I promised that I would post answers to the challenge problems in the learning activities and I have also received requests for 'answers' to the molecular modeling activity (#2). Here are some answers and comments.
Keep in mind: many of the questions in the learning activities are designed to help you build concepts. They are not "problems" in the usual sense and they don't reflect the kinds of problems that might appear on exams. Your book's end-of-chapter problems offer a much better idea of what you can expect exam-wise.