Freezing-point depression of a solvent due

The freezing-point depression of a solvent due to the presence of a solute (Sec. 

Freezing-Point Depression of a Solvent Due to the Presence of a Solute 673 However, for the solutes we use... 

FREEZING-POINT DEPRESSION OF A SOLVENT DUE TO THE PRESENCE OF A SOLUTE THE FREEZING POINT OF LIQUID MIXTURES... 

The depression of the freezing point of a solvent due to the presence of a dissolved solute is an example of a colligative property, that is, a property of a dilute solution that depends on the number of dissolved particles and not on the identity of the particles. Water has a freezing point depression constant, Kf, of 1.86 K kg mol-1. In other words, for every mole of nonvolatile solute dissolved in a kilogram of water, the freezing point of water is lowered by 1.86°C. The change in freezing point, A T, can be calculated from the equation... 

Raoult, Francois-Marie (1830-1901) A French chemist who studied the behaviour of solutions and their physical properties. He is best known for his work on solutions, in particular the depression of freezing points and the depression of a solvent s vapour pressure due to a solute that was shown to be proportional to the solute s molecular weight. Both became ways of determining molecular weights of organic substances. "Raoult s law is named after him. 

In physical chemistry, we apply the term colligative to those properties that depend upon number of molecules present. The principal colligative properties are boiling point elevation, freezing point depression, vapour pressure lowering, and osmotic pressure. All such methods require extrapolation of experimental data back to infinite dilution. This arises due to the fact that the physical properties of any solute at a reasonable concentration in a solvent are... 

The phenomena of freezing-point depression, boiling-point elevation, and osmotic pressure all result from the reduction in the escaping tendency of solvent in a solution due to the addition of solute. They can all be used to determine the activity of the solvent. For example, as long as pure solvent freezes out of the... 

An alternative approach is based on the theoretical foundation described earlier for the colligative properties. If the solution is isotonic with blood, its osmotic pressure, vapor pressure, boiling-point elevation, and freezing-point depression should also be identical to those of blood. Thus, to measure isotonicity, one has to measure the osmotic pressure of the solution and compare it with the known value for blood. However, the accurate measurement of osmotic pressure is difficult and cumbersome. If a solution is separated from blood by a true semipermeable membrane, the resulting pressure due to solvent flow (the head) is accurately measurable, but the solvent flow dilutes the solution, thus not allowing one to know the concentration of the dissolved solute. An alternative is to apply pressure to the solution side of the membrane to prevent osmotic solvent flow. In 1877, Pfeffer used this method to measure osmotic pressure of sugar solutions. With the advances in the technology, sensitive pressure transducers, and synthetic polymer membranes, this method can be improved. However, results of the search for a true semipermeable membrane are still... 

If a small amount of solute (gas, liquid, or solid) dissolves in a liquid solvent and the temperature of the mixture is lowered, a temperature Tf is reached at which the pure solvent begins to separate out as a solid. This temperature is lower than the freezing (or melting) point of the pure solvent, Tm- Here we are interested in estimating the depression of the solvent freezing point, AT = Tm — Tj, due to the addition of the solute. This equilibrium state is, in a sense, similar to the one encountered in the solubility of a solid in a liquid in that there we were interested in. the equilibrium between a solid and a dilute liquid mixture of that component, whereas here we are interested in the equilibrium between a solid and a liquid mixture that is concentrated in that component. 

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