Determination of Molar Mass by Freezing-Point Depression.pdf - CHMI 5ILL DETERMINATION OF MOLAR MAsS BY fREEzING-POINT DEPRESSION 11 Determination of

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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.