SURVEY OF CHEMISTRY I

CHEM 1151

CHAPTER 7

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 7

SOLUTIONS AND COLLOIDS

- A homogeneous mixture of two or more substances

Solvent - The substance present in the greatest quantity

Solute- The other substance(s) dissolved in the solvent

SOLUTION

- Solutions can exist in any of the physical states

Solid Solution- dental fillings, metal alloys (steel), polymers

Liquid Solution- sugar in water, salt in water, wine, vinegar

Gas Solution- air (O2, Ar, etc. in N2),

- NOx, SO2, CO2 in the atmosphere

SOLUTION

- A measure of how much of a solute can be dissolved in a solventat a given temperature

- Units: grams/100 mL

ExampleSolubility of sugar in water at 20 oC is 204 g/100 mL H2O

Three factors that affect solubility- Temperature

- Pressure- Polarity

SOLUBILITY

Unsaturated Solution- More solute can still be dissolved at a given temperature

Saturated Solution- No more solute can be dissolved at a given temperature

Supersaturated Solution- Too much solute has temporarily been dissolved

(more than solute solubility)

Precipitate- Solute (solid) that falls out of solution

SOLUBILITY

- A solution contains a solvent and one or more solutes

- The ratio of solute to solvent may vary in a solution

- Solution properties change with solute to solvent ratio

- The dissolved solutes are present as individual particles (ions, atoms, or molecules)

- Solutes remain uniformly distributed throughout the solution

- Solutes are separated by physical means (evaporation, distillation)

PROPERTIES OF SOLUTIONS

- A solution in which water (H2O) is the solventNaCl solution: solvent is H2O and solute is NaCl

Hydrophilic- Substances that dissolve in water

- Water loving (NaCl)- Usually polar substances

Hydrophobic- Substances that do not dissolve well in water

- Water fearing (hydrocarbons)- Usually nonpolar substances

AQUEOUS SOLUTION

- Ions make aqueous solutions good conductors of electricity

- Solution conductivity indicates the presence of ions

AQUEOUS SOLUTION

Ionic Compounds

- Form ions in aqueous solution (dissociate into component ions)

Example- NaCl solution contains Na+ and Cl- ions

NaCl(aq) → Na+(aq) + Cl-(aq)- Each ion is surrounded by water molecules

- Good conductor of electricity

AQUEOUS SOLUTION

Solvation Process

- Ions in aqueous solution are surrounded by the H2O molecules

- The O atom in each H2O molecule has partial negative charge (δ-) - Attract positive ions

- The H atoms have partial positive charge (δ+)- Attract negative ions

- Cations and anions are prevented from recombining

- Ions disperse uniformly throughout the solution (homogeneous)

AQUEOUS SOLUTION

Molecular Compounds

- Most molecular compounds do not form ions in aqueous solution- The molecules disperse throughout the solution

- Molecules are surrounded by H2O molecules

Example- Sucrose solution contains neutral sucrose molecules

- Each molecule is surrounded by water molecules - Poor conductor of electricity

- A few molecular compounds form ions in aqueous solution- HCl dissociates into H+(aq) and Cl-(aq)

- HNO3 dissociates into H+(aq) and NO3-(aq)

AQUEOUS SOLUTION

- A solution in which another substance other than water is the solvent

ExamplesAlcohol

petroleum etherPentane

Carbon tetrachloride

NONAQUEOUS SOLUTION

The rate at which solutes dissolve can be increased by

- Grinding or crushing solute particles (size reduction)

- Heating

- Stirring or agitation

RATE OF DISSOLUTION

- The amount of solute dissolved in a given quantity of solvent or solution

Percent Concentration- Percent by mass [mass-mass percent, %(m/m)]

mass of solution = mass of solute + mass of solvent

CONCENTRATION OF SOLUTIONS

100%xsolutionofmass

soluteofmass%(m/m)

A sugar solution is made by dissolving 5.8 g of sugar in82.5 g of water. Calculate the percent by mass concentration

of sugar.

