Dear ChE 530 students,

I received two questions by email for which I thought everyone would be interested in the answer.

> Can we use our homeworks during the final exam?

Yes, that's fine. Homeworks and hwk solutions are part of the class material.

> For question #1 on the May 7, 2007 final exam, what exactly is supposed to be > done for part A?

Part of what I was looking for in #1a was each student's ability to recognize the kinds of analysis that are typical for these experiments. For osmometry data, that involves using the y-intercept to determine the molecular weight and the slope to determine the second virial coefficient, incorporating all of the necessary conversion factors (RT with the intercept, for example). For static light scattering (i.e. the Zimm plot), that involves calculating the molecular weight from the common intercept and then using the slopes to calculate the second virial coefficient (from the angle -> 0 line) and the radius of gyration (from the c -> 0 line).

One semi-common homework mistake was using slopes of the wrong lines to obtain B and <(r_g)^2>. The line for zero concentration leaves only the parts of the equation that involve sin^2 (theta/2), molecular weight, and radius of gyration. The line for zero angle leaves only the parts that include concentration c, molecular weight, and second virial coefficient B.

For each method, it is important to understand what kind of molecular weight is obtained (number average, weight average, viscosity average, entire distribution, ...). I haven't provided the answers above, but you can figure them out from the text and your class notes.

Hope this helps, Prof. Greenfield

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