So today, I wanted to say to my coworker that half of the items lying on my labmate's desk are something that I don't know the purpose of, so I said this.
There are many items on his desk, half of which I don't know the purpose (of).
It's a subtle difference, but should I put "of" at the end of the sentence? I checked how Grammarly answers to this, but neither putting "of" nor not putting it was correct. (Not putting it was grammatically incorrect, but putting it make it "not colloquial") So what should I do in this case? Or is there any natural way to express this?
I also thought about saying this:
There are many items on his desk, half of which I don't know what they are for.
but I'm afraid it's even more incorrect because "they" in this sentence has already been mentioned as "half of which".