EDTA (ethylenediaminetetraacetic acid) is a hexadentate ligand, meaning it can form six coordinate bonds with a metal ion.

Here's how EDTA chelates Ca and Fe:

* Calcium (Ca): Calcium has a +2 charge. EDTA can bind to one Ca ion, donating its six electron pairs to form a stable complex.

* Iron (Fe): Iron can exist in different oxidation states (+2 and +3). EDTA can bind to one Fe ion in either oxidation state, again donating its six electron pairs to form a stable complex.

Therefore, EDTA can chelate one mole of Ca and one mole of Fe.