1. He works at/in the library.

2. He works at/in the town hall.
3. He works at/in the hospital.

(Which preposition do I have to use? Are both OK? Which one is popular?)

4. I want to drink iced tea.
5. I want to drink ice tea.
6. I want to drink an iced tea.
7. I want to drink an ice tea.

(Which ones are correct?)

3 answers

well in first 3 both are ok but "IN" is more popular.

5. I want to drink ice tea.
I would disagree with Elvira, favoring 4 over 5.

"Iced tea" would indicate tea with ice in it, while "ice tea" suggests cubes of frozen tea to me.

6 is also good, but it indicates a limitation of one serving or glass of iced tea. It is just more specific than 4.

I hope this helps a little more. Thanks for asking.
Yes, PsyDAG is correct about the difference between "iced" and "ice" in this phrasing.