Question

Questio: Optically pure (S)-lactic acid has specific rotation of [a]D 25 = -12°. What is the percent composition (%R and %S) of a mixture of R and S lactic acid that has a specific rotation of [a]D 25 = +6°.

(Could you please explain how to solve this? Thanks!)

Answer 1

% Optical purity of sample = 100 * (specific rotation of sample) / (specific rotation of a pure enantiomer)

% Optical purity of sample = 100 * (+6)/(-12)

                                       = 50%

This means it has percent of S is 50% more than percent of R.

Thus, R is 25% and S= 25+50= 75%.