Question

$(+)-$ Mandelic acid has a specific rotation of $+{158}^o$ . What would be the observed specific rotation of a mixture of $25\%$ $(-)-$ mandelic acid and $75\%(+)-$ mandelic acid:

• A. $+{118.5}^o$
• B. $-{118.5}^o$
• C. $-{79}^o$
• D. $+79$

Answer 1

The correct option is D $+79$

Concentration of (-) Mandelic acid= $25$%

Concentration of (+) Mandelic acid= $75$%

$\therefore$ Excess of (+) Mandelic acid= $75-25=50$%

Enantiomeric excess= $\frac{\text{Observed specific rotation}}{\text{(sp. rotation of pure (+) enantiomer)}}\times 100$%

$\Rightarrow 50=\frac{x}{158}\times 100$

$\therefore x$ = observed specific rotation= $+79$