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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: boku on September 29, 2008, 11:10:41 PM

Title: EDTA and Hardness of Water Question
Post by: boku 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

Title: Re: EDTA and Hardness of Water Question
Post by: Borek on September 30, 2008, 02:56:21 AM

Will you be able to do the question if concentration should be given in mol/L?

Title: Re: EDTA and Hardness of Water Question
Post by: boku 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

Title: Re: EDTA and Hardness of Water Question
Post by: Borek 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.

Title: Re: EDTA and Hardness of Water Question
Post by: boku 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.

Title: Re: EDTA and Hardness of Water Question
Post by: Borek 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.

Title: Re: EDTA and Hardness of Water Question
Post by: boku 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.

Title: Re: EDTA and Hardness of Water Question
Post by: Borek on October 07, 2008, 03:18:52 AM

Looks OK.

Title: Re: EDTA and Hardness of Water Question
Post by: xoto 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