That's op. 67 no. 2 in g minor. Lol, nevermind you edited.
I know what key the excerpt is in, what I was saying that the only place with a reference to a solo piece titled "Mazur" by Chopin (asides from Op. 5) is in IMSLP, and it's in g. For all I know they could be two completely different pieces... XD oh well