Molar Mass Determination by Depression
Chemistry 121
Experiment 19
Molar Mass Determination y Depression of the Freezing Point
Introduction:
The most commonly used liquid is water. In this experiment we study the equilibria that can exist between pure water and an aqueous solution, and ice, the solid form of water. The heat will transfer from a higher temperature to a lower temperature. In order for water to change states of matter, it takes a certain amount of kinetic energy or heat. The shift from ice to water (solid to a liquid) is called the heat of fusion. The shift from water to ice is called the freezing point of water, which the standard is usually 0°C. This is the point in which water and ice are at equilibrium. The freezing point Tf°, the vapor pressure of water and ice must be equal. If you add a soluble liquid or solid to the equilibrium mixture, the temperature of the ice and the solution will fall until it reaches equilibrium. The vapor pressure of water at 0°C is less than that of a pure liquid. The temperature of the new solution will change the in which it reaches equilibrium or the new freezing point Tf. The new freezing point will be below the freezing point of the pure liquid. ∆Tf is called the freezing point depression, which is the change in freezing point. Tfinal – Tinitial = ∆Tf . The freezing point depression is one of the colligative properties of a solution. Colligative properties include boiling point elevation, osmotic pressure, and vapor pressure lowering. When considering colligative properties it is easier to work on Molality. Molality is the solute concentration.
Molality of A = no. of moles A dissolved/ no. of kg solvent in solution
The boiling point elevation, Tb – kbm = ∆Tb , and the freezing point depression Tf – kfm = ∆Tf , uses the concentration. Kb and Kf are characteristics of the solvent used. They use these characteristics to find the molar mass of an unknown substance. Finding an unknown, finding two different concentrations, then
Experiment 19
Molar Mass Determination y Depression of the Freezing Point
Introduction:
The most commonly used liquid is water. In this experiment we study the equilibria that can exist between pure water and an aqueous solution, and ice, the solid form of water. The heat will transfer from a higher temperature to a lower temperature. In order for water to change states of matter, it takes a certain amount of kinetic energy or heat. The shift from ice to water (solid to a liquid) is called the heat of fusion. The shift from water to ice is called the freezing point of water, which the standard is usually 0°C. This is the point in which water and ice are at equilibrium. The freezing point Tf°, the vapor pressure of water and ice must be equal. If you add a soluble liquid or solid to the equilibrium mixture, the temperature of the ice and the solution will fall until it reaches equilibrium. The vapor pressure of water at 0°C is less than that of a pure liquid. The temperature of the new solution will change the in which it reaches equilibrium or the new freezing point Tf. The new freezing point will be below the freezing point of the pure liquid. ∆Tf is called the freezing point depression, which is the change in freezing point. Tfinal – Tinitial = ∆Tf . The freezing point depression is one of the colligative properties of a solution. Colligative properties include boiling point elevation, osmotic pressure, and vapor pressure lowering. When considering colligative properties it is easier to work on Molality. Molality is the solute concentration.
Molality of A = no. of moles A dissolved/ no. of kg solvent in solution
The boiling point elevation, Tb – kbm = ∆Tb , and the freezing point depression Tf – kfm = ∆Tf , uses the concentration. Kb and Kf are characteristics of the solvent used. They use these characteristics to find the molar mass of an unknown substance. Finding an unknown, finding two different concentrations, then