Why doesn't a coke can collapse when the drink has been finished

The air pressure is equal inside and out, as with any other open container.

It doesn't collapse because it is filled all round with atmospheric pressure. Both internally and externally it acted by atmospheric pressure and so it balances and does not collapse.

The only way it can collapse is when a vacuum is created inside it, or when the internal pressure is less than the atmospheric pressure outside of it.

The coke can is likely to burst when let's say it has been shaked violently and the gases within become violent and exceeds the external atmospheric pressure, hence the coke can is likely to burst.

I hope this helps.

The answers would be: 1: the liquid only contains one compound4: the liquid is a pure substance