the nature of solution

I. The Nature of Solutions

Solutions

A. Definitions

Solution - homogeneous mixture

Solvent - present in greater amount (usually liquid, like H20)

Solute - substance being dissolved

A. Definitions

Solute - KMnO4 Solvent - H2O

B. Solvation

Solvation – the process of dissolving

solute particles are separated and pulled into solution

solute particles are surrounded by solvent particles

B. Solvation

StrongElectrolyte

Non-Electrolyte

solute exists asions only

- +

salt

- +

sugar

solute exists asmolecules

only

- +

acetic acid

WeakElectrolyte

solute exists asions and

molecules DISSOCIATION IONIZATION

View animation online.

B. Solvation

Dissociation› separation of an ionic

solid into aqueous ions

NaCl(s) Na+(aq) + Cl–(aq)

B. Solvation

Ionization› breaking apart of

some polar molecules into aqueous ions

HNO3(aq) + H2O(l) H3O+(aq) + NO3–(aq)

B. Solvation

NONPOLAR

NONPOLAR

POLAR

POLAR

“Like Dissolves Like”“Like Dissolves Like”

B. Solvation

Soap/Detergent› polar “head” with long nonpolar “tail”› dissolves nonpolar grease in polar water

C. Solubility

SATURATED SOLUTION

no more solute dissolves

UNSATURATED SOLUTIONmore solute dissolves

SUPERSATURATED SOLUTION

becomes unstable, crystals form

concentration

C. Solubility

Solubility› maximum grams of solute that will dissolve

in 100 g of solvent at a given temperature› varies with temp› based on a saturated solution

C. Solubility

Solubility Curve› shows the

dependence of solubility on temperature

C. Solubility

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

Gases are more soluble at...

low temperatures &high pressures (Henry’s

Law).EX: nitrogen narcosis,

the “bends,” soda

II. Concentration

Solutions - Part II

A. Concentration

The amount of solute in a solution.

How can we describe or communicate concentration?› Qualitatively through words

(“black” coffee vs. “a light amber shade”)

› Quantitatively through numbers(2 sugars & 1 cream)

A. Calculating Concentration

With Percent (%) % by mass

› Medicated creams› Proactiv contains 2.5%

benzoyl peroxide by mass

% by volume› Usually used when

both solute & solvent are liquids

› Gasoline can contain up to 10% ethanol by volume

With Moles* Molarity (M) Molality (m) Mole Fraction (X)

** Most often used by chemists **

Parts› ppm, ppb - water

contaminants

(don’t talk about in here)

B. Formulas

solution of L

solute of moles(M)molarity

(mL) solution ofvolume

(grams)solute of massm/v %

(grams) solution of mass

(grams)solute of massm/m %

solution ofvolume total

solute ofvolume v/v %

solution of Moles

solute of MolesionMole Fract

Remember to multiply the % formulas by 100 OR move decimal two spots left

B. Formulas

solvent ofkg

solute of moles(m)molality

2211 VMVM

mass of solvent only

1 kg water = 1 L

water

kg 1

mol0.25 0.25m

C. Practice Problems (Molality)

Find 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

0.25 kg water

kg

molm

C. Practice Problems (Molality)

How 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

C. Practice Problems{Concentration by Percent (%)}

At a restaurant I saw a man put 7 packets of sugar into his cup of coffee. What is the percent by mass of sugar in this poor man’s coffee?› 1 packet of sugar = 4 grams› 1 cup of coffee = 180 grams

28 g sugar

(180 + 28) g= 13.5% sugar

C. Practice Problems (Mole Fraction (X) Example)

Find the mole fraction of sugar in the coffee of that now comatose man who has just come down off his 7 sugar packet high.› 0.08 moles sugar› 10 moles coffee

0.08 moles sugar

(10 + 0.08) total molesXsugar =

= 0.0079

Xcoffee = 1 – Xsugar = 1 – 0.0079 = 0.9921

C. Practice Problems (Mole Fraction (X) Example)

What is the mole fraction of each component in a solution in which 3.57 g of sodium chloride, NaCl, is dissolved in

25.0 g of water?

First, convert from mass of NaCl to moles of NaCl. 3.57 g NaCl x 1 mole NaCl = 0.0610857139 mole NaCl

58.44247 g NaClNext, convert from mass of water to moles of water.

25.0 g H2O x 1 mole H2O = 1.387710877 mole H2O 18.01528 g H2O

Substitute these two quantities into the defining equation for mole fraction.

XNaCl = 0.0610857139 mole NaCl = 0.0421630713 ~ 0.042 (0.0610857139 + 1.387710877) mol solution

Xwater = 1.387710877 mole H20 = 0.9578369287 ~ 0.96 (0.0610857139 + 1.387710877) mol solution

 ** Note the sum of the mole fractions for a solution will equal 1. **

2211 VMVM

D. Dilution Preparation of a desired solution by

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

D. Dilution Example

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

E. 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

E. 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!”

Solution Preparation Lab Turn in one paper per team. Complete the following steps:

A) Show the necessary calculations.

B) Write out directions for preparing the solution.

