Monday January 15, 2024     Day 6
Colligative Properties: Van't Hoff Factor and Freezing Point Depression

Textbook Readings

11.4 Colligative Properties
Course Lecture

3.4  pdf  Video*   Colligative Prop: Boiling Pt. Elv.
Freezing Point Depression

 Van't Hoff Factor

Objectives

1.  Describe the differences between ionic (electrolytes) solution and molecular
     (non-electrolytes) solutions.

2.  Determine individual ionic  concentrations and the total ionic concentration (TOC) for
      a given solution.

3.   Describe what's meant by freezing point depression and how it works on a molecular level.

4.   Calculate new freezing point temperatures for solutions of different concentrations.

5.  Predict comparitive solution freezing points given solution concentrations and ionic formulae.

6.   Describe what is meant by the "Van't Hoff factor" and "ion pairing"

7.   Calculate concentration, Van't Hoff factor and solution concentration given any two of
      three variables.

Homework Problems

6.1  Identify the following as either molecular or ionc.
       i.  Ca(NO3)2          ii.  C12H22O11      iii.  CO2      iv. KCl             v.   Na2CO3       vi. Al2(SO4)3

6.2  For each of the following, determine the identity and number of ions produced
       when dissolved in water.
        i. Ca(OH)2           ii. AlCl3                iii. CO         iv. FeBr3          v. Fe3(PO4)2

6.3   Three different aqueous solutions have different solutes and concentrations as follows:

        i.  0.20 m LiBr          ii.   0.25 m CaCl2          iii.  0.15 m AlCl3

        For each solution determine the molal concentration of each ion.

        For each solution determine the total ion concentration (TOC) by adding together
        the individual ion concentrations.

6.4   Each of the solutions in problem 6.3 freezes at a different temperature.  Arrange the
        solutions in order of decreasing freezing point temperature.

6.5  The Van't Hoff factor "i" ideally indicates the number of particles. molecules or ions,
       produced when a solute dissolves.  For example, when NaCl dissolves, it breaks up into
       individual Na+ and Cl- ions.  These two ions indicate a Van't Hoff factor i = 2.

       For each of the following solutes, determine the ideal Van't Hoff factor.


       i. KBr       ii. NO2     iii. CaCl2        iv. Be(NO3)2        v. K2SO4          vi.  Fe3(PO4)2

6.6   What is ion pairing and what effect does it have on the Van't Hoff factor?

6.7   For each of the following choose the Van't Hoff factor most likely to be an experimentally
        determined value for a dilute aqueous solution containing the given solute.

        a.   MgCl2      i = 2.1       or      i = 2.6      or     i = 2.8        or     i = 3.05       or    i = 3.16

        b.   CO2         i = 1.0       or      i = 1.3      or     i = 2.75        or     i = 2.99      or    i = 3.01

        c.   K3PO4     i = 3.1       or      i = 3.85    or     i = 4.56        or     i = 7.80      or    i = 8.19

6.8  How much the freezing point temperature of the pure solvent is lowered when a solute
       is dissolved can be calculated using the following equation:

                                                  ΔTf   =    i   ×   Kf   ×  m

       where i is the Van't Hoff factor, Kf is the freezing point depression constant for the
       specific solvent, and m is the molality of the solution.

       Calculate  ΔTf   & the new freezing point temperature of a 0.25 m  NaCl solution.

       Assume the ideal Van't Hoff factor.   For water, Kf  = 1.86 oC/m

6.9   The freezing point temperature of a 0.45 m CaCl2 solution is -2.3 oC.
        Determine the experimental Van't Hoff factor, iexp, for this solution.

6.10  How many grams of pyrazine (C4H4N2) would have to be dissolved in 1.50 kg of
         carbon tetrachloride to lower the freezing point by 4.4 °C?
         Kf  for carbon tetrachloride is 30. °C


Click and drag the region below for correct answers

6.1    i. Ionic     ii.   Molecular     iii. Molecular    iv.   Ionic     v.  Ionic     vi.  Ionic

6.2    i. One Ca2+ ion and two OH- ions
         ii. One Al3+ ion and three Cl- ions
         iii. Molecular.  No ions produced and CO remains intact when dissolved in water
         iv.  One Fe3+ ion and three Br- ions
         v.  Three Fe2+ ions and two PO43- ions

