You have a racemic mixture of (+)-2-butanol and -butanol. the (+) isomer rotates polarized light by +13.5∘. what is the observed rotation of your mixture?

The degree of rotation of racemic mixture will equal 0° 

Explanation:

Hello,

(-) isomer is in other words the Enantiomer of (+) isomer and as long as the statement indicates that (+)-isomer optical rotation is + 13.5°, the optical rotation of (-)-isomer will be the same value and degree but with the inverse sign which is -13.5°.

In such a way, the degree of rotation of the taken into account racemic mixture will equal 0° 

This is in this way due to the fact that the racemic mixture has equal amount of both enantiomers.

Best regards.