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Colligative Properties

Key Questions

  • You determine colligative properties by making measurements.

    VAPOUR PRESSURE LOWERING

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    The vapor pressure lowering #ΔP# relative to that of pure solvent A is #ΔP = P_A - P#.

    BOILING POINT ELEVATION

    To determine the boiling point, you use an apparatus like the one shown below. You heat the liquid and measure the temperature when it boils.

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    The boiling point elevation #ΔT_b# relative to that of pure solvent #T_b^o# is #ΔT_b = T_b – T_b^o#.

    FREEZING POINT DEPRESSION

    To determine the freezing point, you place the solid in the apparatus shown below and warm the solid until it melts. Then you measure the temperature as it freezes.

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    The freezing point depression #ΔT_f# relative to that of pure solvent #T_f^o# is #ΔT_f = T_f^o – T_f#.

    OSMOTIC PRESSURE

    You use a membrane osmometer to measure the osmotic pressure #Π#.

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  • Colligative properties are physical properties of solutions that depend on the concentration of the particles and not on the kind of particles.

    These properties include the elevation of boiling point, the lowering of freezing point, a reduction of vapor pressure, and osmotic pressure.

    Consider vapor pressure. Pure Solvents that evaporate more easily exhibit a higher vapor pressure than those that do not. Alcohols tend to have a higher vapor pressure than water.

    If you add solutes to a solvent, this impedes the evaporation of the solvent thus reducing the vapor pressure of the solvent.

    If you lower the vapor pressure, then, the solvent will boil at a higher temperature. By adding sugar or table salt to water you will elevate the boiling point of water. Water boils at a temperature above 100 degrees Celsius.

    Similarly, the pure solvents will freeze at a lower temperature. By adding solutes, water will freeze below zero degrees.

    Osmotic pressure is greater for solutions with dissolved solutes than pure solvents. This is why salt is bad for people with hypertension because it raises the osmotic pressure within blood vessels.

    One final important concept to consider. If you have the same concentrations of non electrolytes versus electrolytes, which will solute will increase the colligative aspect. The answer is the electrolytes. Why?

    If you dissolve 1.0 molal sucrose, #C_12H_22O_11# in water and compare the boiling point of this solution to 1.0 molal of NaCl, you find that table salt has greater effect on the elevation of the boiling point. Sucrose is a nonelectrolyte. Table salt, when dissolving in water, dissociates into two ions, sodium ion and chloride ion, which carry charges and therefore are electrolytes.

    Because of dissociation the molal concentration doubles,; you have 1.0 molal of sodium ion and 1.0 molal of chloride ion which adds up to 2.0 molal.

    Hope that this explanation is helpful.

Questions