Colhgative properties

Thinking Critically Explain why the colhgative properties described in this section may not apply... 

The total concentration of all dissolved solute species determines the colhgative properties. As we will emphasize in Section 14-14, we must take into account the extent of ion formation in solutions of ionic solutes. 

Colhgative Properties and Dissociation of Electrolytes Osmotic Pressure... 

The vapor-pressure lowering caused by adding a nonvolatile solute depends on the total concentration of solute particles, regardless of whether they are molecules or ions. Remember that vapor-pressure lowering is a colhgative property, so its value for any solution depends on the concentration of solute particles and not on their kind or identity. 

Vapor-pressure lowering, boiling-point elevation, freezing-point depression, and osmotic pressure are colhgative properties of solutions, that is, they depend only on the number of solute particles that are present and not on their nature. 

The first quantity is a colHgative property called the number-average relative molar mass or simply the number-average molar mass denoted M and defined as the weighed average of the number fraction of each macromolecule by the following equation. 

Like boiling-point elevation and freezing-point depression, osmotic pressure is directly proportional to the concentration of solution. This is what we would expect, bearing in mind that all colhgative properties depend only on the number of solute particles in solution. If two solutions are of equal concentration and, hence, of the same osmotic pressure, they are said to be isotonic. If two solutions are of unequal osmotic pressures, the more concentrated solution is said to be hypertonic and the more dilute solution is described as hypotonic (Figure 13.10). 

The colligative properties of nonelectrolyte solutions provide a means of determining the molar mass of a solute. Theoretically, any of the four colhgative properties are suitable for this purpose. In practice, however, only freezing-point depression and osmotic pressure are used because they show the most pronounced changes. 

The effect of adding a solute to a liquid is to raise the boiling point of the Uquid. Since the change in boiUng point is independent of the identity of the solute and depends only on the number of solute particles, it is our third colhgative property. The scientific term often used to describe the increase in boiling point is boiling point elevation. 

Manning GS. Limiting laws and counterion condensation in polyelectrolyte solutions I. Colhgative properties. J Cbem Pbys 1969 51 924-933. 

Another colhgative property is osmotic pressure, a pressure related to a process called osmosis. Osmosis is the diffusion of solvent particles across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. The osmotic pressure effect is due to the fact that a smaller amount of solute on the less concentrated side of the membrane results in a larger number of solvent molecules available at that surface of the membrane. Thus, there is more diffusion of solvent from the side of lower solute concentration. 

---