WHATEVER, WHOEVER, WHICHEVER, WHEREVER, WHENEVER

whatever » anything that
whoever » anyone who
whichever » any ... which
wherever » at any place
whenever » at any time

You can do whatever you want to.
Whoever leaves last should turn off the lights.
Choose whichever pencil you like.
Wherever I looked there was something interesting to see.
Whenever I go abroad, I take as many pictures as possible.

It is possible to use what instead of whatever in the first sentence above, but it is not possible to use who instead of whoever in the second sentence:

You can do what you want to.
Who leaves last should turn off the lights.