Question

1.Why would it be important to place the same amount of a solution in the polarimeter tube when measuring optical rotation as the amount of solvent when measuring theblank?

a. Only a certain volume of liquid can be placed in a polarimeter tube.

b.So the path length can be easily converted to mL\'s when calculating optical rotation.

c. So the density of the blank can be added to that of the solution when calculating optical rotation.

d. None of the answers shown are correct.

e. Biots law, used to calculate optical rotation, requires that the volume of the blank be subtracted from that of the solution to arrive at a proper density value.

2. The optical rotation of a sample composed of only L-alanine is determined to be [α]obs = 0.33°. If the cell path length was 1.2 dm and the concentration of L-alanine in the sample was 1.3 g/ml what is the specific observed rotation?

3. A mixture of equal amounts of two enantiomers ___________

a. optically active

b.its called a racemic mixture

c. implies that enantiomers are meso form

d. racemic optically active

e. all answers are incorrect

4. Suppose a student is measuring the optical rotation of a 0.541 M pure sample of (S)-(–)-limonene. The path length is 6.51 cm. What optical rotation should the student expect to measure? (S-(–)-limonene [α]D = –122.1°)

Answer 1

1.It be important to place the same amount of a solution in the polarimeter tube when measuring optical rotation as the amount of solvent when measuring theblank because the blank and samples are comparable only in identical conditions.

So the correct answer would be,

d. None of the answers shown are correct.

2. The optical rotation of a sample composed of only L-alanine is determined to be [α]obs = 0.33°. If the cell path length was 1.2 dm and the concentration of L-alanine in the sample was 1.3 g/ml what is the specific observed rotation is calculated as,

specific rotation = 0.33/1.2 x 1.3 = 0.2115 degree

3. A mixture of equal amounts of two enantiomers is,

b.its called a racemic mixture

4. Suppose a student is measuring the optical rotation of a 0.541 M pure sample of (S)-(–)-limonene. The path length is 6.51 cm. the optical rotation should the student expect to measure? (S-(–)-limonene [α]D = –122.1°) would be,

-122.1 = optical rotation x 1000 / 0.541 x 6.51

thus, optical rotation measured would be = 0.430 degrees