Question

The specific rotation of (S)-2-butanol is +13.5. If 0.119 g of its enantiomer is dissolved in 10.0 mL of ethanol and

placed in a sample cell with a length of 1.00 dm,
what observed rotation do you expect?

Answer 1

+15.8°

Step-by-step explanation:

The formula for the observed rotation (α) of an optically active sample is

α = [α]lc

where

l = the cell path length in decimetres

c = the concentration in units of g/100 mL

[α] = the specific rotation in degrees

1. Convert the concentration to units of g/100 mL

`c = \frac{\text{0.119 g}}{\text{10.0 mL}} * (10 )/(10 ) =\frac{\text{1.19 g}}{\text{100 mL}}`

2. Calculate the observed rotation

`\alpha = (+13.3 ^(\circ)) * 1.19 * 1 = +15.8^(\circ)`