Chlorine is a halogen that usually takes on its charge in an oxidative state a lot like what Oxygen does. But Oxygen rarely takes on any other oxidation number other than -2 which is its charge. It is also more electronegative, so then if with Chlorine, Chlorine would have to take the charge that would be opposite or complementary in order to balance and reflect in the overall anion's charge.
For example with 1) OCl- Oxygen takes on its ionic charge of -2 but the overall molecule is reflecting - or -1, So what would Cl have to take on in order to reflect this? A positive 1 or +1. This is because a -2 combined with a +1 will make the overall molecules net charge -1.
1) Cl oxidation number is +1
2) again Oxygen rarely takes on anything other than -2. 4 Oxygens make Oxygen's component -8, Cl must take +7 in order for net molecule to be -1.
3) Cl is here by itself, Cl- so -1
4) Cl2, if Chlorine is diatomic is there a net charge? No so they would balance and cancel each there if theoretically you made one Cl - and the other Cl +, the net charge would be 0.
5) Hydrogen usually takes on a +1 oxidation number, and there is no net charge so Cl is -1.