Question

What observed rotation is expected when a 1.62 M solution of (R)-2-butanol is mixed with an equal volume of a 0.810 M solution of racemic 2-butanol, and the resulting solution is analyzed in a sample container that is 1 dm long? The specific rotation of (R)-2-butanol is –13.9 degrees mL g–1 dm–1. The unit of the answer is in degree, NOT in degrees mL g–1 dm–1.

Answer 1

volume is doubled = concentration is halved

Molarity of (R)-2-butanol = 1.62 / 2 = 0.81 M

concentration of 2-butanol = 0.81 mol/ Lx 74.12 g/mol

                                          = 60.04 g/L

                                          = 0.060 g/mL

specific rotation = - 13.9 deg mL g-1 dm-1

observed rotation = - 13.9 x 1 x 0.060

                             = - 0.834 deg