Molecular mass is a parameter, which is useful in determining the identity of an unknown compound. One technique to determine the molecular mass of an unknown is to measure the effect the compound has on the freezing point of a solvent in which the unknown is dissolved. The freezing point of a solution is a colligative property. That is, it is a property which varies based on the number of particles (solute) dissolved in the solvent and not on the chemical makeup of the particles themselves. Other colligative properties, which also can be used to determine molecular mass, are osmotic pressure, vapor pressure and boiling point.
The nonpolar solvent 2,6-di-tert-butyl-4-methylphenol has a freezing point of approximately 70°C. A quantity of para-dichlorobenzene will be dissolved in the solvent and the effect on the freezing point determined. The freezing point depression constant will be calculated for the solvent. The experiment will be repeated with each of two unknowns and the molecular weight of the unknowns will be determined from the freezing point depression.
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