The chemical symbol #2Cl^-# means that you have 2 moles of #Cl^-# ion. For an ion, the oxidation state is the charge. Since one chloride ion has a charge of -1, then that would be its oxidation state.
The overall oxidation state of the 2 moles #Cl^-# ion is, therefore, -2 (as 2 moles multiplied by the charge -1).
Now for #Cl_2#, the symbol means that there are two #Cl# atoms sharing electrons to form a substance, which is the chlorine gas. Thus we can calculate the formal charge as:
#Cl# atom: 7 valence electrons; therefore, total valence electrons is 14.
The sharing of electrons brings a formal charge of +1 for one of the #Cl# atoms and a charge of -1 to the other #Cl# atom, bringing the overall charge/oxidation state to zero [(+1) + (-1)].