C) Prepare the solution.

For each of the following solutions:1) 100.0 mL of 0.50M NaCl

2) 0.25m NaCl in 100.0 mL of water

3) 100.0 mL of 3.0M HCl from 12.1M concentrate.

III. Colligative Properties Solutions – Part III

A. Definition

Colligative Property› property that depends on the

concentration of solute particles, not their identity

B. Types

Freezing Point Depression (tf)› f.p. of a solution is lower than f.p. of the

pure solvent

Boiling Point Elevation (tb)› b.p. of a solution is higher than b.p. of the

pure solvent

B. Types

View Flash animation.

Freezing Point Depression

B. Types

Solute particles weaken IMF in the solvent.

Boiling Point Elevation

B. Types

Applications› salting icy roads› making ice cream› antifreeze

cars (-64°C to 136°C) fish & insects

C. 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

C. Calculations # of Particles

› Nonelectrolytes (covalent) remain intact when dissolved 1 particle

› Electrolytes (ionic) dissociate into ions when dissolved 2 or more particles

C. 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 = 1tb = kb · m · n

WORK:

m = 0.73mol ÷ 0.225kg

GIVEN:b.p. = ?tb = ?

kb = 3.60°C·kg/moltb = (3.60°C·kg/mol)(3.2m)(1)

tb = 12°C

b.p. = 181.8°C + 12°C

b.p. = 194°C

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

tf = kf · m · n

WORK:

m = 0.48mol ÷ 0.100kg

GIVEN:

f.p. = ?

tf = ?

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

IV. Total and Net Ionic Equations

Solutions – Part IV

Total Ionic Equations

Once you write the molecular equation (synthesis, decomposition, etc.), you should check for reactants and products that are soluble or insoluble (determine states of matter, which stay the same throughout reaction).

Use a solubility table to tell us what compounds are aqueous (dissolved in water), solids, liquids, and/or gases

We usually assume the reaction is in water If the compound is soluble (does dissolve in

water), the the compound splits into its ions If the compound is insoluble (does NOT dissolve

in water), then it remains as a compound

Steps to Writing Net Ionic Equations

1. Write a balanced chemical (molecular) equation2. Consult the solubility rules (along with strong acids

and strong bases) and assign the correct state of matter symbol annotation

3. Write the Total Ionic Equation (T.I.E)a. All compounds that are annotated (aq) break up into

individual cations and anions in that order

4. Eliminate spectator ionsa. Spectator ions-those ions that do not participate in the

chemical reaction but are present in the reaction mixtureb. Spectator ions are in the same form on each side of the

equation arrow

5. Write the Net Ionic Equation (N.I.E)a. The convention is to write the cation first followed by the

anion on the reactants side

Strong Acids and Strong Bases

Strong Acids› Hdrochloric acid -

HCl› Nitric acid - HNO3

› Sulfuric acid - H2SO4

› Hydrobromic acid - HBr

› Hydroiodic acid - HI› Chloric acid - HClO3

› Perchloric acid - HClO4

A strong acid is an acid that ionizes completely in water; weak ones DO NOT ionize!

Strong Bases› Lithium Hydroxide –

Li(OH)› Sodium Hydroxide –

Na(OH)› Potassium Hydroxide –

K(OH)› Calcium Hydroxide –

Ca(OH)2

› Rubidium Hydroxide – Rb(OH)

› Strontium Hydroxide – Sr(OH)2

› Cesium Hydroxide – Cs(OH)

› Barium Hydroxide – Ba(OH)2

Solubility Table

Solubility Rules

Total Ionic Equations

Molecular Equation:K2CrO4 + Pb(NO3)2 PbCrO4 + 2 KNO3

Soluble Soluble Insoluble Soluble

Total Ionic Equation:2 K+ + CrO4 -2 + Pb+2 + 2 NO3

- PbCrO4 (s) + 2 K+ + 2 NO3

-

Net Ionic Equation:CrO4 -2 + Pb+2 PbCrO4 (s)

Potassium chromate mixes with lead (II) nitrate

More ExamplesBalanced Chemical Equation:

Pb(NO3)2(aq) + 2NaI(aq) PbI2(s) + 2NaNO3(aq)

“Complete Ionic” Equation:Pb2+

(aq) + 2NO3-(aq) + 2Na+

(aq)+ 2I-(aq) PbI2(s) + 2Na+

(aq) +

2NO3- (aq)

Cancel the “spectator ions” that appear on both sides of the arrow

Pb2+(aq) + 2NO3

-(aq) + 2Na+

(aq)+ 2I-(aq) PbI2(s) + 2Na+

(aq) +

2NO3- (aq)

“Net Ionic” Equation:Pb2+

(aq) + 2I-(aq) PbI2(s)

Tips/Hints

States of matter assigned in the molecular equation stay the same throughout the T.I.E. and the N.I.E.

Solids, liquids, and gases DO NOT ionize

Gases keep their subscript (it doesn’t become a coefficient like other compounds)› For example: H2, Br2, Cl2, etc. stay like this

They don’t become 2H-1, 2Br -1, 2Cl -1