How can a compound with an optical rotation of +233.0° be discerned from a compound with an optical rotation of -127.0°?
A. The intensity of the light is greater with the positive optical rotation.
B. The sample with +233.0° optical rotation when diluted to half of its original concentration would show an optical rotation of +116.5°.
C. The larger the absolute value of the optical rotation, the greater the density of the compound.
D. It is not possible to distinguish the two compounds from one another.
Answer:
B) The sample with +233.0° optical rotation when diluted to half of its original concentration would show an optical rotation of +116.5°.
When using a polarimeter, an observed optical rotation of +233.0° and -127.0° would result in the same reading (given that a full circle is 360°). To discern one optical rotation from the other, the sample should be diluted to reduce the magnitude of the observed rotation. If the actual optical rotation were in fact +233.0°, then the lower concentration would show a rotation less than +233.0° (less clockwise). If the actual optical rotation were in fact -127.0°, then the lower concentration would show a rotation of lesser magnitude than -127.0° (less counterclockwise). If the solution concentration were cut in half for instance, the observed rotation would be either +116.5° or -63.5°. The change in rotation can therefore determine the original rotation value. The only answer that indicates changing the concentration is choice B. The intensity of the light depends on absorption, not rotation of plane-polarized light, so it's not choice A. The magnitude of rotation does not depend on the solution density or compound density, so eliminate choice C.
I'm having a hard time understanding their reasoning
A. The intensity of the light is greater with the positive optical rotation.
B. The sample with +233.0° optical rotation when diluted to half of its original concentration would show an optical rotation of +116.5°.
C. The larger the absolute value of the optical rotation, the greater the density of the compound.
D. It is not possible to distinguish the two compounds from one another.
Answer:
B) The sample with +233.0° optical rotation when diluted to half of its original concentration would show an optical rotation of +116.5°.
When using a polarimeter, an observed optical rotation of +233.0° and -127.0° would result in the same reading (given that a full circle is 360°). To discern one optical rotation from the other, the sample should be diluted to reduce the magnitude of the observed rotation. If the actual optical rotation were in fact +233.0°, then the lower concentration would show a rotation less than +233.0° (less clockwise). If the actual optical rotation were in fact -127.0°, then the lower concentration would show a rotation of lesser magnitude than -127.0° (less counterclockwise). If the solution concentration were cut in half for instance, the observed rotation would be either +116.5° or -63.5°. The change in rotation can therefore determine the original rotation value. The only answer that indicates changing the concentration is choice B. The intensity of the light depends on absorption, not rotation of plane-polarized light, so it's not choice A. The magnitude of rotation does not depend on the solution density or compound density, so eliminate choice C.
I'm having a hard time understanding their reasoning