Effects of solute on Effects of solute on solventsolvent

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Vapor PressureVapor Pressure

Define:Define: The pressure of the vapor created by The pressure of the vapor created by

the solution evaporating.the solution evaporating.– Must be volatile solutionMust be volatile solution

Evaporates easilyEvaporates easily

Effect of Nonvolatile SoluteEffect of Nonvolatile Solute

Causes vapor pressure to decreaseCauses vapor pressure to decrease– Concentration volatile solvent Concentration volatile solvent

decreasedecrease– Less ability to evaporateLess ability to evaporate

Partial PressurePartial PressureRaoult’s law Raoult’s law

Colligative propertyColligative property Dependent on mole fractionDependent on mole fraction

PPsolnsoln = P = PoopurepureXXpurepure

Vapor Pressure LoweringVapor Pressure Lowering

Subtract Subtract – PressurePressurepurepure solventsolvent - Pressure - Pressuresolutionsolution

Vapor Pressure Lowering FormulaVapor Pressure Lowering Formula

P = PP = PoopurepureXXsolutesolute

EX. Calculate the vapor-pressure EX. Calculate the vapor-pressure lowering of water when 5.67 g of lowering of water when 5.67 g of glucose is dissolved in 25.2 g of glucose is dissolved in 25.2 g of water at 25water at 25ooC. The vapor pressure of C. The vapor pressure of water at 25water at 25ooC is 23.8 mmHg. What is C is 23.8 mmHg. What is the vapor pressure of the solution? the vapor pressure of the solution?

Naphthalene, CNaphthalene, C1010HH88 is used to make mothballs. is used to make mothballs. Suppose a solution is made by dissolving 0.515 Suppose a solution is made by dissolving 0.515 g of naphthalene in60.8 g of chloroform, CHClg of naphthalene in60.8 g of chloroform, CHCl33. . Calculate the vapor-pressure lowering of Calculate the vapor-pressure lowering of chloroform at 20chloroform at 20ooC from the naphthalene. The C from the naphthalene. The vapor pressure of chloroform at 20vapor pressure of chloroform at 20ooC is 156 C is 156 mmHg. Naphthalene can be assumed to be mmHg. Naphthalene can be assumed to be nonvolatile compared with chloroform. What is nonvolatile compared with chloroform. What is the vapor pressure of the solution?the vapor pressure of the solution?

You need to boil a water-based You need to boil a water-based solution at a temperature lower than solution at a temperature lower than 100100ooC. What kind of liquid could you C. What kind of liquid could you add to the water to make this add to the water to make this happen?happen?

Boiling Point ElevationBoiling Point Elevation

The addition of a nonvolatile solute The addition of a nonvolatile solute to a liquid reduces its vapor to a liquid reduces its vapor pressure; therefore, the temperature pressure; therefore, the temperature must be increased in order to must be increased in order to achieve a set vapor pressure in order achieve a set vapor pressure in order to boil. Hence Bp elevation.to boil. Hence Bp elevation.

Boiling point elevationBoiling point elevation,,

Tb is a colligative property of a Tb is a colligative property of a solution equal to the boiling point of solution equal to the boiling point of the solution minus the boiling point the solution minus the boiling point of the pure solvent. (p 523) of the pure solvent. (p 523)

Tb = KTb = KbbCCmm

KKbb = boiling point constant listed on = boiling point constant listed on p523p523

CCmm = molal concentration = molal concentration

Freezing point depressionFreezing point depression

TTff, is a colligative property of a , is a colligative property of a solution equal to the freezing point of solution equal to the freezing point of the pure solvent minus the freezing the pure solvent minus the freezing point of the solution. point of the solution.

TTff = K = KffCCmm

((KKff = F = Fpp constant p 523) constant p 523)

EX. An aqueous solution (dissolved in EX. An aqueous solution (dissolved in HH22O) is 0.0222 O) is 0.0222 mm glucose. What are glucose. What are the boiling point and the freezing the boiling point and the freezing point of this solution?point of this solution?

From the freezing point lowering, you From the freezing point lowering, you can calculate the molal concentration can calculate the molal concentration and from the molality, you can obtain and from the molality, you can obtain the molecular weight. the molecular weight.

Find moles solute (gFind moles solute (gsolventsolvent X X molalitymolality))

ggsolutesolute / moles = MM / moles = MM

Calculation Molar MassCalculation Molar Mass

EX. A solution is prepared by EX. A solution is prepared by dissolving 0.131 g of a substance in dissolving 0.131 g of a substance in 25.4 g of water. The molality of the 25.4 g of water. The molality of the solution is determined by freezing solution is determined by freezing point depression to be 0.056 point depression to be 0.056 mm. . What is the molecular weight of the What is the molecular weight of the substance?substance?

EX. Camphor melts at 179.5EX. Camphor melts at 179.5ooC. Its C. Its freezing point constant is 40freezing point constant is 40ooC/C/mm. A . A 1.07 mg sample of a compound was 1.07 mg sample of a compound was dissolved in 78.1 mg of camphor. dissolved in 78.1 mg of camphor. The solution melted at 176.0The solution melted at 176.0ooC. C. What is the molecular weight of the What is the molecular weight of the compound? If the empirical formula compound? If the empirical formula of the compound is CH, what is the of the compound is CH, what is the molecular formula?molecular formula?

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