# C HAPTER 16 Solutions. S OLUTION F ORMATION Compositions of the solutes and solvents determine if the substance will dissolve. Stirring, temperature,

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C HAPTER 16 Solutions

S OLUTION F ORMATION Compositions of the solutes and solvents determine if the substance will dissolve. Stirring, temperature, and surface area determines how fast the substance dissolves. Stirring and heating increase the rate at which a solute dissolves. (a) A cube of sugar in cold tea dissolves slowly. (b) Granulated sugar dissolves in cold water more quickly than a sugar cube, especially with stirring. (c) Granulated sugar dissolves very quickly in hot tea.

H ENRY ’ S L AW At a given temperature the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid S 1 is the solubility of a gas at one pressure, P 1 ;S 2 is the solubility at another pressure, P 2.

Solubility : Solubility : amount of substance that dissolves in a given amount of solvent at specific conditions of temp. and pressure to produce a saturated solution Unsaturated solution : Unsaturated solution : contains less solute than a saturated solution at a given temperature and pressure Saturated solution : Saturated solution : contains the max amount of solute for a given amount of solvent at a constant temp. and pressure Supersaturated solution : Supersaturated solution : contains more solute than it can theoretically hold at a given temp. Miscible : Miscible : liquids that dissolve completely in one another Immiscible : Immiscible : liquids that are insoluble in one another 16.1 V OCABULARY

M OLARITY Divide the moles of solute by the volume of solution. Molarity(M) : Molarity(M) : the concentration of solute in a solution expressed as the number of moles of solute dissolved in 1 liter of solution

D ILUTION Diluting a solution reduces the number of moles of solute per unit volume, but the total number of moles of solute in solution does not change. Moles of solute = molarity (M) × liters of solution (V) Moles of solute before dilution = moles of solute after dilution Moles of solute M 1 x V 1 = M 2 x V 2 M 1 and V 1 are the molarity and volume of the initial solution, and M 2 and V 2 are the molarity and volume of the diluted solution. Volumes can be liters or milliliters both V 1 and V 2 have to have the same units. Adding solvent to a concentrated solution lowers the concentration, but the total number of moles of solute present remains the same.

P ERCENT C ONCENTRATION Express as a ratio volume of solute to volume of solution mass of solute to mass of solution OR The label clearly shows this solution of isopropyl alcohol from rubbing alcohol which is a 70% solution of isopropyl alcohol.

16.2 V OCABULARY Concentration : Concentration : measurement of amount of solute that is dissolved in a given quantity of solvent (mol/L) Dilute solution : Dilute solution : solution contains a small amount of solute Concentrated solution : Concentrated solution : solution contains a large amount of solute

C OLLIGATIVE P ROPERTY Three important colligative properties of solutions: vapor-pressure lowering boiling-point elevation freezing-point depression. a property of a solution that depends only upon the number of solute particles, and not upon their identities; boiling-point elevation, freezing-point depression, and vapor-pressure lowering are colligative properties

F ACTORS DETERMINING HOW MUCH PROPERTIES OF THE SOLUTION DIFFER FROM PROPERTIES OF THE PURE SOLVENT The decrease in a solution’s vapor pressure is proportional to the number of particles the solute makes in solution. The magnitude of the freezing-point depression is proportional to the number of solute particles dissolved in the solvent and does not depend upon their identity. The magnitude of the boiling-point elevation is proportional to the number of solute particles dissolved in the solvent. Vapor Pressure Freezing Point Boiling Point

16.3 V OCABULARY Freezing point depression Freezing point depression : the difference in temperature between the freezing point of the solution and the freezing point of the pure solvent Boiling point elevation Boiling point elevation : the difference in temperature between the boiling point of a solution and the boiling point of the pure solvent

C ONCENTRATION OF SOLUTION You can express the concentration of solutions in two ways: Unit molalityMole fractions Molality is also known as molal concentration. Unit molality (m) Unit molality (m): the number of moles of solute dissolved in 1 kilogram (1000 g) of solvent

The magnitudes of the freezing-point depression (ΔT f ) and the boiling-point elevation (ΔT b ) of a solution are directly proportional to the molal concentration (m), when the solute is molecular, not ionic. (ΔT f ) is the difference between the freezing point of the solution and the freezing point of the pure solvent m is the molal concentration of the solution ΔT f = K f × m K f is the molal freezing-point depression constant ΔT b = K b × m F REEZING P OINT D EPRESSION AND B OILING P OINT E LEVATION Freezing Point DepressionBoiling Point Elevation (ΔT b ) is the difference between the boiling point of the solution and the boiling point of the pure solvent K b, is the molal boiling-point elevation constant m is the molal concentration of the solution

16.4 V OCABULARY Mole fraction : Mole fraction : the ratio of the moles of solute in solution to the total number of moles of both solvent and solute Molal freezing-point depression constant K f : Molal freezing-point depression constant K f : the change in freezing point for a 1-molal solution of a nonvolatile molecular solute Molal boiling-point elevation constant K b : Molal boiling-point elevation constant K b : the change in boiling point for a 1-molal solution of a nonvolatile molecular solute

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