Question

An aqueous solution containing 4.69 g of an optically pure compound was diluted to 300.0 mL with water and placed in a polarimeter tube 15.0 cm long. The measured rotation was –3.17° at 25 °C. Calculate the specific rotation of the compound.

Answer 1

Specific rotation = observed rotation/ l.c

Given that

obeserved rotation = –3.17°

concentration c = 4.69 g/ 300 mL = 0.0156 g/mL

length of tube = 15 cm = 1.5 dm

Hence,

Specific rotation = observed rotation/ l.c

                              = –3.17° / ( 1.5 dm x 0.0156 g/mL)

                             = - 135.1°

Therefore, specific rotation of the compound = - 135.5°