You cannot look at any one thing in a Vacuum. My DD had a 3.2 her first semester while doing well in ROTC and competing on a DIV sports team . She earned her appointment early in the process.
If your application is strong, a B in your first semester of college chemistry coupled with a good performance in a challenging schedule will not keep you out.
Now if you have a a 3.0 and only take one or two difficult classes, that may be a different story.