Specific rotation is characteristic value for every optically active compound. Observed rotation of a compound may vary as per variation in concentration and length of the sample tube, but the specific rotation is a constant value for every optically active compound. Following is a formula used for calculation of specific rotation:

Practice Question:

The concentration of a compound dissolved in water is 4.5 g per 100 mL of solution.

• A portion of this solution in a 5-cm Polarimeter tube causes an observed rotation of –1.4^o. Calculate the specific rotation of that compound.

          Solution: [α] = α/c x l =  –1.4^o / 4.5/100 x 0.5 dm

                                     = -62.22 ^o

• Predict the observed rotation if the same solution were placed in a 10 cm tube.

         Solution: -62.22 ^o = α/ 1 x 4.5/100

                                         α  = – 2.8⁰

• Predict the observed rotation if 10 mL of the solution were diluted to 20 mL and placed in a 5 cm tube.

         Solution: -62.22 = α/ 0.5 x 4.5/200

                                      α  = – 0.69⁰