Aru ka isn't one word. It's the verb aru+ka (question marker) byouin ga doko ni aru ka (plain question) ="where is the hospital?" wakaranai ="I don't know" byouin ga doko ni aru ka wakaranai =I don't know where the hospital is. If you want to make sentences like "I don't know why/when/where...etc.", make a plain (not polite) question+wakaranai/shiranai...etc. Hope this helps.

If I add some help for you, we usually use "aru no ka wakarimasen." But you can also use "aruka wakarimasen." byoin ga doko ni aru no ka wakarimasen. "no" can make those words as a noun, like "which" I said was true. Hope you can enjoy learning Japanese!