MCAT Organic Chemistry Question 27: Answer and Explanation

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Question: 27

12. A scientist takes a 0.5 M solution of an unknown pure dextrorotatory organic molecule and places it in a test tube with a diameter of 1 cm. He observes that a plane of polarized light is rotated 12° under these conditions. What is the specific rotation of this molecule?

A. –240°
B. –24°
C. +24°
D. +240°

Correct Answer: D

Explanation:

Remember that the equation for specific rotation is In this example, α_obs is +12° (remember that dextrorotatory, or clockwise, rotation is considered positive), c = 0.5 M, l = 1 cm = 0.1 dm. Remember that path length is always measured in decimeters when calculating specific rotation. Therefore, the specific rotation can be calculated as:

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