# Chem! EDTA and Molarity Question. Need help

1. We had a titration lab where we titrated tap water with EDTA. Through our results we should be able to figure out the molarity of Ca and Mg ions. I think I'm doing this right but it seems to easy in comparison with everything else we are doing in lab and lecture so I just want to know if I'm missing something.

2. M=mol/L
MM of EDTA is 292.24g/mol
EDTA solution was prepared by dissolving 0.6005g of EDTA with 0.400L.
0.6005(g/mol)/292.24g=0.00205mol/.400L=0.005137M of EDTA soultion (I think)

3. 0.005137M*0.0290L (the amount it took to titrate the water)=1.474x10^-4 moles. There was 100.00mL of water and 0.50mL of a buffer added but I don't think the buffer volume is counted in the molarity. So, molarity would be
1.474x10^-4mol/100.00mL=1.474x10^-4M. This is for Mg and Ca

For just Ca, 0.005137*0.02175L (the amount it took to titrate the water) =1.117x10^-4 mol. There was 100.00mL of water and 0.50mL of a buffer added but I don't think the buffer volume is counted in the molarity. So, molarity would be
1.117x10^-4 mol/100.00mL=1.117x10^-4 M for Ca solution.

For just Mg: (1.474x10^-4M) - (1.117x10^-4M)= 3.57x10^-5M

Is that all there really is to this or am missing some extra step I should be doing because of the use EDTA?

Borek
Mentor
0.6005(g/mol)/292.24g=0.00205mol/.400L=0.005137M of EDTA soultion (I think)
Concentration is OK (assuming you used a pure, anhydrous EDTA - possible, but not the only option here). Just note you can't write moles=concentration, there should be no equal sign there.

0.0290L (the amount it took to titrate the water)
molarity would be
1.474x10^-4mol/100.00mL=1.474x10^-4M. This is for Mg and Ca
Sum of Mg & Ca, OK.

0.02175L (the amount it took to titrate the water)
That's where you have lost me. Just a moment ago you wrote it took 29.0 mL to titrate the water, so apparently you did here something different. I suppose this is result of titration at much higher pH, where Mg2+ is precipitated as Mg(OH)2. If so, your calculations look OK, just your description is wrong.

Disclaimer: I have not checked math, just the logic.