Like the person above said, talk about your experience with stats in your research setting and what you did/learned. If you've learned a lot about a specific program (R, SPSS, etc) put that in too. C's in inorganic chemistry is going to matter less and like the person above said you should emphasize that you changed your career path and pursued I/O once you found out about it (and highlight your grades and research there). Chemistry doesn't apply to I/O so don't worry *too* much since your other grades are good. Don't make excuses in your personal statement but it's okay to say that you realized biohealth wasn't for you and I/O was more your forte, or something like that. Make sure to look at the stats of the programs you want to apply to so you know how you compare and which make sense.