1. Chemistry: A Molecular Approach, 1st Ed. Nivaldo Tro
Chapter 12 Solutions
Roy Kennedy
Massachusetts Bay Community College
Wellesley Hills, MA
2008, Prentice Hall

2. Solutions solute is the dissolved substance
seems to “disappear”
“takes on the state” of the solvent
solvent is the substance solute dissolves in
does not appear to change state
when both solute and solvent have the same state, the solvent is the component present in the highest percentage
solutions in which the solvent is water are called aqueous solutions
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3. Solution Concentration Molarity
moles of solute per 1 liter of solution
used because it describes how many molecules of solute in each liter of solution
if a sugar solution concentration is 2.0 M, 1 liter of solution contains 2.0 moles of sugar, 2 liters = 4.0 moles sugar, 0.5 liters = 1.0 mole sugar
molarity =
moles of solute
liters of solution
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4. Molarity and Dissociation
the molarity of the ionic compound allows you to determine the molarity of the dissolved ions
CaCl2(aq) = Ca+2(aq) + 2 Cl-1(aq)
A 1.0 M CaCl2(aq) solution contains 1.0 moles of CaCl2 in each liter of solution
1 L = 1.0 moles CaCl2, 2 L = 2.0 moles CaCl2
Because each CaCl2 dissociates to give one Ca+2 = 1.0 M Ca+2
1 L = 1.0 moles Ca+2, 2 L = 2.0 moles Ca+2
Because each CaCl2 dissociates to give 2 Cl-1 = M Cl-1
1 L = 2.0 moles Cl-1, 2 L = 4.0 moles Cl-1
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5. Solution Concentration Molality, m
moles of solute per 1 kilogram of solvent
defined in terms of amount of solvent, not solution
like the others
does not vary with temperature
because based on masses, not volumes
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6. Percent parts of solute in every 100 parts solution
mass percent = mass of solute in 100 parts solution by mass
if a solution is 0.9% by mass, then there are 0.9 grams of solute in every 100 grams of solution
or 0.9 kg solute in every 100 kg solution
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7. Using Concentrations as Conversion Factors
concentrations show the relationship between the amount of solute and the amount of solvent
12%(m/m) sugar(aq) means 12 g sugar  100 g solution
or 12 kg sugar  100 kg solution; or 12 lbs.  100 lbs. solution
5.5%(m/v) Ag in Hg means 5.5 g Ag  100 mL solution
22%(v/v) alcohol(aq) means 22 mL EtOH  100 mL solution
The concentration can then be used to convert the amount of solute into the amount of solution, or vice versa
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8. Preparing a Solution
need to know amount of solution and concentration of solution
calculate the mass of solute needed
start with amount of solution
use concentration as a conversion factor
5% by mass Þ 5 g solute  100 g solution
“Dissolve the grams of solute in enough solvent to total the total amount of solution.”
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9. Example
How would you prepare g of 5.00% by mass glucose solution (normal glucose)?

10. Solution Concentration PPM
grams of solute per 1,000,000 g of solution
mg of solute per 1 kg of solution
1 liter of water = 1 kg of water
for water solutions we often approximate the kg of the solution as the kg or L of water
grams solute
grams solution
x 106
mg solute
L solution
mg solute
kg solution

11. Example
What volume of 10.5% by mass soda contains 78.5 g of sugar? Density of glucose is 1.04g/ml

12. Solution Concentrations Mole Fraction, XA
the mole fraction is the fraction of the moles of one component in the total moles of all the components of the solution
total of all the mole fractions in a solution = 1
unitless
the mole percentage is the percentage of the moles of one component in the total moles of all the components of the solution
= mole fraction x 100%
mole fraction of A = XA =
moles of components A
total moles in the solution
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13. Example
What is the molarity of a solution prepared by mixing 17.2 g of C2H6O2 with kg of H2O to make 515 mL of solution?
What is the percent by mass of a solution prepared by mixing 17.2 g of C2H6O2 with kg of H2O to make 515 mL of solution?
What is the mole fraction of a solution prepared by mixing 17.2 g of C2H6O2 with kg of H2O to make 515 mL of solution?

