The word awhile is considered by many grammarians to be ungrammatical, a solecism, however it is sometimes used in place of the two words a while when it is used as the object of a preposition. "I'm going to rest for awhile."
I always recommend that my students avoid usages that some, but not all, teachers and other experts consider non-standard. Why take unnecessary chances? The word awhile falls into that category. Avoid using it, you'll never be considered wrong if you use the words a while in every instance.