Is it possible to unlearn something?

Certainly, it's possible to forget or 'unlearn" something. However, it depends on the quantity and quality of the material being forgotten exactly how much time and effort would be required. For example, to unlearn something that was fairly simple to learn and has no close connection with anything else in your banks of knowledge should be relatively simple. However, if we're talking about chemical formulas that are interconnected with other sets of formulas with which you are familiar, it might be difficult to separate those which you want to forget and those which you should remember.