Aqueous Solutions

Solution: Homogeneous mixture; solid liquid, or gas

Soluble: Capable of being dissolvedSolute: Substance that is dissolved,

present in lesser amountsSolvent: Dissolving medium; present

in greater amounts

Types of MixturesSolutions: particles < 1nm; cannot be

seen; no scattering of light (salt water)Suspensions: particles > 100 nm; settle to

the bottom (sand & water)Colloids: particles 1-100 nm; suspended

throughout the medium; exhibit Tyndall effect

Tyndall Effect: Scattering of light so that the beam of light can be seen like headlights in fog; seen in colloids

Hydration

Hydrates: ionic compounds with water molecules in their structure specific ratios of water to compoundheating can drive off the water and leave the

anhydrous salt

CuSO4 • 5H2O (copper (II) sulfate pentahydrate

HydratesEffloresce: If a hydrate has a vapor pressure

greater than water vapor, than it will lose its water of hydration

Hygroscopic: Hydrated salts that have a low vapor pressure remove water from moist air to form higher hydratesUsed as drying agents or desiccants Silica gel

Deliquescent: remove sufficient water from the air to dissolve completely and form solutions

Types of SolutesElectrolytes: Conduct an electric current when

dissolvedAll Ionic CompoundsPolar Molecules when dissolved in water (ionized)Weak Electrolyte: fraction of the solute exist as ionsStrong Electrolyte: almost all of the solute exist as

separate ions

Nonelectrolytes: Do not conduct an electric current when dissolvedCovalent compoundsPolar Molecules in a pure state

Dissociation

Sodium Chloride:

NaCl(s) Na+1(aq) + Cl-1

(aq)

Magnesium nitrate:

Mg(NO3)2(s) Mg+2(aq) + 2NO3

-1(aq)

Dissociation is the “breaking apart” of ions as an ionic compound dissolves in water.

Factors Affecting Rate of Dissolution

Surface Area: Greater surface area (smaller particle size) = faster dissolution

Agitation: stirring = faster dissolutionHeating: generally means faster

dissolution

Like Dissolves Like

The polarity of water molecules plays an important role in the formation of solutions of ionic compounds and other polar compounds in water.

The positive ends of the water molecules attract negative ions (dipoles) and the negative ends of water molecules attract positive ions (dipoles), pulling them from the surface of the crystal.

Miscible/Immiscible

Nonpolar molecules do not attract ions of crystalline solids strongly enough to overcome forces holding them together and therefore cannot dissolve polar or ionic compounds.

Miscible : Two liquids mix; soluble in one another (Ex: Ethanol & Water)

Immiscible: Do not mix; insoluble in one another (Ex: Oil & Water)

Relative ConcentrationConcentration: a measure of the amount of

solute that is dissolved in a given quantity of solvent

Unsaturated: Less than the theoretical amount of solute is dissolved; there is room to dissolve more

Saturated: The maximum amount of solute that can be dissolved at a certain temperature has been reached

Supersaturated: More than the theoretical amount of solute is dissolved in solvent at a given temperature

Solubility/RecrystallizationSolubility:

A measure of how much solute can dissolve in a given amount of solvent at a specific temperature

Dilute Solution: The amount of solute dissolved is small in relation to the amount of solvent present.

Concentrated Solution: The amount of solute dissolved is large in relation to the amount of solute present.

At the same time substances are breaking apart, there are also particles attracted to each other and remaining together – RECRYSTALLIZATION

Solution Equilibrium: Rate of dissolution = Rate of recrystallization

Saturation Curve

°C

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0

20

40

60

80

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100

120

160

180

g / 100 mL

10 20 40 60 80 10030 50 70 90

Supersaturated Saturated

Unsaturated

Changing the temperature of a solvent can have an effect on solubility!!

°C

0

0

20

40

60

80

140

100

120

160

180

g / 100 mL

10 20 40 60 80 10030 50 70 90

A

B

Which solute has the greatest solubility at 20°C?

Which solute has the greatest solubility at 50°C?

Gas SolubilityHenry’s Law: S1

/P1 = S2 / P2

The solubility of a gas in a liquid is directly proportional to the partial pressure of that gas on the surface of the liquid.

Open a can of pop!! Carbon dioxide is forced into solution of flavored water at 5-10 atm.

Effervescence: The FIZZING that happens when you open the pop; the release of gas from a solution

Concentrations of Solutions

Molarity = M = mol solute

L solutionMolality = m = mol solute

kg solventPercent by mass [% m/v] = mass solute x 100

mL solutionPercent by volume [% v/v] = vol solute x 100

vol solution

DilutionsYou have the following stock solutions

available: 2.0 M NaCl; 4.0 M KNO3; 0.5M MgSO4

Calculate the stock volumes you must dilute to make the following solutions using

M1V1 = M2V2

500 mL of a 0.50 M sodium chloride solution

2.0 L of a .20 M magnesium sulfate solution

Examples/Molarity

What is the molarity of a solution that contains 212.5 g of sodium nitrate (NaNO3) in 3.0 liters of solution?

What mass of sucrose, C12H22O11 is needed to make 300 mL of a 0.50 M solution?

Examples/MolalityCalculate the molality of a solution made

by dissolving 45.0 g of dextrose, C6H12O6 in 500.0 g of water.

What is the mass of water required to prepare a 1.00 molal solution containing 10.0 g of NaOH?

Calculate the molality of a solution prepared by dissolving 6.3 moles of KCN in 633 g of water

Percent Problems% m/v: Calculate the number of grams of

solute required to make the following solutions:2.5 L of saline solution– 90% NaCl50 mL of 4% magnesium chloride

%v/v: 10 mL acetic acid is diluted with water to a total solution volume of 200 mL. What is the % v/v of acetic acid?

25mL of ethanol and 75 mL of water are mixed. What is the % v/v of the solution?

Colligative Properties

Depend only on the number of particles dissolved in a given mass of solventVapor Pressure LoweringBoiling Point ElevationFreezing Point Depression

Vapor Pressure LoweringA nonvolatile solute has a lower vapor pressure

than a pure solventAs solutes are added the equilibrium is

disrupted and solvent particles form shells around the solute particles. This reduces the number of free solvent particles able to escape the liquid. Equilibrium is eventually re-established at a lower vapor pressure

Decrease in VP is proportional to the number of particles the solute makes in solutionSolute that disassociate in large #s have a greater

effect of VP

Boiling Point ElevationThe difference in temperature between the

boiling point of a solution & that of a pure solvent

Boiling points of solutions are higher than that of pure solvents The presence of solutes increase BP

Additional KE is required for particles to overcome the attractive forces that keep them in liquid

Boiling Point elevation (ΔTb): directly proportional to the molal concentrationΔTb α m ΔTb = Kb x m

Freezing Point DepressionThe Difference in temperature between

the freezing point of a solution and that of the pure solvent

More KE must be withdrawn from a solution than from a pure solution to solidify

Freezing Point Depression (Δ Tf): directly proportional to the molal concentrationΔ Tf = Kf x m