Black pepper is
piper nigrum in Latin. Potassium perchlorate is
kalii perchloratum. Why on Earth is
piper in nominative but
kalii in genitive? To confuse it even further, it
is "kalium perchloratum" in some medical texts, though my guess is that that isn't proper Latin. The only reason might be that potassium perchlorate is perchlorate
of potassium, while black in black pepper isn't anything derived