Collegative Properties

Experiment (5) Collegative Properties Objectives: To determine the molar mass of a non-electrolyte compound in water by measuring the freezing point of pure water and of solution. Introduction: The properties of a solution differ considerably from those of the pure solvent. Those solution properties which depend primarily on the concentration of solute particles rather than their nature are called collegative properties. These properties include vapor pressure lowering, osmotic pressure, boiling point elevation and melting point depression. The relationships among colligative properties and solute concentration are best regarded as limiting laws. They are approached most closely when the solution is very dilute. In practice, the relationships are valid, for nonelectroytes to within a few percent at concentrations as high as 1M. Solutions usually boil at higher temperature and freeze at a lower temperature than pure solvent. A good example of solutes used to decrease the freezing point of water is ethylene glycol or called antifreeze which is added to car's radiator to prevent freeze-up of water in winter and boiling in summer. The change in boiling or freezing point is a direct result of vapor pressure lowering. At any given temperature, a solution of a nonvolatile solute has a vapor pressure lower than that of the pure solvent. Hence, a higher temperature must be reached before the solution boils. Figure (1) illustrates this reasoning graphically. Figure (1) Figure 1 shows that the freezing point of solution is the temperature at which the solvent in solution has the same vapor pressure as the pure solid solvent. This implies that it is pure solvent that separate when the solution freezes. The increase in boiling point, and the decrease in freezing point, , (usually called boiling point elevation) , (Freezing point depression) are directly proportional to the number of solute particles per mole of solvent molecules. Where Kb and Kf are the solvent's molal boiling point and freezing point constants respectively, which have the unit of Co kg mol-1, and m is the molality of the solution which is the number of moles of solute per one kilogram of solvent. m = mole of solute / mass of solvent in kg. Example: What would be the freezing point and boiling point of a solution containing 65.0g of ethylene glycol (C2H6O2) commonly used as an automotive antifreeze, in 200.0 g of water. (for pure water Kf = 1.86Co/m, Kb = 0.52Co/m, Tf = 0Co, Tb = 100Co). Moles of C2H6O2 = mass/molar mass = 65.0g/62.0 g/mol = 1.05 mol M = moles of solute/kg of solvent = 1.05 mol/0.200 kg = 5.24 mol/kg = 1.86oC/m x 5.24m = 9.75oC 0-0.75 = 9.75oC the solution freezes at -9.75oC = 0.52oC/m x 5.24m = 2.70oC 100 + 2.70 = 102.70oC 100 + 2.70 = 102.70oC the solution boils at 102.70oC In this experiment,, you will determine the formula weight of a non-volatile, non-electrolyte compound in water by measuring the freezing point of pure water and of solution. Procedure: A-Freezing point of pure solvent (H2O): 1- Weigh a clean and dry 200-mL test tube. 2- Add 30.0 mL of water on the test tube and assemble the apparatus shown in figure 2. 3- Place the test tube in a 400-mL beaker containing ice, water and table salt. 4- Record the freezing point of water: the temperature at which two phases (ice and water) co-exist. B- Freezing point of solution: 1- Remove the test tube from the cooling bath and melt the solidified water by pouring tap water at its outside. Dry the outside and reweight. 2- Add 2.5 g of the solid unknown and repeat steps 3 and 4 above. 3- Record the freezing point of the mixture. Figure (2): Freezing point apparatus. Data and Results: Trail 1 Trial 2 Volume of water ……………. ……………. Freezing point of water ……………. ……………. Part A. Freezing point of pure water: Part B. Freezing point of Solution; Unknown No:…………… . Volume of water ……………. ……………. Mass of solute ……………. ……………. Freezing point of solutin ……………. ……………. Calculations: Molal freezing point depression constant for water = 1.86oC molDensity of water = 100g mL-1 Mass of water in solution ……………. ……………. Mass of solute in solution ……………. ……………. Molar mass of unknown ……………. ……………. * Show all your calculations for Trail 1.