I'm not English native speaker, that's why I'd like to understand the following thing:
What is the difference between "Chemistry students' group" and "Chemistry student's group"?
What is the meaning of the apostrophe in this case?
When you have a plural word ending with 's', we put apostrophe 'after' the 's'.
Chemistry students' group
A group that consists of many chemistry students. Here, students are 'plural' and you are talking about those plural students' group.
Chemistry student's group
Here, you are talking about a chemistry student (one). The group belongs to him/her.
So, to conclude, apostrophe after 's' talks about more than one student (students) and before 's' talks about a single student's group.