The specific rotation of a pure substance is +2.5°. What is the specific rotation of a mixture containing 7 5% of this isomer and 25% of the (-) isomer?

Specific rotation of a mixture containing 75% of (+) and 25% of the (-) isomer= Specific rotation  of pure substance ×% of d−% of l100=+2.5×75−25100=+2.5×0.5=+1.25∘