# Ratio of copper(II) ions to EDTA?

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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• They're asking for the ration of ions i.e. for each $Cu^{2+}$ ion, how many $EDTA^{6-}$ are there. – Binary Geek Mar 16 '15 at 18:04
• So all that would be is 1:1, that's it? – ahemmetter Mar 16 '15 at 18:05
• Yes that's right. – Binary Geek Mar 16 '15 at 18:05
• @BinaryGeek EDTA has a maximum of a -4 charge. It can never reach -6. – LDC3 Mar 17 '15 at 1:23