
  4. Nov 28, 2006 #3

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You've forgotten to include the charges on the product ions.

    The concentration of water initially is not zero, but that's not important for the calculation - it is assumed there's a sufficient excess of water to not matter.

    So you've got [HOCl] = 0.175-x, [H3O+] = x = [OCl-]

    Now go back to the question and look at the expression for the Ka of HOCl.

    Well, what are you supposed to get when you neutralize an acid with a base?

    Correct.

    Once you figure out the correct equation for (3), this will make more sense. See hint for (3).
     
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