14B FP Depression.doc - CHM151LL DETERMINATION OF MOLAR MASS BY FREEZING-POINT DEPRESSION | 1 Determination of Molar Mass by Freezing-Point Depression

# 14B FP Depression.doc - CHM151LL DETERMINATION OF MOLAR...

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CHM151LL: D ETERMINATION OF M OLAR M ASS BY F REEZING -P OINT D EPRESSION | 1 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 ( ∆T f ) depends only on the number ratio of solvent and solute molecules (or ions) in the solution, not on their chemical identity. This makes ∆T f one of the colligative properties of solutions. If we use molal concentration c m defined so that 1 m = 1 mol solute 1 kg solvent æ è ç ö ø ÷ , then the freezing-point depression follows a simple direct proportionality relationship: ∆T f = K f c m (1) where the proportionality constant, K f , is called the molal freezing-point depression constant. Lauric acid (the solvent in this experiment) has a reported K f = 3.9 °C·kg/mol = 3.9 °C/ m . In this experiment, you will determine the freezing point of the pure solvent, CH 3 (CH 2 ) 10 COOH (lauric acid). You will then find the freezing point of a lauric acid solution that contains a measured mass of solute C 6 H 5 COOH (benzoic acid) and determine the freezing-point depression. Using the experimental ∆T f value and the reported K f 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 benzoic acid, leads 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 room temperature) using 400-mL beakers filled to about the 300-mL level. Obtain a sample of pure lauric acid in a sealed test tube.  • • • 