13 Chapt13 Soln2 - Pennsylvania State Universitycourses.chem.psu.edu/chem110spring/lecture...

Solutions Colligative Properties •  vapor pressure lowering

Raoult’s Law: PA = XAP°A •  boiling point elevation

!Tb = Kbm •  freezing point depression

!Tf = Kfm •  osmotic pressure

!

" =nV#

$ %

&

' ( RT = M RT

Colloids

1 Mary J. Bojan Chem 110

Test Question Which of the following aqueous solutions has the greatest total concentration of ions?

1.  0.2M NH4NO3 2.  0.2M Pb(NO3)2 3.  0.2M Na2SO4 4.  0.2M AlPO4 5.  0.2M AlBr3 6.  0.2M CH3COOH (acetic acid)


COLLIGATIVE PROPERTIES are properties that depend on the number of solute particles

(collections) in solution but not on the identity of the solute.


Examples of Colligative properties •  vapor pressure lowering •  boiling point elevation •  freezing point depression •  osmotic pressure

Example: What is the total particle concentration in a 0.2M solution of Pb(NO3)2 ? (salt, strong electrolyte) [Pb+2]= [NO3

"]=

total concentration of particles =

COLLIGATIVE PROPERTIES


What is the total particle concentration 0.2M solution of CH3COOH (acetic acid)? (weak acid, weak electrolyte) CH3COOH(aq) ! H+(aq) + CH3COO"(aq)

When a non volatile solute is dissolved in a volatile solvent, the VAPOR PRESSURE of

the solvent will be LOWERED

VAPOR PRESSURE LOWERING


rate of evaporation ! vapor pressure of SOLVENT !

DEMO: VAPOR PRESSURE LOWERING


Quantitatively: for dilute solutions the partial pressure of the solution PA is related to the mole fraction and the vapor pressure of the

pure solvent P°A Raoult’s Law: PA = XAP°A

PA = vapor pressure of solution XA = mole fraction of solvent P°A = vapor pressure of pure solvent


Vapor pressure lowering is a colligative property It depends on the concentration but not on the nature of the solute.

Lowering of the vapor pressure results in changes in the melting and boiling points.

BOILING POINT ELEVATION

FREEZING POINT DEPRESSION !Tb = Kbm Kb = molal boiling point elevation constant !Tf = Kfm Kf = molal freezing point depression constant

m = molality of the solution


Kb and Kf are tabulated for different solvents

Eg. water Kb= 0.52°C/m Kf= 1.86°C/m benzene Kb= 2.53°C/m Kf= 5.12°C/m

Don’t forget that these are colligative

properties!!!

Example

What is the freezing point of seawater assuming all of the salinity* is due to NaCl?

(MW of NaCl = 58.5 g/mol)

*ocean salinity is ca. (35 g salt)/(1 kg seawater) mainly: Cl–, Na+, SO4

2", Mg2+, Ca2+, K+


Osmosis: flow of molecules through a membrane

Osmotic pressure: # Pressure needed to stop the flow of molecules through a membrane

!

" = nV#

$ %

&

' ( RT = MRT

units of atm molarity


Osmosis through a cell membrane

Hypertonic: solution: water moves out of cell

Hypotonic solution: water moves into cell; can cause cell to rupture.

Isotonic: solution that has same osmotic pressure as cell. 11 Mary J. Bojan Chem 110

Reverse Osmosis: exert pressure on a concentrated solution, forcing solvent

molecules through a membrane. Solute molecules are trapped in concentrated portion.)

Used to purify water.


An aqueous solution contains 3.50 mg of protein dissolved in sufficient water to form 5.00 mL of solution. The osmotic pressure of the solution at 25°C was found to be 1.54 torr.

Calculate the molar mass of the protein.


The size of particles in a mixture influences the properties of the mixture.


Particle Size small large solution Colloidal dispersion mixture uniform uniform non-uniform molecules Particle size = 20-2000 Å Sedimentary

particles EXAMPLES

NaCl(aq) Milk (fat particles) silt Sugar(aq) Fog (water droplets)

Colloid: homogeneous mixture of particles defined by the size of the particles

•  Gravity does not cause particles to settle •  between solution and heterogeneous mixture •  particles are very large molecules or aggregates •  phases are mutually insoluble

Mary J. Bojan Chem 110 15

How light interacts with matter: Light scattering

Tyndall effect: light is scattered when $ of light % particle size

For colloids (size~ 200nm) $ is in visible region: Light passes through colloidal suspensions with scattering (milk, fog).


For molecules (~5Å) $ in X-ray region •  visible light passes through solutions without scattering •  Atmosphere scatters small $ light

Explains why the sky is blue

Beautiful colors found in stained glass windows are made using Gold Colloids

•  Elixir of Life, a potion made from gold (ancient Rome)

•  Used to stain glass


Solutions of gold nanoparticles: colors are related to size of the gold particles suspended in solution

13 Chapt13 Soln2 - Pennsylvania State Universitycourses.chem.psu.edu/chem110spring/lecture...

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