A solution containing 0.04 g/mL of a pure S enantiomer in a 1 dm polarimeter rotates plane polarized light by +2$\degree$. What is the specific rotation of the R isomer?

Question

Quizplus · Accepted Answer

The specific rotation of a compound is given by the following formula:

α = α_observed / (c*l)

where α is the specific rotation, α_observed is the observed rotation in degrees, c is the concentration of the solution in g/mL, and l is the path length of the sample tube in dm.

In this case, we are given that the solution contains 0.04 g/mL of the S enantiomer, and that it rotates plane polarized light by +2°F10S°. The path length is not given, but it is usually assumed to be 1 dm in polarimetry.

So, we can calculate the specific rotation of the S enantiomer using the formula above:

α(S) = +2°F10S° / (0.04 g/mL * 1 dm) 
α(S) = +50°F10S° / g*cm^-3*dm

However, we want to know the specific rotation of the R enantiomer. We know that enantiomers have equal and opposite specific rotations, so we can use the formula:

α(R) = -α(S)

to find the answer.

α(R) = -(+50°F10S° / g*cm^-3*dm) 
α(R) = -50°F10S° / g*cm^-3*dm

Therefore, the answer is choice A) -50°F10S°.

A Solution Containing 0 °\degree° What Is the Specific Rotation of the R Isomer

A Solution Containing 0 $\degree$ What Is the Specific Rotation of the R Isomer