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Topic: EDTA and Hardness of Water Question  (Read 14276 times)

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Offline boku

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EDTA and Hardness of Water Question
« on: September 29, 2008, 11:10:41 PM »
I have been working on this problem for a while and am still unclear how to solve it. I have finished my lab assignment without solving this problem (it is a practice problem only) but still want to know the answer.

Question: Find the titer value of an EDTA solution in mg CaCO3/mL if 50 mL of CaCO3 standard with .8 mg CaCO3/mL requires 45 mL of the EDTA titrant to reach a calmagite endpoint.  The answer box says it should have units of mg CaCO3/mL

I guess I am most confused by the units of the answer and don't really know how to proceed. Any help would be appreciated.

Thanks

Offline Borek

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Re: EDTA and Hardness of Water Question
« Reply #1 on: September 30, 2008, 02:56:21 AM »
Will you be able to do the question if concentration should be given in mol/L?
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Offline boku

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Re: EDTA and Hardness of Water Question
« Reply #2 on: September 30, 2008, 09:37:25 AM »
So the concentration of CaCO3 in mol/L?

Then I would do the M1 x V1=M2 x V2 correct?

But that would give me mol/L of EDTA and the answer is supposed to have units of mgCaCO3/mL

Or am I thinking incorrectly?

Thanks for the help

Offline Borek

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Re: EDTA and Hardness of Water Question
« Reply #3 on: September 30, 2008, 09:55:49 AM »
Example:

You have 0.8765 M EDTA solution.

That means that during titration 1mL of this solution will titrate 0.8765x10-3 mole of Ca2+.

0.8765x10-3 mole of Ca2+ is 0.03513g, or 35.13 mg Ca2+.

That in turn means that your original EDTA solution has titer value of 35.13 mg Ca2+/mL.
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Offline boku

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Re: EDTA and Hardness of Water Question
« Reply #4 on: September 30, 2008, 03:08:39 PM »
Okay, I understand how to do it with the M of EDTA, but is there a way to find the titer value of EDTA with only the information given in the problem?

Given info:  .8 mg CaCO3/mL standard CaCO3
                50 mL of standard CaCO3.
                45 mL of EDTA solution used to reach endpoint.

So is there a way to find the EDTA titer value with only that information?

Thanks again for the help.

Offline Borek

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Re: EDTA and Hardness of Water Question
« Reply #5 on: September 30, 2008, 03:20:01 PM »
It is all about converting between masses and numbers of moles.

You may either convert your mass of CaCO3 to moles and calculate molarity of EDTA (using balanced reaction equation), then proceed with reverse calculation as shown; or you may go much easier way - calculate total mass of Ca2+ titrated and divide it by the volume of EDTA used.
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Offline boku

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Re: EDTA and Hardness of Water Question
« Reply #6 on: October 06, 2008, 08:57:42 PM »
It's been a while, but here is what I did to get a titer value.

(.8 mg CaCO3/mL)(50 mL CaCO3)= 40mg CaCO3/(45mL EDTA sol.)=0.889mgCaCO3/mL EDTA sol.

so 0.889mgCaCO3/mL EDTA sol. is my titer value.

If I had another water sample at 100mL and it required 14.9 mL of EDTA titrant I could find the ppm CaCO3 by doing the following:

(0.889mgCaCO3/mL EDTA)(14.9 mL EDTA)=13.2461mgCaCO3/(100mL water sample)=.132461mgCaCO3/mL

Which would end up being 132.5 ppm CaCO3.

I think this is correct, let me know if it is wrong. Thanks again for the help.

Offline Borek

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Re: EDTA and Hardness of Water Question
« Reply #7 on: October 07, 2008, 03:18:52 AM »
Looks OK.
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Offline xoto

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Re: EDTA and Hardness of Water Question
« Reply #8 on: January 02, 2010, 05:23:09 AM »
i think what you are asking for is rather the concentration of EDTA, and that value is 2.5976mg/ml

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