Molecular%20weight%20of%20non-volatile%20solid%200708

# Molecular%20weight%20of%20non-volatile%20solid%200708 -...

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APSC 175 Matter and Energy – Lab Sep-Dec 2007 University of British Columbia Okanagan - Engineering 1 DETERMINATION OF THE MOLECULAR WEIGHT OF A NON-VOLATILE SOLID Pre-lab Assignments 1. In an experiment, a 1.40 g sample of an unknown organic compound was dissolved in 14.3 g of naphthalene. The measured depression of the freezing point of naphthalene was 3.41 K. Calculate the molar mass of the unknown. 2. Calculate the freezing temperature of an aqueous solution that boils at 101.5 o C. K b for water is 0.52 K . kg/mol and K f for water is = 1.86 K . kg/mol. Introduction Colligative properties of a solution depend on the concentration of solute particles rather than on the nature of the solute, i.e. for a given solvent the colligative constants will be the same for all solutes. The four important colligative properties are: vapour pressure lowering , boiling point elevation , freezing point depression and osmotic pressure . The effects of these properties, although usually small, can be very important, especially in biological systems. At the freezing point of a pure solvent, molecules in the liquid phase solidify (freeze) at the same rate as molecules in the solid phase liquefy (melt). In a solution, the presence of non-volatile solute molecules will reduce the rate at which solvent molecules are freezing. Hence, in order to restore the equilibrium between the solid and liquid phase, the temperature has to be lowered. This depression in

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