Question

Question #3c1e2

Answer 1

You mean the oxidation number of `Co` in `"cobaltous chloride"`, `CoCl_2` is `+II`. Of course you may mean the oxidation state of inorganic `C` in phosgene gas, `Cl_2C(=O)`, which is `+IV`.

Explanation:

The oxidation number is the charge left on the central atom when all of the bonding species are removed from it. Chlorine binds here as the ion chloride, `Cl^-`. For a neutral molecule the sum of the oxidation numbers is zero:

`Co^("Oxidation number")-2xx1=0`

The halogens normally have a `-I` oxidation number.

Of course if you want the oxidation number of `C` in `Cl_2C=O` this is `+IV`.