Bookiemom, at D's school, the last four years running, the class average on the Ochem final has been a range of 25 - 29 correct answers out of 100. Everyone gets an F, then the curve is applied.
D's final in Ochem 2, she told me that when the exam ended, three or four students walked outside, sat down on the curve outside the classroom and began to cry. They were convinced their dreams of med school were over.
D ended up with solid B's, but with very hard work, and with a tutor supplied by the athletic department. That coupled with B's in the rest of the pre-med core - physics, cal and bio - and she no longer believes med school is an option (but she doesn't want to be a doctor any more anyway).
I do believe a B will do it - but - from what (little) I know of med school admissions, it gets very problematic with anything less than A's, unless the MCAT score is stellar. As to Ochem 2, someone recommended to my D that she audit the class for an entire semester first, then take it, because this would make it easier to get the coveted A, but, D didn't do that.
As far as med school, D's school has an entire department exclusively for post-bacc students, the majority of those students being people who have completed their undergrad degrees and are either retaking the hard sciences, or taking further coursework in preparation for med school applications. So, a B or even C in chemistry etc. doesn't have to mean the end of med school aspirations.