If the specific rotation of (r)-2-butanol is −13.5° then
the specific rotation of (s)-2-butanol is simply the opposite in sign 13.5°. In
a 3:1 mixture of (s):(r) 2-butanol, 75% is (s) while 25% is r, so the specific
rotation of the mixture is:
mixture rotation = 0.75 * (13.5°) + 0.25 * (- 13.5°)
mixture rotation = 6.75°
