ComputerUsing Freezing-Point Depressionto Find Molecular WeightWhen a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the number of moles of solute added. This property, known as freezing-point depression, is a colligativeproperty; that is, it depends on the ratio of solute and solvent particles, not on the nature of the substance itself. The equation that shows this relationship is:T= Kf • mwhere Tis the freezing point depression, Kfis the freezing point depression constant for a particular solvent (3.9°C-kg/mol for lauric acid in this experiment1), and mis the molality of the solution (in mol solute/kg solvent).In this experiment, you will first find the freezing temperature of the pure solvent, lauric acid, CH3(CH2)10COOH. You will then add a known mass of benzoic acid solute, C6H5COOH, to a knownmass of lauric acid, and determine the lowering of the freezing temperature of the solution. In an earlier experiment, you observed the effect on the cooling behavior at the freezing point of adding a solute to a pure substance. By measuring the freezing point depression, T, and the mass of benzoicacid, you can use the formula above to find the molecular weight of the benzoic acid solute, in g/mol.OBJECTIVESIn this experiment, you willDetermine the freezing temperature of pure lauric acid.Determine the freezing temperature of a solution of benzoic acid and lauric acid.Examine the freezing curves for each.Calculate the experimental molecular weight of benzoic acid.Compare it to the accepted molecular weight for benzoic acid.Figure 1MATERIALScomputerutility clampVernier computer interface18 150 mm test tubeLoggerProlauric acidTemperature Probebenzoic acid400 mL beakerthermometerring standPROCEDURE
Computer 151.Obtain and wear goggles.2.Connect the Temperature Probe to the computer interface. Prepare the computer for data collection by opening the file “15 Freezing Pt Depression” from the Chemistry with Vernierfolder.