# Does a 75:25 racemic mixture rotate light?

Discussion in 'MCAT Study Question Q&A' started by stester77s, 05.15.14.

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1. ### stester77s 2+ Year Member

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Hi, if we have an unequal mixture of two enantiomers, say 75% d and 25% l, and pure d rotates light by 15 degrees (and l rotates light by -15 degrees), would that racemic mixture rotate plane polarized light? If so, by what degree.

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3. ### CzarcasmHakuna matata, no worries. 2+ Year Member

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Well, if it's an unequal mixture, it's not a racemic mixture (an equal mixture of both enantiomers). If they were, the opposing rotations would cancel out and wouldn't rotate plane polarized light. This is an enantiomeric excess problem. If you have 25% l and 75% d, well, 25% of 75% of d will cancel out with 25% l. Therefore, 50% excess of d will remain. If it d rotates +15 degrees, then 50% of +15 = (+15)(.5) = +7.5%