Other answers have shown (or correctly claimed, at least) that the example questions are both wrong. They, however, do not answer the question, which is about whether a contraction allows a sentence to end in a proposition.
A contraction is irrelevant to the use of a preposition at the end of a sentence. In English stranded propositions are actually allowed. The common belief that they are not allowed is the product of an attempt to make English more like Latin, in which they are not allowed. (See here for the story and for the defense of propositions at the end of sentences.)
The problem, then, with "Where is it at?" is not that it ends in a proposition. The problem is that where does not require at. We could test this by rearranging the question to avoid the alleged error of ending in a preposition: "At where is it?" We do not say this.