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Acid-base titration

  1. Mar 29, 2005 #1
    Hello! Here is a acid-base titration question which I am a bit confused at, please help me with it.

    If 25cm^3 solution containing 0.5M sodium carbonate and 0.5M sodium hydrogencarbonate is titrated with 1M HCl using phenolphthalein indicator. Upon the addition of 12.5cm^3 HCl, the phenolphthalein is decolourized. After the 1st end point, methyl orange indicator is added and the solution is further titrated with HCl. What is the volume of HCl needed the change the colour of methyl orange from yellow to orange?

    My teacher told me it was 37.5 cm^3, but I don't understand it. I think it is 25 cm^3.....
     
  2. jcsd
  3. Mar 29, 2005 #2

    Borek

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    Staff: Mentor

    Write down reaction that takes place before first endpoint.

    How much HCl do you need to reach first endpoint?

    Write down reaction that takes place before second endpoint.

    How much HCl do you need to reach second endpoint?

    Calculate total amount of HCl needed.

    Try these questions and - if you will be still not able to find out why it is 37.5 cc - post your answers so that we will be able to show you where you have gone wrong.

    Unless someone decide to show you results of calculations before checking out if you are not able to do it by yourself :(


    Chemical calculators for labs and education
    BATE - pH calculations, titration curves, hydrolisis
     
  4. Mar 29, 2005 #3
    God I swear why do they still do A-B titrations in school. They are totally worthless, not one chemist in the real world does A-B titrations like they are taught to undergraduates.
     
  5. Mar 30, 2005 #4
    Hello Borek, Thank you for your help. however, I am still not able to understand why it is 37.5 cc. Here are my steps:
    The reaction that happens to reach the first end point is:
    sodium carbonate + HCl --> sodium hydrogencarbonate + NaCl
    Since there are 0.025*0.5= 0.0125 mol of sodium carbonate, 0.0125 mol of HCl is needed. That is, 12.5 cc of HCl is needed to reach the first end point.

    The reaction that happens to reach the second end point it:
    sodium hydrogencarbonate + HCl --> NaCl + water + carbon dioxide
    Since there are 0.0125 mol sodium hydrogencarbonate originally,adds up with the 0.0125 mol of sodium hydrogencarbonate formed before the first end point, there will be totally 0.025 mol of sodium hydrogencarbonate. Therefore, 0.025 mol, and thus, 25 cc of HCl is needed to reach the second end point.

    Therefore the total amount of HCl added is 37.5 cc.
    But, in my notes, it says " Upon the addition of 37.5 cc of HCl, the methyl orange changes from yellow to orange". I don't think it is right, because that just means 50 cc of HCl is needed in total...or I have mistaken something? please tell me.
     
  6. Mar 30, 2005 #5
    Just as a question what does methyl orange actually indicate exactly? Like when does it actually change color?
    It looks like you might be right... though, maybe your teacher had mistaken the total HCl added instead....
     
  7. Mar 30, 2005 #6

    Borek

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    Staff: Mentor

    It is not 'upon addition of 37.5 cc more'!

    Titration goes as follows:
    0 cc titrant added - phenolophtaleine visible
    12.5 cc titrant added - phenolophtaleine color disappears
    now you add methyl orange and the solution is yellow
    total of 37.5 cc titrant added - solution changes to orange

    So you have to add either 25 cc of titrant to the solution that have reached first end-point, or total of 37.5 cc of titrant to the not yet titrated solution.

    Sorry if my explanation is not readable or wrong - I did my best :blushing: but English is my second language and perhaps I have missed something.


    Chemical calculators for labs and education
    BATE - pH calculations, titration curves
     
  8. Mar 30, 2005 #7
    I think it is about 4.0 that makes the methyl orange to change from yellow to orange.

    It is not 25 cc of the titrant added which makes the solution to reach the first end point...it should be the second end point.. :wink:
     
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