Experiment 4 COLLIGATIVE PROPERTIES MOLAR MASS DETERMINATION

Experiment 4: COLLIGATIVE PROPERTIES: MOLAR MASS DETERMINATION BY FREEZING POINT DEPRESSION Objective: The purpose of the experiment is to become familiar with colligative properties and use them to determine the molar mass of a substance. Background and Introduction: Solutions are homogeneous mixtures that contain two or more substances. In simple two component (a.k.a. binary) solutions, the major component is called the solvent and the minor component is called the solute. Because the solution is composed primarily of solvent, the physical properties of a solution resemble those of the solvent. Some of these physical properties, called colligative properties, are independent of the nature of the solute and depend only on the concentration of the solute in the solution. Colligative properties include freezing point depression, boiling point elevation, vapor pressure lowering, and the generation of osmotic pressure. Vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed container. When the vapor pressure of a liquid is equal to the atmospheric pressure, the liquid boils. All liquids (and solids for that matter) have a tendency to evaporate into a gaseous form, and all gases have a tendency to condense back to their liquid or solid form. Experiments have found that dissolution of a non-volatile solute lowers the vapor pressure of the solvent, which in turn, raises the boiling point and lowers the freezing point of the solution relative to that of the pure solvent. You may be familiar with some of the common, everyday applications of colligative properties. Antifreeze is added to the coolant (water) in automotive engines to raise the boiling point and lower the freezing point so the engine can operate optimally across a wider temperature range than when water alone is used as the coolant. We spread salt on icy steps in the winter to lower the freezing point and melt the ice. These effects are expressed qualitatively by the colligative property law, which states that the freezing point and boiling point differ from those of the pure solvent by amounts that are proportional to the molal (containing 1 mole of solute per kilogram of solvent) concentration of solute. The relationship between the lowering of the freezing or boiling point and concentration is shown below: ar,-Kf-mn and ATb = Kb.ml Where AT, and AT, are the freezing point depression and boiling point elevation amounts, Kr and K, are constants specific for a solvent, and m is the molality of the solution. The Krconstant for cyclohexane, the solvent used in this experiment, is 20.4°C/m 20

Solution

Question is well received torelatesolute concentrationwith molalityand depressionin freezing point. Alsocorrectlylabmethodis described for determinationof density of water. It is worth point to tell that only non volatiesolutes are preferred incolligiative properties calculation. Method is well describers.