# Colligative Properties Colligative properties depend only on the number of solute particles present, not on the identity of the solute particles. Among.

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Colligative Properties Colligative properties depend only on the number of solute particles present, not on the identity of the solute particles. Among colligative properties are  Vapor pressure lowering  Boiling point elevation  Freezing point depression  Osmotic pressure

Vapor Pressure Lowering

The vapor pressure necessary to achieve equilibrium with the pure solvent is higher than that required with the solution. Consequently, as the pure solvent seeks to reach equilibrium by forming vapor, the solution seeks to reach equilibrium by removing molecules from the vapor phase. A net movement of solvent molecules from the pure solvent to the solution results. The process continues until no free solvent remains.

Boiling Point Elevation and Freezing Point Depression Solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent. For example, the addition of salt to water causes the water to freeze below its normal freezing point (0°C) and to boil above its normal boiling point (100°C).

Bioling point elevation At the normal boiling point of the pure liquid, the vapor pressure of the liquid, Po = 1 atm. The addition of a solute to the liquid again ALWAYS LOWERS the vapor pressure of the solution relative to the vapor pressure of the pure liquid at the same temperature. In order to bring the vapor pressure of the solution back up to the applied pressure Po = 1 atm., the temperature of the solution must be increased by an appropriate amount so that the vapor pressure is equal to the applied pressure. This is the ELEVATION of the boiling point.

Freezing point depression The freezing point of a solution is the temperature at which the first crystals of pure solvent begin to form.

Freezing point depression

Basically, adding solute (such as NaOH) to a solvent (such as water) causes the solute to "hold on" to solvent molecules that would otherwise escape from the liquid (in the case of boiling point raising) or bind to other like solvent molecules during the formation of the solid phase (in the case of freezing point reduction).

Boiling Point Elevation and Freezing Point Depression In both equations,  T does not depend on what the solute is, but only on how many particles are dissolved.  T b = K b  m  T f = K f  m

Osmosis Semipermeable membranes allow some particles to pass through while blocking others. Because only solvent can pass through the semipermeable membrane, the driving force of osmosis is the inequality of the chemical potentials of solvent on opposing sides of the membrane. So, direction of osmotic flow from pure solvent where chemical potential is highest because there is more moles of it, to the concentrated solution, where number of moles and therefore the chemical potential is low

Osmosis

. In biological systems, most semipermeable membranes (such as cell walls) allow water to pass through, but block solutes.

Osmotic Pressure To stop osmosis, the chemical potential of the solvent in the more concentrated solution can be increased by forcing the molecules closer together under an externally applied pressure.

Osmotic Pressure The pressure required to stop osmosis, known as osmotic pressure, , is nVnV  = ( ) RT = MRT where n is number of moles of solute, V volume of solution, M is the molarity of the solution T is thermodynamic temperature R is gas constant

Osmotic Pressure If the osmotic pressure is the same on both sides of a membrane (i.e., the concentrations are the same), the solutions are isotonic.

Classification of Solutions According to Their Osmotic Pressure: Hypertonic Hypotonic Isotonic

Blood and fluids of eye, nose and bowel are the major concern for a pharmacist in the manufacture of preparations to be mixed with these biological fluids e.g. ophthalmic, parenteral, nasal and enema.

Osmosis in Blood Cells If the solute concentration outside the cell is greater than that inside the cell, the solution is hypertonic. Water will flow out of the cell, and crenation (shrinking) results.

Osmosis in Cells If the solute concentration outside the cell is less than that inside the cell, the solution is hypotonic. Water will flow into the cell, and hemolysis results.

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