Solutions lab lecture practice questions (expts 1) fall 2016 PDF

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Solutions lab lecture practice questions (expts 1) fall 2016...

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SOLUTIONS to practice questions from EXPT 1 lab lecture notes (mp range determination) 1) For what reason(s) would someone be interested in determining the melting point (mp) range of a compound? There are three principal reasons for determining the mp range of a solid: a) determination of its PURITY b) establishing an unknown solid's IDENTITY (including mixed mp range determination) c) reporting NEW PHYS. PROP. DATA on a compound never before isolated/made

2) What would be the effect of an insoluble (immiscible) impurity (e.g., sand or charcoal) on the mp range of an otherwise pure compound? An insoluble impurity has no effect on the mp range. Recall, for melting/freezing point depression the impurity must dissolve in the melting solid.

3) Three vials, labelled A, B & C, contain substances with nearly identical mp ranges. What specific experimental steps could you take to prove that the contents of A, B and C were different substances, and what observations would lead you to that conclusion? Make mixtures of A + B, B + C, and A + C and determine their mp ranges. If A ≠ B ≠ C, the mp ranges of all the mixtures should be broad and depressed.

4) Too rapid heating of a sample during mp range determination causes an inaccurate observed mp range. Exactly how would the observed range compare with the true mp range if you made this error, and explain why this would be so. Heating > ≈2 oC/min during a mp range determination would result in an abnormally low mp range. For a thermometer to read the correct temperature it must be in equilibrium with the system. If you raise the heat too rapidly the temperature of the thermometer will lag behind that of the sample and, even though your sample will melt at the temperature that it should, the reading on the thermometer will have not yet reached that temperature....