6.3      i.  0.20 m LiBr      [Li+]  =    0.20 m      +        [Br-] = 0.20 m         =        TOC   = 0.40 m
          ii.  0.25 m CaCl2    [Ca+2]  =  0.25 m      +        [Cl-] = 0.50 m          =        TOC   = 0.75 m   
         iii.  0.15 m AlCl3    [Al+3]  =  0.15 m       +        [Cl-] = 0.45 m          =        TOC   = 0.60 m  

6.4   Lowest Tf:     0.25 m CaCl2   ....    0.15 m AlCl3   ..... 0.20 m LiBr  ....highest Tf

6.5  i. KBr                iideal  = 2           ii. NO2         iideal  = 1            iii. CaCl2         iideal  = 3    
 
       iv. Be(NO3)2      iideal  = 3           v. K2SO4     iideal  = 3            vi.  Fe3(PO4)2   iideal  =  5

6.6  Ion pairing occurs when dissolved oppositely charged ions momentarily bump into one
       forming one particle from what had been two independent ions.  Although infrequent,
       enough ions experience this to lower the particle concentration and thus lower the Van't
       Hoff factor slightly.

6.7      a.   MgCl2  iexp = 2.8              b.   CO2         iexp = 1.0               c.   K3PO4      iexp= 3.85 

6.8        ΔTf   = 0.93oC      Tf    =     0oC    - ΔTf      =      -0.93oC

6.9        iexp  = 2.7  (less than the ideal value i = 3 as we would expect).

6.10      17.6 grams of pyrazine would be required
Tuesday January 16, 2024   Day 7
Colligative Properties: Boiling Point Elevation
Freezing Point Depression and Molar Mass Determination

Textbook Readings

11.4 Colligative Properties
Course Lectures

3.4  pdf  Video*   Colligative Prop: Boiling Pt. Elv
Boiling Point Elevation
Freezing Point Depression (again)
and Molar Mass Determination
Objectives

1.   Describe what's meant by boiling point elevation and how it works on a molecular level.

2.   Calculate new boiling point temperatures for solutions of different concentrations.

3.   Use boiling point information to calculate molality for the solution, moles
       of solute and solute molar mass.

4.  Use freezing point information to calculate molality for the solution, moles
      of solute and solute molar mass.

Homework Problems

7.1   What is the boiling point elevation (ΔTb) when 11.4 g of ammonia (NH3) is dissolved in 200. g
         of water?    Kb for water is 0.52 °C/m.

7.2   How many grams of fructose (C6H12O6) must be dissolved in 937 g of acetic acid to raise the
          boiling point by 9.1 °C?      (Acetic acid:  Kb  = 3.08 °C/m)

7.3   0.64 g of adrenaline, a molecular compound,  in 36.0 g of CCl4 results in a ΔTb = 0.49 °C.
         What is adrenaline's molar mass?  (Carbon tetrachloride:  Kb = 4.95oC/m)

7.4  When 2.58 g of a molecular compound was dissolved in 40.0 g of benzene, the solution freezes
        at 36.7 °C.   (Benzene:   Kf = 5.12 °C/m        Tf = 41.9°C)
       Determine the molar mass of the molecular compound.


Answers:  Click and drag in the space below

7.1. Δt = 1.74 °C      7.2. 499 gram      7.3. 180 g/mol        7.4. 63.5 g/mol
Wednesday  January 17, 2024   Day 8
Colligative Properties: Partial Pressures, Raoult's Law
and Vapor Pressure Lowering

Textbook Readings

Raoult's Law
Course Lectures

3.3  pdf  Video*   Colligative Prop: Raoults Law
Objectives

1.  Describe how the presence of a solute
    lowers the vapor pressure of a solvent
    on a molecular level.

2. Correctly calculate mole fraction given
   gram amounts of solute and solvent (review).

3.  Use the Clausius-Clapeyron Equation to
    determine liquid vapor pressures at
    different temperatures 		Partial Pressures of Gases and Mole Fractions

4. Use mole fractions to determine the vapor pressures of a solution

5. Calculate the mass of solute required to reduce the vapor pressure of the solvent to a
     predetermined level.


Homework Problems

8.1  The vapor pressure of a liquid always increases as the temperature increases. 
       What's happening on a molecular level?

8.2   The vapor pressure of water at 298K is 23.8 mmHg. Use the Clausius-Clapeyron Equation to
        determine the  temperature when  the vapor pressure of water reaches 1075 mmHg.

                             Useful Information:  H2O: ΔHvap = 40.65 kJ/mol       Tb = 373.15 K

8.3   When table salt is added to a pot of boiling water, the boiling momentarily stops. 
         Why is this (there are two reasons)?

