If the the word "chemists" refers to only one chemist, then the possessive word should be chemist's (the apostrophe goes in between the<em> t </em>and the last <em>s</em>). 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