CH 11: Properties of Solutions

1. Describing Solutions – concentration units2. Energetics of solution formation3. Factors impacting solubility – P and T4. Colligative Properties of solutions

• BP elevation• FP depression• Osmotic pressure• Vapor Pressure

Terms

• Solution – homogeneous mixture• Solvent – generally the larger component of

the solution– Determines the physical state of the solution

• Solute – generally the smaller component of the solution– Solute is dispersed in the solvent

Solution Composition

• Concentrated solution – relatively large amount of solute

• Dilute solution – relatively small amount of solute

Solution Composition

• Unsaturated solution –solution with less than the maximum amount of solute that will normally dissolve at a given temperature

• Saturated solution - solution with maximum amount of solute that will normally dissolve at a given temperature

Solution Composition

• Super-saturated solution - solution with more than the maximum amount of solute that will normally dissolve at a given temperature

Concentration Units

Molarity (M) = moles solute/Liters solution

Molality (m) = moles solute/kg solvent

Mass % = Mass solute/mass solution x100%

Mole fraction () = moles A/total moles

Normality

Normality (N) = # gram equivalent wt./L solution– Gew = mass of an acid or base that produces 1

mole of H+ or OH -1

Normality = Molarity x (# H+ or OH -1 /mol)– 1 M HCl = 1 N HCl– 1 M H2SO4 = 2 N H2SO4

For acid base titrations:(NAcid) (VAcid) = (NBase) (VBase)

Practice!

• Start by writing definitions for the concentration unitsM =m =Mass % =Mole fraction =

Starting with Molarity

Solution: – 3.75 M H2SO4 solution with a density of 1.23 g/mL

Calculate: – Mass % – Molality– mole fraction of H2SO4

Starting with Masses

Solution: – A solution is made by combining 66.0 grams of

acetone (C3H6 O) with 146.0 grams of water.– Solution has a density of 0.926 g/mL

Calculate: – Molarity – need volume of solution– Mass %– Molality– Mole fraction of acetone

Starting with Mass %

Solution: – 35.4 % H3PO4

– Density of 1.20 g/mL

Calculate: – Molarity – Molality– Mole fraction of H3PO4

Starting with Molality

Solution: – 2.50 m HCl solution– Density of 1.15 g/mL

Calculate: – Molarity – need _______ – Mass %– Mole fraction of HCl

Solution Formation

Formation of a solution involves 3 steps1. Separate the solute particles• expand the solute

2. Separate the solvent particles• Expand the solvent

3. Form the solution– Solute and solvent interact

Solution Formation

• Each step of solution formation involves energy and has a H.

H1 = energy needed to separate the solute

H2 = energy needed to separate the solvent

H3 = energy released when solution forms

Solution Formation

HsolutionH1 + H2 + H3

Solutions form when the Hsolution is a small value – see page 504

Factors Impacting Solubility

• Structure – like dissolves like– #44 on page 533

Factors Impacting Solubility

• Pressure– Pressure has little impact on the solubility of

liquids and solids– Pressure has a significant impact on the solubility

of gases in a liquid• The higher the pressure of gaseous solute above a

liquid the higher the concentration of the gas in the solution

Henry’s Law• Henry’s Law: C = kP

C = Concentration of dissolved gask = solution specific constantP = partial P of the solute gas above

the solution• What does Henry’s Law mean?– No calculations required.

Page 506

Temperature and Solubility

• Temperature has variable effects on the amount of solid that will dissolve in an aqueous solution!– See figure 11.6 page 508

• Solutes will dissolve more rapidly at higher temperatures

Temperature and Solubility

• The solubility of a gas in water decreases as temperature increases.– See figure 11.7 on page 508

– Thermal pollution – read the story on page 510 when you get a chance

Vapor Pressure of Solutions

• See Raoult’s Law on page 510

• Psolution = solventsolvent P0 solventsolvent

Colligative Properties

• Colligative properties– properties of a solution that depend upon the

amount of dissolved solute, not the identity of the solute.• Freezing point depression• Boiling point elevation• Osmotic Pressure

• Note: I will be weaving section 11.7 and the van’t Hoff factor (i) into my consideration of these properties and not consider it separately.

Colligative Properties

FP = Kf m i

BP = Kb m i

See page 517 for needed constants

1. Calculating the bp or fp of a solution2. Calculating the molar mass of a solute from

fp or bp data

Osmotic Pressure

• Osmotic Pressure () is often used to determine the molar mass of large biological molecules

= MRTi