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One question in a test I am preparing for is:

What is the ratio of copper(II) ions to EDTA in a $\ce{[Cu(EDTA)]^{2-}}$ complex?

I know that EDTA forms up to 6 coordinate bonds with the central atom, but is that what they are asking for with "ratio"? According to this website, it seems that copper actually bonds with all 6 free electron pairs.

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  • $\begingroup$ Welcome to chemistry.SE! If you had any questions about the policies of our community, you can ‎visit the help center or take a ‎‎tour of the website.‎ And, "homework" tag is for questions when you want a more "teacher" approach. It doesn't mean "homework" literally. $\endgroup$ – M.A.R. Mar 16 '15 at 18:01
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    $\begingroup$ They're asking for the ration of ions i.e. for each $Cu^{2+}$ ion, how many $EDTA^{6-}$ are there. $\endgroup$ – Binary Geek Mar 16 '15 at 18:04
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    $\begingroup$ So all that would be is 1:1, that's it? $\endgroup$ – ahemmetter Mar 16 '15 at 18:05
  • $\begingroup$ Yes that's right. $\endgroup$ – Binary Geek Mar 16 '15 at 18:05
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    $\begingroup$ @BinaryGeek EDTA has a maximum of a -4 charge. It can never reach -6. $\endgroup$ – LDC3 Mar 17 '15 at 1:23
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The ratio of Cu to EDTA is the number of copper ions per EDTA ion. In this case there is one EDTA ion for every copper ion so the ratio is 1:1.

Copper bonds with six electron pairs on EDTA to form the complex shown below where the central metal species is copper(II):

enter image description here

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