Season 7, Episode 15.
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No source is for certain why Mason Musso and Chelsea Kay broke up. Although, they are for certain that she hurt him deeply and it hurt him to let her go.
There is no last episode of glee at the moment, the latest episode is season 4 episode 4: The Break Up
In the episode "Regarding Henry"
Hero Time
It is implied that they broke up in "The Job" but it is specifically addressed in "Fun Run"