Question

An aqueous solution containing 6.83 g of an optically pure compound was diluted to 200.0 mL with water and placed in a polarimeter tube 15.0 cm long. The measured rotation was –4.89° at 25 °C. Calculate the specific rotation of the compound.

Answer 1

The specific rotation of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter [10 cm] and a sample concentration of 1 gram per 1 millilitre.

Specific rotation = observed rotation/ (tube length in decimeters x concentration)

[] = / (l x c)

In this equation, ℓ is the path length in decimeters and c is the concentration in g/mL, for a sample at a temperature T (given in degrees Celsius) and wavelength λ (in nanometers).

= -4.89°;

c = 6.83 g in 200 mL = 6.83/200 = 0.03415 g mL^-1;

ℓ =15/10 = 1.5 decimeter

[α] = -4.89^o / (0.03415 x 1.5) = --95.5^o