"Cuál vs Qué" is a little more complicated than just "what vs which". Unless you are asking for a definition, "cuál" is required when preceeding "es" and is still translated as "what".
Here is a link to explain:
Precedent to the word 'es' you would always use cual.
Refer to these links for more information: http://spanish.about.com/cs/grammar/a/que_vs_cual.htm https://www.duolingo.com/comment/1936132