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Posted by on Sunday, November 29, 2009 at 5:20pm.

Analysis of Tap water (Hardness)

50.00 mL of tap water was transferred to a 250 mL erlenmyer flask. 10mL of NH3/NH4^+ ph 10 buffer was added to the flask, as well as 6 drops of eriochrome black T indicator. This soln was then titrated with EDTA soln(0.00641M)

Given equations:
1. Ca^2+ + Mg^2+ --> Ca^2+ + Mg(Indicator)

2. Mg(Indicator) + Ca^2+ + EDTA^4- --> CaEDTA^2- + MgEDTA^2- + free indicator


For the volume of titrant used (9.95 mL) calculate the:

a) Ca^2+ + Mg^2+ in sample (mol)
b)[Ca^2+] + [Mg^2+] of sample (M)

The equations 1 & 2 were given, and I believe we are supposed to make an equation for the reaction from this i.e. add them so that:

Ca^2+ + Mg^2+ + EDTA --> MgEDTS^2- + CaEDTA-

So by using the given volume of titrant (EDTA=9.95 mL) we could find the mol of Ca and Mg...i'm just not sure how to go about doing this (the stoichiometry is confusing me since the concentration they are looking for is Ca and Mg added...)

  • Chemistry - , Sunday, November 29, 2009 at 6:08pm

    moles EDTA = M x L.
    moles Ca + moles Mg = moles EDTA

    molarity Ca + Mg = moles/0.050 L = ??

  • Chemistry - , Sunday, November 29, 2009 at 6:17pm

    thank you so much for your help!!

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