8.4   1.5 moles of cherry Kool-Aid are added to a pitcher containing 2 liters of water on a nice
        day at 25o C. The vapor pressure of water alone is 23.8 mm Hg at 25o C.
        What is the new vapor pressure of Kool-Aid?

8.5   Calculate the vapor pressure of a solution made by dissolving 50.0 g glucose,  C6H12O6 , in
        500. g of water. The vapor pressure of pure water is 47.1 torr at 37°C

8.6   Calculate the vapor pressure of a solution made by dissolving 50.0 g CaCl2 and
        50.0 g  C6H12O6 , in 500. g of water. The vapor pressure of pure water is 47.1 torr at 37°C

8.7   What are the partial pressures of benzene and toluene in a solution in which the
         mole fraction of benzene is 0.6?

         What is the total vapor pressure?

        The vapor pressure of pure benzene and toluene are  is 95.1 mm Hg and  28.4 mm Hg at 25oC..

Click and drag the region below for answers

8.1   As temperature increases, so do the kinetic energies of solvent species.
        This makes it possible for more liquid phase species to leave the liquid's surface and become
        gas phase species where their presense increase the vapor pressure of the gas.

8.2  388.1 K

8.3  Reason #1:  The salt is initially cooler than the boiling water.  The heat energy required to
                            warm the salt is sacrificed by the boiling water which momentarily stops boiling
                            until the boiling temperature is restored.  In other words, whenever something
                            cool is added to boiling water, the boiling stops until things warm up again.

       Reason #2:  The presense of the dissolved salt lowers the vapor pressure of water. 
                            Higher temperatures are then required to restore the vapor pressure of
                            water to atmospheric pressure and the pot boils again.

8.4 - 8.7  Solutions available in text book.  Click HERE
Thursday January 18, 2024   Day 9
Colligative Properties: Osmosis & Osmotic Pressure

Textbook Readings

11.4 Colligative Properties
Course Lectures

3.2  pdf  Video*   Colligative Prop: Osmotic Pressure


Qualitative Osmosis Explanation


 		Osmotic Pressure
Objectives

1.  Describe on a molecular level how osmotic pressure develops.

2. Define the terms hypotonic, isotonic, hypertonic, and semipermeable membrane and how
     they relate to the osmotic process.

3. Predict the solvent flow in osmotic situations and the effect osmosis has on the relative
    volumes of vliquid either side of a semipermeable membrane.

4. Calculate osmotic pressure given solution concentrations and temperatures

5. Determine the concentrations and solute amounts required to produce a specific osmotic pressure.

6. Use osmotic pressure information to obtain solute molar mass.

Homework Problems

9.1 What is the osmotic pressure of a solution prepared by adding 13.65 g of sucrose (C12H22O11)
     to enough water to make 250 mL of solution at 25 °C?

9.2 Sea water contains dissolved salts at a total ionic concentration of about 1.13 mol/L.

    What pressure must be applied to prevent osmotic flow of pure water into sea water through
    a membrane permeable only to water molecules at 25 °C? ?

9.3 The osmotic pressure of a benzene solution containing 5.0 g of polystyrene per liter was
     found to be 7.6 torr at 25°C.
     Estimate the average molecular weight of the polystyrene in this sample.

Click and drag the region below for correct answers

9.1    3.9 atm           9.2   27.6 atm        9.3   12 200 g/mol
Friday January 19, 2024   Day 10
Colloids

Textbook Reading

11.5 Colloids
Course Lectures
Objectives

1.  Describe the differences between suspensions,
     solutions and colloidal mixtures.

2.  List the different colloidal dispersions their
    dispersed phase & medium and give
    examples of each.  (sol, aerosol,
    emulsion, gel and foam)

3. Describe the operation of an emulsifying
    agent and the cleaning action of soap.

 		Types of Colloids and their Properties

4.  Describe the two ways colloidal particles can be made and provide examples of each.

5.  Describe how colloidal dispersions interactions with charged surfaces (smoke cleanup)
     and light (Tyndall effect)

Homework Problems

10.1  Identify the dispersed phase and the dispersion medium in each of the following colloidal
         systems:   starch dispersion, smoke, fog, pearl, whipped cream, floating soap, jelly, milk, and ruby.

10.2  Distinguish between dispersion methods and condensation methods for preparing colloidal systems.

10.3  How do colloids differ from solutions with regard to dispersed particle size and homogeneity?

10.4  Explain the cleansing action of soap.

10.5  How can it be demonstrated that colloidal particles are electrically charged?

 
See textbook for answers.
End Week 2