Chapter 15Solutions

Like Dissolves Like

Definitions

Solution - Solution - homogeneous mixture

Solvent Solvent - present in greater amount

Solute Solute - substance being dissolved

Solvation SolvationSolvation

– the process of dissolving

solute particles are separated and pulled into solution

solute particles are surrounded by solvent particles

Solvation DissociationDissociation

– separation of an ionic solid into aqueous ions

– movie

NaCl(s) Na+(aq) + Cl–(aq)

Solvation

Molecular Molecular SolvationSolvation– molecules

stay intact

C6H12O6(s) C6H12O6(aq)

Solvation

IonizationIonization– breaking apart of

some polar molecules into aqueous ions

HNO3(aq) + H2O(l) H3O+(aq) + NO3–(aq)

Solvation

StrongElectrolyte

Non-Electrolyte

solute exists asions only

- +

salt

- +

sugar

solute exists asmolecules

only

- +

acetic acid

WeakElectrolyte

solute exists asions and

molecules

Making solutionsIn order to dissolve, the solvent

molecules must come in contact with the solute.

Stirring moves fresh solvent next to the solute.

The solvent touches the surface of the solute.

Smaller pieces increase the amount of surface area of the solute.

Solution FormationNature of the solute and the solvent

–Whether a substance will dissolve–How much will dissolve

Factors determining rate of solution...–stirred or shaken (agitation)–particles are made smaller– temperature is increased

Why?

Temperature and Solutions

Higher temperature makes the molecules of the solvent move around faster and contact the solute harder and more often.

– Speeds up dissolving.Usually increases the

amount that will dissolve (exception is gases)

How Much?Solubility- The maximum amount of

substance that will dissolve at a specific temperature (g solute/100 g solvent)

Saturated solution- Contains the maximum amount of solute dissolved. movie

Unsaturated solution- Can still dissolve more solute

Supersaturated- solution that is holding more than it theoretically can; seed crystal will make it come out. movie

Solubility

SATURATED SOLUTION

no more solute dissolves

UNSATURATED SOLUTIONmore solute dissolves

SUPERSATURATED SOLUTION

becomes unstable, crystals form

concentration

Cloud SeedingEver heard of seeding the clouds

to make them produce rain?Clouds- mass of air

supersaturated with water vaporSilver Iodide (AgI) crystals are

dusted into the cloudThe AgI attracts the water,

forming droplets to attract others

LiquidsMiscible means that two liquids can

dissolve in each other–water and antifreeze, water and ethanol

Partially miscible-

slightly dissolveImmiscible can’t mix

–oil and vinegar

Solvation

NONPOLAR

NONPOLAR

POLAR

POLAR

DetergentsDetergents– polar “head” with long nonpolar “tail”– can dissolve nonpolar grease in polar water

““Like Dissolves Like”Like Dissolves Like”““Like Dissolves Like”Like Dissolves Like”

Solubility?For solids in liquids, as the

temperature goes up-the solubility usually goes up.

For gases in a liquid, as the temperature goes up-the solubility goes down.

For gases in a liquid, as the pressure goes up-the solubility goes up.

Mentos and Diet CokeSolubility can also be affected by other

factors such as a reduction in water surface tension and the formation of nucleation sites. Nucleation sites are places where gas molecules gather to form a bubble. Mentos contains compounds that reduce surface tension and at the same time the rough surface of the dissolving tablet provides nucleation sites. The combination in rather dramatic as evidenced in the video clip.

Solubility SolubilitySolubility

– maximum grams of solute that will dissolve in 100 g of solvent at a given temperature

– varies with temp– based on a saturated solution

Solubility Solubility Solubility

CurveCurve– shows the

dependence of solubility on temperature

Solubility

Solids are more soluble at...Solids are more soluble at...– high temperatures.

Gases are more soluble at...Gases are more soluble at...– low temperatures.– high pressures (Henry’s Law).– EX: nitrogen narcosis, the “bends,” soda

The Bends

Shallow Water Blackout

DVD Movie

Gases in liquids...Henry’s Law - says the solubility of a

gas in a liquid is directly proportional to the pressure of the gas above the liquid

– think of a bottle of soda pop, removing the lid releases pres.

