Physical Properties of SolutionsUnit 10

Why are some compounds more effective in melting ice than others?

Solutions in the World Around Us

Definitions

Review from Chapter 4:Solution: homogeneous mixture of two or more substances.

Solute: the substance being dissolved.

Solvent: the substance doing the dissolving

•Colloid – solution of solid particles small enough to be suspended in solution•Example: milk, paint, smoke•Colloids may look clear or almost clear

when dilute enough

•Tyndall effect – scattering of light by particles in a colloid or suspension▫Causes the beam of light to become visible

▫Why you can see rays from the sun (particles in the air scatter the light)

•Suspension – a mixture from which some of the particles settle out slowly upon standing▫Particles are too big to be dissolved▫Example: sand in water, dust in air▫Suspensions can be filtered

Concentration•Concentration of a solution: the quantity

of a solute in a given quantity of solution (or solvent).

▫A concentrated solution contains a relatively large amount of solute vs. the solvent

▫A dilute solution contains a relatively small concentration of solute vs. the solvent “Concentrated” and “dilute” aren’t very quantitative .

Need different concentration units to describe different properties of solutions:

• Molarity (review)• Mass Percent• Molality

Solution Concentration

Example 1: What is the molarity of a solution made by dissolving 12.5 g of oxalic acid in 456 mL of solution?

solution of liters

solute of moles (M) Molarity

Molarity (M)

Example 2: How many grams of sodium carbonate are needed to prepare .250 L of an aqueous 0.300 M soln.?

solution of liters

solute of moles (M) Molarity

Molarity (M)

100 * solution of mass

solute of mass mass bypercent

Mass Percent

Ex. 4: What is the % by mass of the solute in a soln. made by adding 1.20 g of methyl alcohol to 16.8 g of H2O?

solventof kilogramssoluteof moles

)( Molality mMolality (m)

Example 5: A solution contains 15.5 g of urea, NH2CONH2, in 74.3 g of water. Calculate the molality of the urea.

Why use Molality??Molality (m) is the number of moles of solute per one kilogram of solvent (not solution!)

EOS

Molality does NOT change with temperature!

Molarity (M) varies with temperature due to the expansion or contraction in the volume of the solution

Dilution of Solutions

Example 6: How would you prepare 0.250 L of 0.300 M Na2CO3 starting with 1.33 M solution?

2211 VMVM

Vocabulary of Solutions

Liquids that mix in all proportions are called miscible.

When there is a dynamic equilibrium between an undissolved solute and a solution, the solution is

saturated

EOS

A saturated solution is one in which a given amount of solvent has dissolved the absolute maximum amount of solute at that temperature.

Vocabulary

EOS

A solution which contains less solute than can be held at equilibrium is unsaturated

Solubility Graphs or Curves

The concentration of the solute in a saturated solution is the solubility of the solute, and may be shown on a graph or curve.

Solubility CurvesExample: What mass of solute will dissolve in 100mL of water at the following temperatures.

1.KNO3 at 70°C 

2.NaCl at 100°C 

Solubility CurvesExample: What term - saturated, unsaturated, or supersaturated – best describes:

A solution that contains 70g of NaNO3 per 50 g H2O at 30°C

Solubility CurvesExample: What term - saturated, unsaturated, or supersaturated – best describes:

A solution that contains 70 g of dissolved KCl per 200 g H2O at 80°C.

Solubility CurvesExample: Determine the molality of a saturated NaCl solution at 25°C.

Ionic Compounds

EOS

The attractions of water dipoles for ions pulls the ions out of the crystalline lattice and into aqueous solution.

The Solubilities of GasesMost gases become less soluble in liquids as the temperature increases

Example: can of soda going flat on a hot day

Effect of Pressure

When Pgas drops, solubility drops.

Also known as Henry’s Law.

Colligative PropertiesWhen adding a solute to a solvent, the properties of

the solvent are modified.

• Vapor pressure decreases• Melting point decreases• Boiling point increases• Osmosis is possible (osmotic pressure changes)

These changes are called COLLIGATIVE

PROPERTIES.

Colligative PropertiesDefinition:

Colligative Properties depend only on the

NUMBER of solute particles relative to

solvent particles, not on the KIND or

IDENTITY of solute particles.

FP Depression and BP Elevation—Two Colligative Properties

The presence of the solute lowers (depresses) the freezing point of the solvent (Tf) and increases (elevates) the boiling point of the solvent (Tb)

EOS

Examples: adding salt to water allows the water temperature to exceed 100 oC, thereby cooking food faster; placing salt on ice lowers the freezing point, allowing ice to melt at a lower temperature.

Elevation of Boiling Point Elevation in BP = ∆Tb = Kb•m•i

kb = constant; characteristic of solvent

i = van’t Hoff factor = 1 FOR ALL NONELECTROLYTES!

m = molality

∆Tb = number of degrees BP goes UP

Lowering (depression) of Freezing Point

Depression in FP = ∆Tf = Kf•m•i

kf = constant; characteristic of solvent

i = van’t Hoff factor = 1 FOR ALL NONELECTROLYTES!

m = molality

∆Tf = number of degrees FP goes DOWN

Sample ProblemExample: Calculate the boiling point of solution that contains 50.0 g of glucose, C6H12O6, in 400 g of water. The molal boiling point constant of water is 0.52oC/m.

▫Calculate the molality first!▫i = 1 since glucose is a NONELECTROLYTE.

Freezing Point Depression • Freezing point of a solution is lower than the

freezing point of the pure solvent.

• Dissolving substances lowers the freezing point of a solvent.

• Ex: Icy pavement - throw down CaCl2 or NaCl, and the water will then freeze at a lower temperature

Ex: Antifreeze: a solution of ethylene glycol in water1. Prevents car’s radiator from freezing in the winter.2. Prevents car’s radiator from boiling over in the summer

The more ethylene glycol in the water, the lower the freezing point, and the higher the boiling point.

Freezing Point Depression

The freezing point of a solution is LOWER than that of the pure solvent.

FP depression = ∆TFP = KFP•m

Pure waterPure water Ethylene glycol/water Ethylene glycol/water solutionsolution

Colligative Properties of ElectrolytesNonelectrolytes vs. electrolytesNonelectrolytes produce only molecules in solution; electrolytes produce ions.

NaCl Na + + Cl –

The greater the product of molality and number of ions, the greater the boiling point elevation or

freezing point depression.

Modified Equation: Van’t Hoff Factor

∆T = K•m•ii = van’t Hoff factor = number of particles produced per formula unit.

Compound Theoretical Value of i

Sugar (NONELECTROLYTE) 1NaCl 2CaCl2 3

Sample Problem

Example. Rank the following aqueous solutions in order of lowest to highest melting point:

▫0.010 m C6H12O6

▫0.0050 m MgCl2

▫0.0080 m HCl

▫0.0040 m Al2(SO4)3