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• CHMI 5 ILL: DETERMINATION OF MOLAR MAsS BY fREEzING-POINT DEPRESSION 11 Determination of Molar Mass by Freezing-Point Depression OBJECTIVES: • Gain familiarity with colligative properties of nonelectrolyte solutions • Find the molar mass of a solute by the method of freezing-point depression • Evaluate the accuracy of the method by comparison.to a molar mass calculated from a molecular formula. DISCUSSION: When a solution forms, the freezing point of the solution is lower than the freezing point of the pure solvent The magnitude of this.freezing-point depression (llT,n depends only on the number ratio of solvent and solute molecules (or ions) in the solution, not on their chemical identity. This makes llT1one of the colligativeproperties ·of solutions. If . ( ~~~) we use molal concentration m defined so that m = kg solvent , then the freezing-point depression follows a simple direct proportionality relationship: llT1= KJ m (1) where the proportionality constant, KJ, is called the molal freezing-point depression constant. Laurie acid (the solvent in this experiment) has a reported KJ = 3.9 °C·kg/mol = 3.9 °Clm. In this experiment, you will determine the freezing point of the pure solvent, CH3(CH2)1oCOOH (lauric acid). You will then find the freezing point of a lauric acid solution that contains a measured mass of solute CdlsCOOH (benrojc acid) and determine the freezing-point depression. Using the experimental llT1-value and the reported KJ value in equation (1) will enable you to find the amount (in moles) of benzoic ,acid in your solution. This value, along with the known mass of benroic acid, le~ds to the molar mass determination. PROCEDURE: Part I-Freezing point of pure lauric' acid 1. Set up two water baths ( one above 70°C and one at roo~ temperature) using_ 400-mL beakers filled to about the 300-mL level. Obtain a sample of pure lauric acid in a sealed test tube. Remove the stopper with care and then place this tube in the hot water bath to melt the solid lauric acid. 2. Once the lauric acid has melted completely, insert a thermometer or temperature probe into the hot liquid lauric acid. Check that the thermometer reads above 55 °C. 3. Remove the test tube/thermometer assembly from the hot bath and clamp it in the air or suspended in the room temperature bath.