lab 2 - Freezing Point Depression Revised 4/11/06 USING...

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Freezing Point Depression Revised 4/11/06 USING FREEZING POINT DEPRESSION TO FIND MOLECULAR WEIGHT Adapted from "Chemistry with Computers" Vernier Software, Portland OR, 1997 OBJECTIVES For you, the student, to write…. INTRODUCTION When a solute is dissolved in a solvent, the freezing point temperature is lowered in proportion to the number of moles of solute added. This property, known as freezing point depression is a colligative property ; that is, it depends of the ratio of solute and solvent particles and not on the nature of the substance itself. The equation that shows this relationship is: Δ T = K f · m where Δ T is the freezing point depression, K f is the freezing point depression constant specific for a given solvent (3.90 °C-kg/mol for lauric acid in this experiment), and m is the molality of the solution. Molality (m) is used because it is independent of the volume changes that can occur with variations in temperature, unlike molarity (M). molality (m) = moles of solute / kg of solvent In this experiment, the freezing temperature of the pure solvent, lauric acid, CH 3 (CH 2 ) 10 COOH is found first. Next the freezing point of a solution prepared from a mixture of an unknown organic acid and lauric acid is measured. By measuring the masses of the unknown organic acid and lauric acid in the mixture and calculating freezing point depression, Δ T, and, the equations above can be used to find the molecular weight of the unknown acid. 1
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Freezing Point Depression Revised 4/11/06 molecular weight = mass / moles Look up the melting point (freezing point) of pure lauric acid (CH 3 (CH 2 ) 10 COOH, also called "dodecanoic acid"). Physical constants and properties of compounds in "The Merck Index" or the "CRC Handbook of Chemistry and Physics" or go online. Copies of Merck and CRC are available in the reference department of the Science Library. Complete all calculations in lab ; no post lab is due next week for this experiment. PROCEDURES 1. Work in pairs. Wear safety goggles and lab aprons at all times. Keep the hot acids in the hoods. Important: This lab is "messy" and part of your score will be based on how well you clean your lab bench, equipment, hood, and balance areas. The TA will be observing closely to see that all areas are kept clean. 2. Prepare the computer for data collection by opening "Exp 15" from the "Chemistry with Computers" experiment files of Logger Pro. Connect the temperature probe to Channel 1 and select "Exp 15 Stainless-Steel Temp". The horizontal axis has time scaled from 0 to 10 minutes. Change the time intervals by pulling down “Experiment” from the menu bar to Data Collection. Then select Sampling and under Sampling Speed select every 10 seconds (6 samples per minute). This will give better results when determining the freezing point of the unknown solution.) Make sure the temperature is displayed to two decimal places by selecting the “Data” menu and choosing “Column Options” and “Temperature”. Click on the “Options” tab.
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lab 2 - Freezing Point Depression Revised 4/11/06 USING...

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