To describe the relationship in between solute concentration and also the physical properties that a solution. To understand that the total variety of nonvolatile solute particles determines the diminish in vapor pressure, increase in boil point, and decrease in freezing point of a systems versus the pure solvent.

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Many that the physics properties of remedies differ substantially from those that the pure substances disputed in previously chapters, and these differences have essential consequences. For example, the limited temperature selection of liquid water (0°C–100°C) severely limits its use. Aqueous solutions have actually both a lower freezing point and a higher boiling allude than pure water. More than likely one the the most familiar applications the this phenomenon is the enhancement of ethylene glycol (“antifreeze”) to the water in an vehicle radiator. This solute lowers the freezing point of the water, avoiding the engine from cracking in really cold weather indigenous the expansion of pure water on freezing. Antifreeze also enables the cooling device to operate at temperatures greater than 100°C there is no generating sufficient pressure come explode.

Changes in the freezing allude and boiling suggest of a equipment depend mainly on the number of solute particles present rather 보다 the type of particles. Such properties of services are called colligative nature (from the Latin colligatus, meaning “bound together” as in a quantity). As we will certainly see, the vapor pressure and also osmotic pressure of remedies are likewise colligative properties.

When we recognize the variety of particles in a solution, it is necessary to psychic that not all remedies with the same molarity save the exact same concentration the solute particles. Consider, because that example, 0.01 M aqueous options of sucrose, (NaCl), and also (ceCaCl_2). Due to the fact that sucrose dissolves to give a equipment of neutral molecules, the concentration of solute particles in a 0.01 M sucrose systems is 0.01 M. In contrast, both (ceNaCl) and (ceCaCl_2) space ionic compounds the dissociate in water to yield solvated ions. Together a result, a 0.01 M aqueous solution of (ceNaCl) contains 0.01 M Na+ ions and 0.01 M (Cl^−) ions, for a total particle concentration of 0.02 M. Similarly, the (ceCaCl_2) solution includes 0.01 M (Ca^2+) ions and 0.02 M (Cl^−) ions, for a total particle concentration the 0.03 M.These values are correct because that dilute solutions, where the dissociation the the compound to kind separately solvated ion is complete. At greater concentrations (typically >1 M), especially with salts of small, very charged ions (such as (Mg^2+) or (Al^3+)), or in services with less polar solvents, dissociation to give separate ions is often incomplete. The sum of the concentration of the dissolved solute particles dictates the physical properties that a solution. In the adhering to discussion, we must as such keep the juniorg8.comistry nature of the solute firmly in mind.

Boiling allude Elevation

Recall that the normal boiling suggest of a substance is the temperature in ~ which the vapor pressure equals 1 atm. If a nonvolatile solute lowers the vapor push of a solvent, it have to also influence the cook point. Since the vapor push of the systems at a provided temperature is less than the vapor pressure of the pure solvent, achieving a vapor press of 1 atm because that the solution needs a higher temperature 보다 the regular boiling allude of the solvent. Hence the boiling allude of a solution is constantly greater than that that the pure solvent. We can see why this must be true by compare the phase diagram for an aqueous equipment with the phase diagram for pure water (Figure (PageIndex1)). The vapor pressure of the equipment is less than the of pure water at every temperatures. Consequently, the liquid–vapor curve because that the systems crosses the horizontal line matching to ns = 1 atm in ~ a greater temperature 보다 does the curve because that pure water.

Figure (PageIndex1): step Diagrams of Pure Water and an Aqueous equipment of a Nonvolatile Solute. The vaporization curve for the equipment lies below the curve for pure water at every temperatures, which outcomes in boost in the boiling point and a to decrease in the freezing point of the solution.

The boiling allude of a systems with a nonvolatile solute is constantly greater 보다 the boiling point of the pure solvent.

The size of the rise in the boiling suggest is pertained to the size of the decrease in the vapor pressure. Together we have actually just discussed, the diminish in the vapor pressure is proportional to the concentration the the solute in the solution. For this reason the magnitude of the increase in the boiling allude must additionally be proportional to the concentration that the solute (Figure (PageIndex2)). We can define the boiling suggest elevation ((ΔT_b)) as the difference between the boiling points of the solution and the pure solvent:

<ΔT_b=T_b−T^0_b labeleq1>

where (T_b) is the boiling allude of the solution and (T^0_b) is the boiling suggest of the pure solvent. We have the right to express the relationship between (ΔT_b) and also concentration together follows

<ΔT_b = mK_b labeleq2>

where m is the concentration the the solute expressed in molality, and (K_b) is the molal boiling allude elevation continuous of the solvent, which has units of °C/m. Table (PageIndex1) list characteristic Kb values for several frequently used solvents.


Video (PageIndex1): Freezing point depression is exploited to remove ice native the regulate surfaces the aircraft.

Exercise (PageIndex5)

Arrange this aqueous services in bespeak of enhancing freezing points: 0.2 m (NaCl), 0.3 m acetic acid, 0.1 m (ceCaCl_2), and also 0.2 m sucrose.


0.2 m (ceNaCl) (lowest freeze point)

Determining Molar Mass from Freezing point Depression

A solution of 4.00 g of a nonelectrolyte liquified in 55.0 g of benzene is uncovered to freeze in ~ 2.32 °C. What is the molar fixed of this compound?


We have the right to solve this problem using the complying with steps.

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Determine the adjust in freezing point from the observed freezing point and the freezing point of pure benzene (Table (PageIndex1)).

(mathrmMoles: of: solute=dfrac0.62:mol: solute1.00cancelkg: solvent×0.0550cancelkg: solvent=0.035:mol)

Determination of a Molar Mass from Osmotic Pressure

A 0.500 together sample of one aqueous equipment containing 10.0 g of hemoglobin has actually an osmotic pressure of 5.9 torr at 22 °C. What is the molar fixed of hemoglobin?


Here is one set of steps that can be used to resolve the problem:


Convert the osmotic press to atmospheres, then identify the molar concentration native the osmotic pressure.

(mathrmmolar: mass=dfrac10.0:g1.6×10^−4:mol=6.2×10^4:g/mol)