Homework Help: Chemistry

Posted by Lauren--PLEASE HELP 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

QUESTION TO BE ANSWERED

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 - DrBob222, 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 - Lauren--PLEASE HELP, Sunday, November 29, 2009 at 6:17pm
thank you so much for your help!!

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