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Properties of Solutions

Brown, LeMay Ch 13AP Chemistry

CaCl2

(aq)

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13.1: Types of Solutions

Example Solvent Solute

Air (g in g)

Soda (g in l)

H2 in Pt (g in s)

Alcoholic beverages (l in

l)

Sea water (s in l)

Brass (s in s)

N2 O2

H2O CO2

Pt H2

H2O C2H5OH

H2ONaCl

(one of many salts)Copper

(55% – 90%)Zinc

(10% – 45%)

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When do solutions form? Solutions form (the solute and solvent will mix)

when: Energy: solute-solvent interactions are stronger

than solute-solute or solvent-solvent interactions.

Disorder: Solutions result in a more disordered state than the separate solute and solvent states, since molecules will be “mixed” that were once “well organized”.

NaCl (s) + H2O (l) → Na+ (aq) + Cl- (aq) Ion-dipole interactions > H-bonds (H2O···H2O)

< Ionic bonds (Na+ Cl-) The increase in disorder also drives the dissolving

process.

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13.2: Ways to Express Concentration

Mass Percent

Mole Fraction: commonly used for gases

Molarity: commonly used for solutions

Molality: commonly used for colligative properties

100solvent mass solute mass

solute mass%

mol total

A molAX

solution Liters

solute molM

solvent kg

solute molm

Does not vary with T

Varies with T

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13.3: Solubility Vocabulary

Solvation: dissolving; the interactions between solute and solvent Hydration: solvation with water

Crystallization: “un-dissolving”; process by which solute particles leave the solvent. Solute + solvent ↔ solution

(equilibrium)

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13.3: Solubility Vocabulary Saturated: a solution that is in equilibrium

with undissolved solute (appears as solution and crystals)

Solubility: the amount of solute needed to form a saturated solution

Unsaturated: a solution containing less than the saturated amount (appears as solution only)

Supersaturated: a solution containing more than the saturated amount, yet appears unsaturated.

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13.4: Factors Affecting Solubility

1. “Like dissolves like.” Miscible: liquids that mix (polar or ionic

solute with polar solvent, or nonpolar with nonpolar)

Immiscible: liquids that do not mix (polar or ionic solute with nonpolar solvent)

Covalent network solids do not dissolve in polar or nonpolar solvents.

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13.4: Factors Affecting Solubility

2. Pressure: does not significantly affect solubility of liquids and solids

Gases: increased P means increased solubility

Henry’s law: Cg = k Pg

Cg = solubility of gas in solution (M)

k = Henry’s law constantPg = partial pressure of gas over solution

William Henry(1775-1836)

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13.4: Factors Affecting Solubility

3. Temperature Most solids: increased T means

increased solubility * Exception: Ce2(SO4)3

Gases: increased T means decreased solubility

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13.5: Colligative Properties• Properties that are dependent on the number of

solute particles present in solution

1. Vapor pressure lowering: the greater the concentration of a nonvolatile solute, the lower the vapor pressure of the solvent

Solute takes up surface area Introduction of solute-solvent IMF

Raoult’s law: PA = XA P°A

PA = vapor pressure of solvent vapor abovesolution (solute A is nonvolatile)

XA = mole fraction of solute AP°A= normal vapor pressure of solvent

François-Marie Raoult(1830-1901)

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Ideal solution: described by Raoult’s law

Has low concentration of solute Solute and solvent have similar types of

IMF & molecular sizes

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Extension of the Liquid Phase

2. Boiling point elevation:Tb = i Kb m Kb (H2O) = 0.51 ºC•kg/mol

3. Freezing point depression:Tf = i Kf m Kf (H2O) = 1.86 ºC•kg/mol

i = van’t Hoff factor:Unitless constant associated with the degree of dissociation of a solute in a solvent Jacobus van’t

Hoff(1852-1911)

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Ideal i values i = 1 Substances which do not ionize in solution

Ex: sucrose (sugar) i = 2 Substances which ionize into 2 ions

Ex: NaCl i = 3 Substances which ionize into 3 ions

Ex: MgCl2

Ex: Determine the solute “equivalent molality” (factoring in i) for the following solutions:

1-m sucrose 1-m NaCl 1-m CaCl2

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Osmotic Pressure ()4. Pressure required to prevent osmosis

of solute particles Applied on

solution side to stop net movement of solvent from the pure solvent side.

Osmosis: net movement of solvent toward the solution with the highest solute concentration

= iMRT = (n/V)RT

R = 0.0821 L-atm/mol-K

Prevents flow of solute particles

Solvent “wants” to flow

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13.6: Colloids

Mixtures containing particles intermediate between: A solution (homogeneous, < 10 Å)

and A suspension (heterogeneous, >

2000 Å)

Tyndall effect: scattering of lightseen in a colloid

John Tyndall(1820-1893)