Solution &Solubility

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Solution Solutions are homogeneous mixtures of two or more pure

substances. In a solution, the solute is dispersed uniformly throughout

the solvent. A homogenous mixture of a solute dissolved in a solvent. The solubility (ability to dissolve) of a solute in a solvent is

dependent on the

a. Temperature For solid solutes

as temperature increases, solubility increases. For gas solutes

:as temperature increases, solubility decreases.

b. Pressure For solid solutes

as pressure increases, solubility remains the same.

For gas solutes:as pressure increases, solubility increases

c. Nature of Solute/Solvent “Like dissolves in like.”

Solute Type Non Polar Solvent

Polar Solvent

Non Polar (Fat Grease)

Soluble (Soap) Insoluble (Water)

Polar Insoluble Soluble (Water)

Ionic (Salt) Insoluble Soluble (Water)

High Solubility-Soluble Low Solubility-insoluble

Solubility Maximum grams of solute that will dissolve in 100 g of

solvent at a given temperature. Solubility= Varies with temperature Based on a saturated solution

Gram of solute 100 g of Water

The bond between solubility and temperature can be expressed by a solubility curve. The solubility curves of several compounds are shown on the right. Such curves disclose the maximum amount of solute that can be dissolved in 100 grams of water over a range of temperatures.

The solubility of most compounds increases as temperature increases, although exceptions do exist.

Types of Solution1. An unsaturated solution is a solution in which more

solute can be dissolved at a given temperature.2. A saturated solution is a solution containing the

maximum amount of solute that will dissolve at a given temperature.

3. A supersaturated solution is a solution that contains more solute than would dissolve in a saturated solution at a given temperature.

SATURATED SOLUTIONno more solute

dissolves

UNSATURATED SOLUTION

more solute dissolves

SUPERSATURATED SOLUTIONbecomes unstable, crystals form

increasing concentration

Saturated Solvent holds as much solute as is possible at

that temperature. Dissolved solute is in dynamic equilibrium

with solid solute particles.

Types of Solution

Unsaturated Less than the maximum amount of solute for that

temperature is dissolved in the solvent.

Saturated

Supersaturated Solvent holds more solute than is normally possible at that

temperature. These solutions are unstable; crystallization can usually be

stimulated by adding a “seed crystal” or scratching the side of the flask.

Solubility Rules1. Not all ionic compounds are water soluble.2. There are some general rules for compounds that

are water soluble: Group 1 ionic compounds and ammonium (NH4+) are

always water soluble Group 17 ionic compounds are water soluble except

when paired with Ag, Pb, and Hg ions

1. A salt is soluble in water if it contains any one of the following ions:

NH4+ Li+ Na+ K+ or NO3

-

Examples: Soluble salts

LiCl Na2SO4 KBr Ca(NO3)2

Solubility Rules2. Salts with Cl- are soluble, but not if the positive ion is :

Ag+, Pb2+, or Hg22+.

Examples:

Soluble Insoluble

MgCl2 AgCl , PbCl2

3. Salts with SO42- are soluble, but not if the positive ion

is Ba2+, Pb2+, Hg2+ or Ca2+. Examples:Soluble Not soluble

MgSO4 BaSO4 PbSO4

The stronger the attraction between solute and solvent molecules, the greater the solubility.

Like dissolves like (the substances have similar intermolecular attractive forces.)

Polar substances tend to dissolve in polar solvents. Non-polar substances do not dissolve in polar solvents.

Factors Affecting Solubility

Chemists use the axiom “like dissolves like”: Polar substances tend to dissolve in polar solvents. Nonpolar substances tend to dissolve in nonpolar

solvents.

Factors Affecting Solubility

The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.

Glucose (which has hydrogen bonding) is very soluble in water, while cyclohexane (which only has dispersion forces) is not.

Factors Affecting Solubility

Vitamin A is soluble in nonpolar compounds (like fats).

Vitamin C is soluble in water.

Factors Affecting Solubility

Concentration Concentration of a solution can be expressed either

quantitatively or qualitatively (dilute or concentrated).Methods of Expressing Concentrations of Solutions

Mass Percentage Mass percentage is expressed using the equation:

Mass % of component = X

Mass of component in solution Total mass of solution 100

Parts per Million (ppm)ppm =

mass of A in solutiontotal mass of solution

106

Parts per Billion (ppb)ppb =

mass of A in solutiontotal mass of solution

109

Mole Fraction This is how many moles of substance are in the solution.

