'Whom' is used when you are referring to the object of the verb. 'Who' is used when you are referring to the subject of the verb.'Who is there?''Who broke that vase?''For whom is that parcel...
I have thought about this when conversing with people who consider English a second language. This awkward construction might be considered an idiom. It is commonly used but is not properly...
"At" and "in" can be used in any prepositional phrase, such as "The piano can be found "in" the music room" or "the sonata was played "at" the end of the recital.