Optical Purity Calculator
Find the optical purity (and enantiomeric excess) of a sample of chiral molecules by comparing its measured specific rotation to that of the pure enantiomer.
What optical purity measures
When a sample contains a mixture of two enantiomers (mirror-image forms of a chiral molecule) rather than just one, the mixture’s ability to rotate plane-polarized light is weaker than that of the pure enantiomer alone. Optical purity compares the two rotations to tell you how enantiomerically pure a sample is.
The formula
Optical purity is calculated as OP% = (observed specific rotation ÷ specific rotation of the pure enantiomer) × 100. For most simple two-component mixtures, optical purity is numerically equal to enantiomeric excess (ee%), which is the percentage by which one enantiomer exceeds the other (50% + ee/2 of the major form and 50% − ee/2 of the minor form).
Note: This relationship assumes rotation is measured under matching conditions (same solvent, concentration, temperature, and wavelength) and that the reference rotation value for the pure enantiomer is accurate. It is intended for general chemistry education and lab planning, not as a substitute for calibrated instrument analysis.