Enter the angle through which plane-polarized light is rotated, the mass concentration of the solutions, and the path length.
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Specific Rotation Formula
The following formula is used to calculate the specific rotation.
SR = a/(y*l)
- Where SR is the specific rotation (deg*mL/(g*dm))
- a is the angle through which plane-polarized light is rotated (degrees)
- y is the mass concentration (g/mL).
- l is the path length (dm)
To calculate specific rotation, divide the angle by the product of the mass concentration times the path length.
Specific Rotation Definition
What is specific rotation? A specific rotation is defined as “the change in orientation of monochromatic plane-polarized light, per unit distance-concentration product, as the light passes through the sample of a compound in solution.” (Source)
Example Problem
How to calculate specific rotation?
- First, determine the angle.
For this problem, the angle through which the light is rotated is 25 degrees.
- Next, determine the mass concentration.
In this example, the mass concentration of the compound in the solutions is 50g/mL.
- Next, determine the path length.
In this example, the total length of the path is 3dm.
- Finally, calculate the specific rotation.
Using the formula above, the specific rotation is found to be 25/(50*3) = .1666
- (deg*mL*g^-1*dm^-1)
About Specific Rotation
Is specific rotation concentration dependent? A specific rotation is dependent on the mass concentration of the compound in the solution.
Can specific rotation be negative? A specific rotation can be negative if the rotation counterclockwise is considered negative. The absolute specific rotation will always be positive.