14. A water sample is found to contain the pollutant chlorobenzene with a concentration of 15 ppb (by mass). What volume of this water contains 5.00 x 102 mg of chlorobenzene? Assume density of 1.00 g/ml

15. Spontaneous Mixing
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16. Mixing and the Solution Process Entropy
formation of a solution does not necessarily lower the potential energy of the system
the difference in attractive forces between atoms of two separate ideal gases vs. two mixed ideal gases is negligible
yet the gases mix spontaneously
the gases mix because the energy of the system is lowered through the release of entropy
entropy is the measure of energy dispersal throughout the system
energy has a spontaneous drive to spread out over as large a volume as it is allowed
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17. Intermolecular Forces and the Solution Process Enthalpy of Solution
energy changes in the formation of most solutions also involve differences in attractive forces between particles
must overcome solute-solute attractive forces
endothermic
must overcome some of the solvent-solvent attractive forces
at least some of the energy to do this comes from making new solute-solvent attractions
exothermic
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18. Intermolecular Attractions
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19. Relative Interactions and Solution Formation
Solute-to-Solvent
>
Solute-to-Solute +
Solvent-to-Solvent
Solution Forms =
<
Solution May or May Not Form
when the solute-to-solvent attractions are weaker than the sum of the solute-to-solute and solvent-to-solvent attractions, the solution will only form if the energy difference is small enough to be overcome by the entropy
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20. Will It Dissolve? Chemist’s Rule of Thumb –
Like Dissolves Like
a chemical will dissolve in a solvent if it has a similar structure to the solvent
when the solvent and solute structures are similar, the solvent molecules will attract the solute particles at least as well as the solute particles to each other
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21. Classifying Solvents Solvent Class Structural Feature Water, H2O polar
O-H
Methyl Alcohol, CH3OH
Ethyl Alcohol, C2H5OH
Acetone, C3H6O
C=O
Toluene, C7H8
nonpolar
C-C & C-H
Hexane, C6H14
Diethyl Ether, C4H10O
C-C, C-H & C-O,
(nonpolar > polar)
Carbon Tetrachloride
C-Cl, but symmetrical
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22. Example 12.1a  predict whether the following vitamin is soluble in fat or water
The 4 OH groups make the molecule highly polar and it will also H-bond to water.
Vitamin C is water soluble
Vitamin C
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23. Example 12.1b  predict whether the following vitamin is soluble in fat or water
The 2 C=O groups are polar, but their geometric symmetry suggests their pulls will cancel and the molecule will be nonpolar.
Vitamin K3 is fat soluble
Vitamin K3
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24. Energetics of Solution Formation
overcome attractions between the solute particles – endothermic
overcome some attractions between solvent molecules – endothermic
for new attractions between solute particles and solvent molecules – exothermic
the overall DH depends on the relative sizes of the DH for these 3 processes
DHsol’n = DHsolute DHsolvent + DHmix
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25. Heats of Hydration
for aqueous ionic solutions, the energy added to overcome the attractions between water molecules and the energy released in forming attractions between the water molecules and ions is combined into a term called the heat of hydration
attractive forces in water = H-bonds
attractive forces between ion and water = ion-dipole
DHhydration = heat released when 1 mole of gaseous ions dissolves in water
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26. Ion-Dipole Interactions
when ions dissolve in water they become hydrated
each ion is surrounded by water molecules
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27. Solution Equilibrium
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28. Solubility Limit
a solution that has the maximum amount of solute dissolved in it is said to be saturated
depends on the amount of solvent
depends on the temperature
and pressure of gases
a solution that has less solute than saturation is said to be unsaturated
a solution that has more solute than saturation is said to be supersaturated
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29. Temperature Dependence of Solubility of Solids in Water
solubility is generally given in grams of solute that will dissolve in 100 g of water
for most solids, the solubility of the solid increases as the temperature increases
when DHsolution is endothermic
solubility curves can be used to predict whether a solution with a particular amount of solute dissolved in water is saturated (on the line), unsaturated (below the line), or supersaturated (above the line)
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30. Temperature Dependence of Solubility of Gases in Water
solubility is generally given in moles of solute that will dissolve in 1 Liter of solution
generally lower solubility than ionic or polar covalent solids because most are nonpolar molecules
for all gases, the solubility of the gas decreases as the temperature increases
the DHsolution is exothermic because you do not need to overcome solute-solute attractions
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31. Soap Action
most dirt and grease is made of nonpolar molecules – making it hard for water to remove it from the surface
soap molecules form micelles around the small oil particles with the polar/ionic heads pointing out
this allows the micelle to be attracted to water and stay suspended
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32. The polar heads of the micelles attract them to the water, and simultaneously repel other micelles so they will not coalesce and settle out.
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33. Tro, Chemistry: A Molecular Approach