JEE Main levelSingle correct optionDifficulty 2 of 3st130
A sample is 85% (R) and 15% (S). Pure (R) has [α]=−42.0. The expected specific rotation of the sample is
A−29.4
B−35.7
C−6.3
D−42.0
Answer: A
ee = 85 - 15 = 70%, so [α]=0.70×(−42.0)=−29.4. The value −35.7 uses the 85% fraction directly, which is wrong because the 15% (S) cancels an equal amount of (R).