Question

1.) The specific rotation, [?]_D, for sucrose is +67. What is the observed rotation for a solution of 0.70 g of sucrose in 10 mL of water in a sample tube having a pathlength of 10 cm?

___degrees .

2.) The observed rotation of a solution of 1.7 g of a compound in 10 mL of water is +13 degrees. If the pathlength is 10 cm, what is the specific rotation of the compound?

Answer 1

1) The relation between observed and specific rotation is as follows,

[?]_l^T = ? / lc

where [?]_l^T = specific rotation

? = observed rotation

l = Pathlength of sample tube in dm ( 10 cm = 1 dm)

c = conc. of solution in g per 100 mL

From the data provided we know that,

[?]_l^T = +67
l = 10 cm = 1 dm

c = 0.7 g in 10 mL = 7 gm in 100 mL

Plugging these values in the equation we get,

67 = ? / 1 x 7

=> ? = +469 degrees

2) This is similar to the previous question. We again need to plug the values provided to get the unknown. Only this time the unknown is specific rotation

The data provided is,

? = +13
l = 10 cm = 1 dm

c = 1.7 g in 10 mL = 17 gm in 100 mL

=> [?]_l^T = 13 / 1 x 17 = 0.764 degrees