The slight difference between 'awhile' and 'a while'

Look at the following sentences and tell me which is wrong?
1 Please wait for awhile and I'll be with you soon.
2 Please wait for a while and I'll be with you soon.

The first sentence is wrong because 'awhile' needs no preposition in front of it.
Hence the first sentence should be rendered as:
Please wait awhile and I'll be with you soon.

Both wait awhile and for a while means wait for a moment.