What chemical compounds might be present in drinking water?

# 3.2.3 Prediction of formulas

For an element in its highest oxidation number (and sometimes others), it is possible to predict the formula of its orthoacid from its coordination number considerations. The condensed oxoacid formulas are then easily derived by subtracting the appropriate number of water molecules.

Third and fourth-row elements prefer four-coordination in their oxoanions. For example, using the oxidation number approach, you can see that when phosphorus(V) coordinates four O2− ions to form a complex ion of stoichiometry PO4, the resultant charge on this oxoanion is (+5 − 8) = −3. Thus, the formula of the corresponding neutral oxoacid is H3PO4.

Predict the formula of the oxoacid formed by sulfur(VI).

## Answer

Sulfur(VI) coordinates four O2− ions to form a complex ion of stoichiometry SO4, with the resultant charge (+6 − 8) = −2, so the oxoacid is H2SO4.

The chlorine(VII) oxoanion, with stoichiometry ClO4, has the charge (+7 − 8) = −1, so only one proton is required for the formation of the neutral oxoacid HClO4.

For oxoanions of the second Period three-coordination is the norm, as it allows for the formation of a planar assembly with extensive π bonding, e.g. carbonate, CO3 2−.

The above arguments often fail to predict the formulas of oxoacids of elements in lower oxidation numbers, because the preferred coordination number is either not achieved, or is achieved only by formation of a direct link between the central atom and hydrogen.

OpenLearn - What chemical compounds might be present in drinking water?
Except for third party materials and otherwise, this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence, full copyright detail can be found in the acknowledgements section. Please see full copyright statement for details.