% 6.6 100%xg 82.5) (5.8

g 5.8%(m/m)

CONCENTRATION OF SOLUTIONS

volume of solution ≠ volume of solvent + volume of solute

- Due differences in bond lengths and angles

- The amount of solute dissolved in a given quantity of solvent or solution

Percent Concentration- Percent by volume [volume-volume percent, %(v/v)]

CONCENTRATION OF SOLUTIONS

100%xsolutionofvolume

soluteofvolume%(v/v)

Calculate the volume percent of solute if 345 mL of ethyl alcohol is dissolved in enough water to produce 1257 mL

of solution

CONCENTRATION OF SOLUTIONS

% 27.4 100%xmL 1257

mL 345%(v/v)

- Units are specified because they do not cancel

- The amount of solute dissolved in a given quantity of solvent or solution

Percent Concentration- Mass-volume percent [%(m/v)]

CONCENTRATION OF SOLUTIONS

100%xsolutionofvolume

soluteofmass%(m/v)

The concentration of a solution of NaCl is 0.92 %(m/v)used to dissolve drugs for intravenous use. What is the

amount, in grams, of NaCl needed to prepare 41.50 mL of the solution?

g solute = [%(m/v)] x [volume of solution (mL)]/[100 %]

= [(0.92 % g/mL) x (41.50 mL)]/(100 %)

= 0.38 g

CONCENTRATION OF SOLUTIONS

Molarity (M: molar)

- The number of moles of solute per liter of solution

Lsolutionofvolume

solutemolesMolarity

- A solution of 1.00 M (read as 1.00 molar) contains 1.00 mole of solute per liter of solution

CONCENTRATION OF SOLUTIONS

Calculate the molarity of a solution made by dissolving 2.56 g of NaCl in enough water to make 2.00 L of solution

- Calculate moles of NaCl using grams and molar mass- Convert volume of solution to liters

- Calculate molarity using moles and liters

NaClmol0.0438NaClg58.44

NaClmol1xNaClg2.56

mol/L)(orM0.0219solutionL2.00

NaClmol0.0438Molarity

CONCENTRATION OF SOLUTIONS

After dissolving 1.56 g of NaOH in a certain volume of water, the resulting solution had a concentration of 1.60 M. Calculate the

volume of the resulting NaOH solution

- Convert grams NaOH to moles using molar mass- Calculate volume (L) using moles and molarity

NaOHmol0.0380NaOHg41.00

NaOHmol1xNaOHg1.56

solutionL0.0237NaOHmol1.60

solutionLxNaOHmol0.0380solution Volume

CONCENTRATION OF SOLUTIONS

Mole Fraction

- Fraction of moles of a component of solution

CONCENTRATION OF SOLUTIONS

components all of moles total

component of molesfraction mole

Given that the total moles of an aqueous solution of NaCl andother solutes is 1.75 mol. Calculate the mole fraction of NaCl

if the solution contains 4.56 g NaCl.

NaClmol0.0780NaClg58.44

NaClmole1xNaClg4.56NaClMoles

CONCENTRATION OF SOLUTIONS

0446.0 totalmol 1.75

NaCl mol 0.0780fraction mole

DILUTION

Consider a stock solution of concentration M1 and volume V1

If water is added to dilute to a new concentration M2 and volume V2

moles before dilution = moles after dilution

M1V1 = M2V2

Calculate the volume of 3.50 M HCl needed to prepare 500.0 mL of 0.100 M HCl

(3.50 M)(V1) = (0.100 M)(500.0 mL)

V1 = 14.3 mL

CHEMICAL ANALYSIS

Calculate the concentration of NaOH solution if 24.50 mL of this base is needed to neutralize 12.00 mL of 0.225 M HCl solution

- Write balanced equation and determine mole ratio

- Calculate moles of HCl (convert mL to L)