Equation:

S1 S2

P1 P2

=

Henry’s Law Example

Concentration is...a measure of the amount of solute

dissolved in a given quantity of solventA concentrated solution has a large

amount of soluteA dilute solution has a small amount of

solute– thus, only qualitative descriptions

But, there are ways to express solution concentration quantitatively

Concentration The amount of solute in a solution. Describing Concentration

– % by mass - medicated creams– % by volume - rubbing alcohol– ppm, ppb - water contaminants– molarity - used by chemists– molality - used by chemists

Molarity - Most ImportantThe number of moles of solute in 1

Liter of the solution. Note: not solvent.

M = moles/Liter; such as 6.0 molarWhat is the molarity of a solution with

2.0 moles of NaCl in 250 mL of solution?

Dilution

Adding water to a solution

2211 VMVM

DilutionPreparation of a desired solution by

adding water to a concentrate.Moles of solute remain the same.

DilutionThe number of moles of solute doesn’t

change if you add more solvent!The # moles before = the # moles afterSince M x L = moles thenM1 x V1 = M2 x V2

M1 and V1 are the starting concentration and volume.

M2 and V2 are the final concentration and volume.

Stock solutions are pre-made to a known Molarity

Dilution What volume of 15.8M HNO3 is required to

make 250 mL of a 6.0M solution?

GIVEN:

M1 = 15.8M

V1 = ?

M2 = 6.0M

V2 = 250 mL

WORK:

M1 V1 = M2 V2

(15.8M) V1 = (6.0M)(250mL)

V1 = 95 mL of 15.8M HNO3

Preparing Solutions

250 mL of 6.0M HNO3 by dilution

– measure 95 mL of 15.8M HNO3

95 mL of15.8M HNO3

water for

safety

250 mL mark

– combine with water until total volume is 250 mL

– Safety: “Do as you oughtta, add the acid to the watta!”

Making SolutionsPour in a small amount of solventThen add the solute (to dissolve it)Carefully fill to final volume.Also remember: M x L = moles of

soluteHow many moles of NaCl are

needed to make 8.0 L of a 0.75 M NaCl solution?

Practice2.0 L of a 0.88 M solution are diluted

to 3.8 L. What is the new molarity?You have 150 mL of 6.0 M HCl. What

volume of 1.3 M HCl can you make?You need 450 mL of 0.15 M NaOH.

All you have available is a 2.0 M stock solution of NaOH. How do you make the required solution?

Making Solutions10.3 g of NaCl are dissolved in a

small amount of water, then diluted to 250 mL. What is the concentration?

How many grams of sugar are needed to make 125 mL of a 0.50 M C6H12O6 solution?

Percent Solutions...Percent means parts per 100, soPercent by volume:

= Volume of solute x 100 Volume of solution

indicated %(v/v)What is the percent solution if 25 mL of CH3OH is diluted to 150 mL with water?

Percent SolutionsPercent by mass:

= Mass of solute(g) x 100 Mass of solution(g)

Indicated %(m/m)More commonly used4.8 g of NaCl are dissolved in 82 g of

solution. What is the percent of the solution?

Colligative Properties

Depend only on the number of dissolved particles

Not on what kind of particle

Colligative ApplicationsCommon Applications

– salting icy roads– making ice cream– antifreeze

• cars (-64°C to 136°C)• fish

Solute particles weaken IMF in the solvent.

Boiling Point Elevation

Boiling Point ElevationThe vapor pressure determines

the boiling point.Lower vapor pressure = higher

boiling point.Salt water boils above 100ºCThe number of dissolved

particles determines how much, as well as the solvent itself.

Depends on # of PiecesElectrolytes form ions when they

are dissolved = more pieces.NaCl Na+ + Cl- (= 2 pieces) More pieces = bigger effect

Vapor Pressure DecreasedThe bonds between molecules keep

molecules from escaping.In a solution, some of the solvent is

busy keeping the solute dissolved.Lowers the vapor pressure

Freezing Point DepressionSolids form when molecules make

an orderly pattern.The solute molecules break up the

orderly pattern. Makes the freezing point lower.Salt water freezes below 0ºCHow much depends on the number

of solute particles dissolved.

