SolutionsSolutions

Review of Properties of WaterReview of Properties of Water

Is a polar moleculeHas extensive

hydrogen bondingMany of water’s

unusual properties are due to hydrogen bonding

How aqueous solutions are formedHow aqueous solutions are formed

Solvation

The process by which cations and anions become surrounded by water molecules

Occurs at the surface of the solute

Solvation Process

16.1 Solution Formation16.1 Solution Formation

The nature of the solvent and solute determine solution formations

“Like dissolves like”◦Polar dissolves polar/ionic◦Non-polar dissolves non-polar

Rate of solution formationRate of solution formation

Temperature◦Kinetic energy of solvent & solute

Amount of surface area exposed to solvent◦Particle size◦Stirring

Surface Area and Particle SizeSurface Area and Particle Size

6 m2 x 8 = 48 m2 24 m2

SolubilitySolubility

Expresses the degree to which a substance dissolves

Usually expressed in g solute/100 g solvent

Examples◦Solubility of KNO3 is 85 g/100 g water at 20°C

Important Factors Affecting Important Factors Affecting SolubilitySolubility

Temperature◦For most solid solutes, solubility increases with

increasing temperature◦↑ T → ↑ Solubility◦For gases, solubility decreases with increasing

temperature◦↑ T → ↓ Solubility

Pressure◦No effect on solubility of solids◦For gases, ↑ P → ↑ Solubility

Solubility and TemperatureSolubility and Temperature

Solubility and TemperatureSolubility and Temperature

Types of solutionsTypes of solutions

Saturated◦Contain maximum amount of solute at a given

temperature◦Dissolved solute in equilibrium with undissolved◦All solutions are saturated solutions on a solubility

curveUnsaturated

◦Contain less than maximum solute◦No undissolved solute present

Supersaturated◦Contain more than the maximum amount of solute

at a given temperature◦How could this happen?

Henry’s Law and Solubility of Henry’s Law and Solubility of GasesGases

At a given temperature, solubility of a gas is directly proportional to the partial pressure of the gas

2

2

1

1

P

S

P

SxPkS

Saturated, Unsaturated and Saturated, Unsaturated and Supersaturated solutionsSupersaturated solutions

16.2 Concentrations of Solutions16.2 Concentrations of Solutions

Concentration tells how much solute is dissolved in an amount of solution or solvent

Many ways of expressing concentrationMost common is molarity (M)Concentrated solutions have much soluteDilute solutions have little solute

MolarityMolarity

Is the moles of solute per liter of solution

What is the molarity of a solution containing 0.9 g NaCl dissolved in 100 mL of solution?

L

mol

solution liters

solute molesM

MolarityMolarity

To make a 0.5-molar (0.5M) solution, first add 0.5 mol of solute to a 1-L volumetric flask half filled with distilled water.

16.2

MolarityMolarity

Swirl the flask carefully to dissolve the solute.

16.2

MolarityMolarity

Fill the flask with water exactly to the 1-L mark.

16.2

Molarity ProblemMolarity Problem

What mass of CaCl2 is needed to make 0.50 L of 2.5 M solution?

138.6 gExplain how to prepare the solution.

Making DilutionsMaking Dilutions

The total number of moles of solute remains unchanged upon dilution, so you can write this equation.

M1 and V1 are the molarity and volume of the initial solution, and M2 and V2 are the molarity and volume of the diluted solution.

Making DilutionsMaking Dilutions

Making a Dilute Solution

Percent ConcentrationPercent Concentration

Sometimes concentration is expressed as a percent

Example: the label on a bottle of bleach reads 8.25% sodium hypchlorite (NaClO)

What does this mean?

Percent ConcentrationPercent Concentration

Volume percent means

What is the V% when 15.0 mL ethanol is dissolved in 65.0 mL water?

18.8%, why not 23.1%

Percent ConcentrationPercent Concentration

Mass percent means

What is the m% when 2.50 g of KCl is dissolved in 75.0 mL of water?

3.23%, why not 3.33%

Colligative Properties of SolutionsColligative Properties of Solutions

Solutes change the properties of solventsColligative properties depend upon the

concentration of solute particles in solution

Important colligative propertiesImportant colligative properties

Vapor pressure loweringBoiling point elevationFreezing point depression

Vapor Pressure LoweringVapor Pressure Lowering

Colligative Properties and Number Colligative Properties and Number of Solute Particlesof Solute Particles

Colligative properties depend on the total moles of solute particles and not upon their identity

1 mol glucose 1 mol glucose◦(= 1 mol particles)

1 mol NaCl 1 mol Na+ + 1 mol Cl- ◦(= 2 mol of particles)

1 CaCl2 1 Ca 2+ + 2 Cl-

◦(= 3 mol particles)

Colligative Properties and Number Colligative Properties and Number of Solute Particlesof Solute Particles

Changes in Pvap, bp, fp are Changes in Pvap, bp, fp are proportional to the number of solute proportional to the number of solute particles dissolved in the solventparticles dissolved in the solvent