Question

An aqueous solution containing 8.65 g of an optically pure compound was diluted to 600.0 mL with water and placed in a polarimeter tube 20.0 cm long. The measured rotation was –2.05° at 25 °C. Calculate the specific rotation of the compound.

Answer 1

Given:

8.65 g of an optically pure compound was diluted to 600.0 mL with water.

polarimeter tube length 20.0 cm long.

measured rotation was –2.05° at 25 °C.

concentration of the solution = (8.65 gm /600 ml) = 0.0144 gm/ml

length of the polarimeter tube = 20 cm

1 decimeter = 10 cm ,

20 cm = 2 decimeter

For solutions

For solutions, a slightly different equation is used:

In this equation, α is the measured rotation in degrees, l is the path length in decimeters, c is the concentration in g/mL, T is the temperature at which the measurement was taken (in degrees Celsius), and λ is the wavelength in nanometers.

Specific rotation = (–2.05°) / (2)(0.0144 ) = (–2.05°) / (0.0288 ) = -71.18

the specific rotation of the compound is -71.18° at 25 °C. we can also say as -71.18 deg cm² g^-1