Answer:
If the the word "chemists" refers to only one chemist, then the possessive word should be chemist's (the apostrophe goes in between the t and the last s). If it refers to multiple people, then the possessive word should be chemists' (with the apostrophe on the very end). In this situation, it looks more likely for it to be a singular chemist, so I would go with the first answer: chemist's
Hope this helps!