Freezing Point Depression

Properties of Water Ice is less dense than water

movie

Molality

solvent ofkg

solute of moles(m)molality

mass of solvent only

1 kg water = 1 L waterkg 1

mol0.25 0.25m

Molalitya new unit for concentrationm = Moles of solute

kilogram of solventm = Moles of solute

1000 g of solvent What is the molality of a solution

with 9.3 mole of NaCl in 450 g of water?

MolalityFind the molality of a solution containing 75 g

of MgCl2 in 250 mL of water.

75 g MgCl2 1 mol MgCl2

95.21 g MgCl2

= 3.2m MgCl2

0.250 kg water

kg

molm

MolalityHow many grams of NaCl are req’d to make a

1.54m solution using 0.500 kg of water?

0.500 kg water 1.54 mol NaCl

1 kg water

= 45.0 g NaCl

58.44 g NaCl

1 mol NaCl

kg 1

mol1.5 1.5m

Preparing Solutions 500 mL of 1.54M NaCl

500 mLwater

45.0 gNaCl

– mass 45.0 g of NaCl– add water until total

volume is 500 mL– mass 45.0 g of NaCl– add 0.500 kg of water

500 mLmark

500 mLvolumetric

flask

1.54m NaCl in 0.500 kg of water

Types of Property Change Boiling Point ElevationBoiling Point Elevation (tb)

– b.p. of a solution is higher than b.p. of the pure solvent

Freezing Point DepressionFreezing Point Depression (tf)

– f.p. of a solution is lower than f.p. of the pure solvent

Boiling Point Change?The size of the change in boiling

point is determined by the molality.Tb = Kb x m x nTb is the change in the boiling pointKb is a constant determined by the

solvent.m is the molality of the solution.n is the number of pieces it falls into

when it dissolves.

What About Freezing?The size of the change in freezing

point is also determined by molality.Tf = Kf x m x nTf is the change in freezing pointKf is a constant determined by the

solvent.m is the molality of the solution.n is the number of pieces it falls into

when it dissolves.

Calculations n = the # of Particlesn = the # of Particles

– Nonelectrolytes (covalent)• remain intact when dissolved • 1 particle n=1

– Electrolytes (ionic)• dissociate into ions when dissolved• 2 or more particles n=2 or more

Boiling and Freezing Points and Constants

Boiling–Point Elevation and Freezing–Point Depression

Calculations

t: change in temperature (°C)

k: constant based on the solvent (°C·kg/mol)

m: molality (m)

n: # of particles

t = k · m · n

Calculations At what temperature will a solution that is composed

of 0.73 moles of glucose in 225 g of phenol boil?

m = 3.2mn = 1

WORK:

m = 0.73mol ÷ 0.225kg

GIVEN:b.p. = ? tb = ?

tb = kb · m · n

kb = 3.60°C·kg/mol

tb = (3.60°C·kg/mol)(3.2m)(1)

tb = 12°C

b.p. = 181.8°C + 12°C

b.p. = 194°C

Calculations Find the freezing point of a saturated solution of

NaCl containing 28 g NaCl in 100. mL water.

m = 4.8m

n = 2

WORK:

m = 0.48mol ÷ 0.100kg

GIVEN:

f.p. = ?

tf = ?

tf = kf · m · n

kf = 1.86°C·kg/mol

tf = (1.86°C·kg/mol)(4.8m)(2)

tf = 18°C

f.p. = 0.00°C - 18°C

f.p. = -18°C

ProblemsWhat is the boiling point of a

solution made by dissolving 1.20 moles of NaCl in 7.50e2 g of water?

What is the freezing point?What is the boiling point of a

solution made by dissolving 1.20 moles of CaCl2 in 750. g of water?

What is the freezing point?

Mole FractionThis is another way to express

concentrationIt is the ratio of moles of solute to

total number of moles of solute + solvent

nsolute

nsolute + nsolvent

X =

Mole Fraction ProblemWhat is the mole fraction of solute in

a 35.5 percent by mass aqueous solution of formic acid (HCOOH)?

Molar MassWe can use changes in boiling

and freezing to calculate the molar mass of a substance

Find: 1) molality 2) moles, and then 3) molar mass

MM Example ProblemAn aqueous solution made using

500. mL of solvent has a freezing point of -5.00°C. If 245 g of covalent solid was used, what is the molar mass of the solute?

If the empirical formula of the solute above is CH2O, what is the molecular formula?