Mole fraction of component = Moles of component Total moles of all componentsMolarit

y Relates the volume of solution to the quantity of

solute that it contains.

Molarity = Moles solute

Liters solution

Molality(m) mol of solute

kg of solventm =

Because both moles and mass do not change with temperature, molality (unlike molarity) is not temperature dependent.

Changing Molarity to Molality

If we know the density of the solution, we can calculate the molality from the molarity, and vice versa.

Raoult’s Law

Psoln = Vapor pressure of the solution

solvent = Mole fraction of the solvent

Psolvent = Vapor pressure of the pure solvent

The presence of a nonvolatile solute lowers the vapor pressure of a solvent.

Psoln = solventPsolvent

The presence of a nonvolatile solute lowers the concentration of solvent molecules in the solution, thus lowering the vapor pressure.

Demonstration of Raoult’s Law

Water is transported to the solution due to difference in vapor pressure.

An Aqueous Solution and Pure Water in a Closed Environment

Demonstration of Raoult’s Law

Ideal Solution...a solution that obeys Raoult’s law

Psoln = solventPsolvent

Vapor Pressure of a Solution Containing a Nonvolatile Solute

Ideal solutions• obey Raoult’s law exactly• solute-solute, solvent-

solvent, and solute-solvent interactions are identical

• do not exist

Real Solutions• are nonideal• may approach ideal

behavior if solute and solvent are similar

• Example: sucrose in water

Vapor Pressure of an Ideal Solution

Two Volatile SolventsPtotal = PA + PB = XAPA

0 + XBPB

0

Raoult’s Law Vapor Pressure for a Solution of Two Volatile Liquids

IDEAL SOLUTION

NONIDEAL SOLUTIONS

Positive Deviation

Negative Deviation

Colligative PropertiesDepend only on the number, not on the

identity, of the solute particles in an ideal solution.

Boiling point elevation Freezing point depression Osmotic pressureBoiling Point

Elevation

Kb = Molal boiling point elevation constant

m = Molality of the solute

A nonvolatile solute elevates the boiling point of the solvent.

T = Kbmsolute

Freezing Point Depression

Kf = Molal freezing point depression constant

m = Molality of the solute

A nonvolatile solute depresses the freezing point of the solvent.

T = KfmsolutePhase Diagram for an Aqueous Solution

In effect, a dissolved solute acts to extend the liquid range of the solvent

Boiling-Point Elevation and Freezing-Point

Depression

Colligative Properties of Electrolyte Solutions

T = imK = iMRT

i = moles of particles in solution

moles of solute dissolved

van’t Hoff factor, “i”, relates to the number of ions per formula unit.

NaCl = 2, K2SO4 = 3

Data for 0.05 m Solutions

Electrolyte i (expected) i (observed)

NaCl 2.0 1.9

MgCl2 3.0 2.7

FeCl3 4.0 3.4

Osmosis Some substances form semipermeable membranes,

allowing some smaller particles to pass through, but blocking other larger particles.

In biological systems, most semipermeable membranes allow water to pass through, but solutes are not free to do so.

In osmosis, there is net movement of solvent from the area of higher solvent concentration (lower solute concentration) to the are of lower solvent concentration (higher solute concentration).

Osmosis

• The pressure required to stop osmosis, known as osmotic pressure, , is

where M is the molarity of the solution

If the osmotic pressure is the same on both sides of a membrane (i.e., the concentrations are the same), the solutions are isotonic

nV

= ( )RT = MRT

Osmotic Pressure

Colloids Colloids are particles that are large on the molecular

scale but still small enough to remain suspended indefinitely in a solvent system. (aka colloidal dispersions.)

They are intermediate between solutions and heterogeneous mixtures.

They have the ability to scatter light; known as the Tyndall effect.Hydrophilic and Hydrophobic

Colloids The most important colloids are those in which water is the solvent, or dispersing agent.

Hydrophilic colloids are water loving and are found in the human body and help keep molecules suspended in water. (enzymes, antibodies)

Hydrophobic colloids are water fearing and must be stabilized before they can be mixed into water. (droplets of oil.)

Colloids in Biological SystemsSome molecules have a

polar, hydrophilic (water-loving) end and a nonpolar, hydrophobic

(water-hating) end.

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