Which of the following mixtures is optically inactive overall? (Tartaric acid = 2,3-dihydroxybutanedioic acid; RR and SS are the chiral forms.)

A1 mol (2R,3R)-tartaric acid + 1 mol meso-tartaric acid
B1 mol (2R,3R)-tartaric acid + 1 mol (2S,3S)-tartaric acid
C2 mol (2R,3R)-tartaric acid + 1 mol (2S,3S)-tartaric acid
D1 mol (2S,3S)-tartaric acid + 1 mol meso-tartaric acid

Answer: B

meso-Tartaric acid is inactive, so its presence does not cancel anything. Only equal amounts of the (R,R) and (S,S) enantiomers cancel (racemate). The other mixtures contain an excess of one chiral enantiomer and remain active.

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