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prepare a 10-mg sample of the partially racemized Asp from the skull sample in 10 mL
of water and measure its optical rotation in a precision polarimeter in solution in a 1-dm
cell. What observed optical rotation do you find for this sample? Show your work.
Biot’s law is [a]observed = [a]20(concentration in g/mL)(cell length in dm)
The concentration in g/mL is 0.001, since a 10-mg sample was diluted into 10 mL.
Therefore, for pure unracemized L-Asp, the expected observed rotation would be
[a]observed = (+5 deg mL g–1 dm–1)(0.001 g mL–1)(1 dm) = 0.0050 degrees
In a partially racemized sample, if y% is D-Asp, then 2y% of the optical activity of L-Asp MCMP 204 / Exam 2 / Spring 2012 ANSWERS Page 8 has disappeared, because the y% of D-Asp has negative rotation that cancels y% of the
positive rotation of L-Asp. Therefore, for the 6/94 sample found here, the percent remaining
optical activity is 100–2(6) = 88. Therefore, the observed optical activity of the partially
racemized sample should be (0.88)(0.0050) = 0.0044 degrees....
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This note was uploaded on 06/18/2012 for the course MCMP 204 taught by Professor Berkstrom during the Spring '08 term at Purdue University-West Lafayette.
- Spring '08