Molar Mass Determination by Depression of the Freezing Point February 7, 2012 Adrienne Oxley Lab Partner: Everett Spell Title page includes the name of the experiment, the date, your name and your lab partner’s name.
Introduction: This experiment focused on colligative properties, properties that are based on the concentration of solute particles present and not the chemical identity of the solute. Freezing point depression was used to study the molar mass of a substance.1This colligative property is based on the idea that a solution freezes at a lower temperature than the pure solvent.1,2By adding unknown impurities to a solution of water and studying the effects this solute “impurity” has on the freezing point, valuable information about the identity of the substance can be obtained. The following equation describes the idea of freezing point depression: ΔTf= iKfm The ΔTfterm indicates the change in temperature observed for the solution relative to that of the pure substance (the freezing point depression), kfis a term specific to the identity of the solvent being used (in this case water is used and kfis 1.86 ⁰C/m), and mis the molality of particles in the solution (a measure of moles of solute per kg of solvent). By measuring a change in temperature, the molality of an aqueous solution can be calculated and this information can subsequently be used to determine molar mass (provide the initial mass of sample was recorded). Figure 1 shows a typical phase diagram for a pure solvent (the smaller line inside) and the effects on the phase diagram when a solute is added to make a solution (outer line). The equilibrium lines present in this phase diagram are shifted in the presence of a Figure 1. Phase diagram illustrating fp depression3In text references are shown as superscripts which correspond to a numbered reference found at the end of the report (in the reference section) Introduction includes explanation of concepts found in key equations. All variables used are clearly defined (see highlighted portion) Use figures whenever possible to illustrate points (Note: if you use a figure, you need to reference it and refer to it in the text –there should be explanation of the figure).