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CH 11: Properties of Solutions

- Describing Solutions – concentration units
- Energetics of solution formation
- Factors impacting solubility – P and T
- 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 (cA) = 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)

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 HClsolution
- Density of 1.15 g/mL

Calculate:

- Molarity – need _______
- Mass %
- Mole fraction of HCl

Solution Formation

Formation of a solution involves 3 steps

- Separate the solute particles
- expand the solute
- Separate the solvent particles
- Expand the solvent
- Form the solution
- Solute and solvent interact

Solution Formation

- Each step of solution formation involves energy and has a DH.

DH1 = energy needed to separate the solute

DH2 = energy needed to separate the solvent

DH3 = energy released when solution forms

Solution Formation

DHsolution = DH1 +DH2 +DH3

Solutions form when the DHsolution 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 gas

k = solution specific constant

P = 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= csolvent P0solvent

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.

Calculating the bp or fp of a solution

- Calculating the molar mass of a solute from fp or bp data

Osmotic Pressure

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

P = MRTi

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