Question

(+) Mandelic acid has a specific rotation of $+{158}^{\circ }$ what would be the observed specific rotation of a mixture of 25 $\%$ (-) mandelic acid and 75 (+) mandelic acid ?

• A. + 118.5°
• B. -118.5°
• C. - 79°
• D. +79°

Answer 1

The correct option is D

+79°

Specific rotation of the mixture

= $\left(\frac{75}{100}\right)(+{158}^{\circ })+\left(\frac{25}{100}\right)(-158)=+{79}^{\circ }$