Welcome to Flyertalk
Anas1059 and a warm welcome to the QR Forum.
In terms of the A350, you will find seating and cabin info here -
http://www.flyertalk.com/forum/qatar...seating-q.html
The F fare, is showing because there is likely an F cabin on your onward journey. If so, it is showing you the combined fares irrespective of the fact that there is no F cabin on the A350.
Safe travels,
M