Question

1) The specific rotation, [α]_D, for sucrose is +67. What is the observed rotation for a solution of 0.50 g of sucrose in 10 mL of water in a sample tube having a pathlength of 10 cm? Answer in degrees.

2) The observed rotation of a solution of 1.3 g of a compound in 10 mL of water is +11 degrees. If the pathlength is 10 cm, what is the specific rotation of the compound?

Answer 1

we know that

conc (C) = mass / volume

so

conc (C) = 0.5 / 10

conc (C) = 0.05 g / ml

conc (C) =

now

given

path length (l) = 10 cm = 1 decimeter

now

we know that

specific rotation = observed rotation / C x l

so

67 = observed rotation / 0.05 x 1

observed rotation = 3.35

2)

now

conc (C) = 1.3 / 10 = 0.13 g / ml

path length = 10 cm = 1 deci meter

so

specific rotation = observed rotation / C x l

specific rotation = 11 / 0.13 x 1

specific rotation = 84.6

so

the specific rotation of the compound is 84.6