Question

(+) - Mandelic acid has a specific rotation of +160°. The observed specific rotation of a mixture
(C) 4 and 3
(D) 3 and 3
of 40% (-) mandelic acid and 60%(+)-mandelic acid is
(A) - 320
(B) +320
(C) +640
(D) +260​

Answer 1

Correct option is

D

+79

Concentration of (-) Mandelic acid= 25%

Concentration of (+) Mandelic acid= 75%

∴ Excess of (+) Mandelic acid= 75−25=50%

Enantiomeric excess=

(sp. rotation of pure (+) enantiomer)

Observed specific rotation

×100%

⇒50=

158

x

×100

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