"Where is the ketchup?"
Translation:Missä ketsuppi on?
There's no bigger difference in meaning. In my ears, the latter [Missä on ketsuppi(a)?] you would probably say when you are in a new place, as a guest or in a restaurant, and you are not quite sure if there is any ketchup, but the first one is the question when you ask when you know there is ketchup in the household but not sure where THE ketchup is (you mean the exact bottle of it you've seen earlier).