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Homework Help: EDTA titration? NEED HELP

  1. Jan 20, 2008 #1
    EDTA titration?!!!URGENT NEED HELP

    we did a lab in school to determine water hardness by EDTA titration. I did three trials , first one require 38mL , 2nd - 50 and third-42 the question was to averge the volumes of EDTA used in the last two titrations and to calculate the moles of EDTA required for the titration. do u just averge the two volumes and multiply it by the molar concentraion of EDTA to get the moles?

    and how do u calculate the molarity of metal ion present in the water sample? i kno that EDTA binds the metal ions in a 1 to 1 ratio.

    and for the question above how can u change it to ppm Ca

    also when there is a question asking to draw a lewis dot formula of EDTA and show the 6 electron pairs donated by the lewis , am i just supposed to draw the lewis structure?

    how does the reaction of EDTA w/metal ions fit into the bronsted lowry and lewis theories?
    i kno that Bronsted-Lowry acids are defined as proton donors. (Protons are basically the same thing as positive hydrogen ions.) A Lewis acid though is defined as an electron pair acceptor.

    For bases, Bronsted-Lowry bases are defined as proton acceptors, and Lewis bases are defined as electron pair donors.

  2. jcsd
  3. Jan 20, 2008 #2


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    If you titrated the same replicate samples, then your precision is absolutely terrible. Did you really titrate three completely different samples, from each of three individual sources?

    Yes, the EDTA anion coordinates in a 1 to 1 ratio with Calcium or Magnesium ion.

    milligrams per liter is often taken as parts per million.
  4. Jan 20, 2008 #3
    i titrated with the same sample
  5. Jan 20, 2008 #4


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    Your final answer depends on how you want the units. You might want units as calcium carbonate, or just as calcium ion. The typical unit is probably as calcium carbonate, even if there be any magnesium present.

    You moles of EDTA which would be the same as moles of titrated metal ions, will be based on your titrant volume and your titrant concentration. Use the formula weight of calcium carbonate (if this is the kind of unit that you want to use) to convert to mass of calcium carbonate (remember, 1:1 mole ration for EDTA to metal ion). Now you need to use the volume of sample and the mass of calcium carbonate to find your sample concentration.
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