- Determine moles of NaOH

-Calculate molarity of NaOH

NaOH + HCl → NaCl + H2O

1 mole NaOH : 1 mole HCl

Volume HCl = 12.00 mL = 0.01200 L

mole HCl = 0.225 M x 0.01200 mL = 0.00270 mol = mole NaOH

NaOHM0.110NaOHL1

NaOHmL1000x

NaOHmL24.50

NaOHmol0.00270NaOHMolarity

CHEMICAL ANALYSIS

How many grams of KOH are needed to neutralize 25.00 mL of 0.250 M H2SO4 solution

- Write balanced equation and determine mole ratio

- Calculate moles of H2SO4

- Determine moles of KOH

- Calculate grams of KOH using molar mass

CHEMICAL ANALYSIS

2KOH + H2SO4 → K2SO4 + 2H2O

2 mole KOH : 1 mole H2SO4

mole H2SO4 = 0.250 M x 0.02500 L = 0.00625 mol

Mole KOH = 2 x 0.00625 mol = 0.0125 mol

KOHg0.701KOHmol1

KOHg56.11xKOHmol0.0125KOHmass

CHEMICAL ANALYSIS

- Substances whose aqueous solutions contain ions NaCl(aq) → Na+(aq) + Cl-(aq)

- Two categories: strong and weak electrolytes

Strong Electrolytes- Solutes that completely or nearly completely

ionize when dissolved in water

Salts: NaCl, NH4Cl, KBr, NaNO3

Strong acids: HCl, HNO3, H2SO4

Strong Bases: NaOH, KOH, Ca(OH)2

ELECTROLYTES

- Substances whose aqueous solutions contain ionsNaCl(aq) → Na+(aq) + Cl-(aq)

- Two categories: strong and weak electrolytes

Weak Electrolytes- Only a small fraction of solutes ionize when

dissolved in water (exhibit a small degree of ionization)

Weak acids: acetic acid (HC2H3O2), citric acid (C6H8O7)Weak bases: ammonia (NH3) methylamine, cocaine, morphine

ELECTROLYTES

- Single arrow is used to represent ionization of strong electrolytesH2SO4(aq) → H+(aq) + HSO4

-(aq)- Ions have no tendency of recombining to form H2SO4

- Double arrow is used to represent ionization of weak electrolytesHC2H3O2(aq) ↔ H+(aq) + C2H3O2

-(aq)- This implies reaction occurs in both directions

- Chemical equilibrium is when there is a balance in both directions

ELECTROLYTES

NONELECTROLYTES

- Substances whose aqueous solutions do not contain ions

ExamplesMany molecular compounds

Sucrose (C12H22O11) ethanol (C2H5OH)

COLLIGATIVE PROPERTIES

- Physical properties of a solvent changes when a solute is added

- Four physical properties change based on the amount of solute added but not the solute’s chemical identity

- These are known as the Colligative Properties- Vapor-pressure lowering- Boiling-point elevation

- Freezing-point depression- Osmotic pressure

Osmolarity (osmol) = (M) x (i)i = number of particles produced from the dissociation

of one formula unit of solute

- The number of particles present determines the osmotic pressure

- NaCl dissociates in solution to produce 2 particles (Na+ and Cl-)

- Glucose does not dissociate - The osmotic pressure of NaCl is twice that of glucose

- Solutions with higher osmotic pressure take up more water than solutions with lower osmotic pressure

COLLIGATIVE PROPERTIES

COLLOIDAL DISPERSION

- A substance is dispersed in another substance but not dissolved(typically cloudy)

- Dispersed particles are intermediate in size between those of a

true solution and an ordinary heterogeneous mixture

- Dispersed phase is used in place of solute

- Dispersing medium is used in place of solvent

ExamplesBlood, milk, smoke, fog, cheese, shaving cream

- The dispersed phase do not settle out under the influence of gravity and cannot be filtered out with filter paper

- Difficult to distinguish with the naked eye but scatters a beam of light

(Tyndall effect)

COLLOIDAL DISPERSION

- The passage of ions and small molecules through semipermeable membranes

- The semipermeable membrane is known as the dialyzing membrane

- The membrane holds back colloid particles and large molecules but allows solvent, hydrated ions, and small

molecules to pass through

- This technique is used to clean blood of people with kidneymalfunction

DIALYSIS