I'm having difficulties in understanding two questions of the following passage. Could someone here please help me with it? The passage is from Princeton Review MCAT Science Workbook Ochem - passage 4 (advanced passage) "A research chemist synthesized Compound A, which was shown to be a powerful analgesic upon clinical testing. Further evidence indicated only one stereoisomer of A was the acitve agent. The researcher was instructed to separate Compound A into its different stereoisomers to help determine which one was the analgesic. Compound A, which showed an optical rotation of 0 degree, was treated with the R stereoisomer of Compound B (melting point 55C). Two easily separated solids, C and D, whose melting points were 101C and 90C, respectively, were isolated from the reaction mixture. Compound C and D were then independently treated with warm aqueous acid. After workup, Compound C gave Compounds E and B, which Compound D yielded F and B. Testing of Compounds E and F determined that E was the active analgesic, while F was biologically inactive." Question: 1. Compound A can be described as: a. a meso compound b. a racemic mixture c. an equal mixture of two diastereomers d. an optically pure compound Answer is B. The explanation given is "from the information given, Compound A is a mixture of two stereoisomers. With an optical rotation of 0 degree, it must be a mixture of enantiomers, which is racemic mixture". My concern is, I could tell only from the text that it is a kinda mixture. However, if consider calling it "compound A", shouldn't it be a pure chemical? 2. Compound C and D are best described as: a. enantiomers b. tautomers c. diastereomers d. rotamers Answer is C. Explanation: "Compound C has configuration R, R while compound D has configuration S, R. They are different at only one center and are therefore diastereomers. My concern is that I couldn't tell how to figure out the configuration of C and D just based on the passage. I could only guess they are probably diastereomers. Thank you very much!