Assignment 6 Solutions.doc - Chemistry 221 Assignment 6 1 A sample of a pure liquid in a 10-cm(that’s 1 dm tube is placed in the polarimeter and the

Assignment 6 Solutions.doc - Chemistry 221 Assignment 6 1 A...

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Chemistry 221 Assignment 6 1. A sample of a pure liquid in a 10-cm (that’s 1 dm) tube is placed in the polarimeter and the reading is +45 o . How could you tell that it is really +45 o and not -315 o ? After all, that would look the same on the dial (think about it, there are 360 degrees in a circle). As a start, consider the lab you did a couple of weeks ago. Consider what’s going on in the tube. Each molecule encountered by the light rotates it a little bit. All you see is the sum of the interactions, so you can’t tell in which direction the rotation took place. If there were fewer (or more) encounters, though, there would be a different sum. Imagine cutting the path length by two thirds. Now, only one third as many encounters would occur, and the rotation would be only one third as much. If the original rotation had been +45 o , therefore, the new value would be only +15°. If the original value had been -315°, the new value would be -105°. In real experiments on new compounds, it is necessary to carry out a series of measurements at various concentrations to establish the specific rotation.
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