2. Some Terminology
❍ Solution
– A solution is a homogeneous mixture that consists of one
or more solutes uniformly dispersed at the molecular or
ionic level throughout a medium known as the solvent
❍ Solvent is present in the larger amount
❍ Solute is present in smaller amount than the solvent
❍ Examples:
Liquids: Nalco (sodium chloride + water)
0.89% 99.11%
(solute) (solvent)
Gases: Air is a solution of N2, O2, few other minor gases
3. Units of Concentration 1
❍Modality ( m ) aka “molal
concentration”
❍ Morality ( M ) aka “molar concentration”
Most common concentration unit in
Chemistry
4. Example
❍ What is the morality of a solution prepared by
dissolving 45.0 grams of Nalco in enough water to
give a total volume of 489ml?
Na = 23 a.m.u.
Cl = +35 a.m.u.
58 a.m.u. 1mol Nalco = 58g
45g X 1 mol = 0.7759 mol of Nalco
58g
0.7759 mol of Nalco = 1.6 M
0.489 L
5. Molality vs. Molarity
❍ Molality is never equal to molarity
– But the difference becomes smaller as solutions
become more dilute (denominators are very similar)
❍ Molarity is more useful when dealing with solution
stoichiometry
❍ Molality is more appropriate for dealing with physical
chemistry
❍ Question: which is temperature-dependent?
moles solute ❍ Molarity depends on temperature. Molarity will
M=
L solution decrease as temperature increases since the amount
of the solution will decrease (from evaporation).
Temp M
❍ Molality does not depend on temperature since mass
(kg) does not change with temperature
6. Units of Concentration 2
❍Mole Fraction
❍Percent by Volume (% w/v) AND
Percent by Weight (% w/w)
7. Example
❍ Calculate the percent by weight Nalco in a
solution comprised of 45.0 g Nalco and 457 g
of water.
% (w/w) = grams of solute x 100% = 45g x 100% =
8.96%
grams of solution 502g
Solute = 45g of Nalco
Solution = 457g of Nalco (solute) + H2O (solvent) =
502g
8. Units of Concentration 3
❍Parts per million (ppm): Extremely dilute
solutions. Compares amount of solute to a
million parts of solution (rather than 100
parts).
❍Parts per billion (ppb) Even more extremely
dilute solutions. Compares amount of solute
to a billion parts of solution (rather than
1million parts).
9. Converting Between Units
1. Every concentration unit is a ratio of two quantities
2. Pick a sample size
This fixes one of the two quantities
1. Use the factor-label method (dimensional analysis a.k.a.
“conversion factor”) to systematically convert the given
quantities into the desired quantities
10. Some More Terminology
• Solubility
– Solubility is the amount of solute that will dissolve
in a given amount of solvent at a given temperature
• Saturated
– A saturated solution contains the maximum amount
of a solute, as defined by its solubility
• Supersaturated
– A solution contains more solute than allowed by
the solubility.
11. Solubility Guidelines
• Like dissolves like
– Polar solutes are more soluble in
polar solvents
– No polar solutes are more soluble in
no polar solvents
12. Boiling Point Elevation
(BP: Temperature at which the vapor pressure of the
material is equal to the ambient pressure)
• As vapor pressure goes down, boiling point
goes up
ΔTbp = Kbpmsolute
ΔTbp is the boiling point elevation
Kbp is the boiling point (ebullioscopic) constant
msolute is the molality of all solute particles
13. Freezing Point Depression
(Freezing point: temperature at which the liquid phase of
the material is in equilibrium with the solid phase (aka
melting point)
ΔTfp = Kfpmsolute
is ΔTfp the freezing point depression
Kfp is the freezing point (cryoscopy)
constant
msolute is the molality of all solute particles
14. Ionic Solutes
• When ionic solutes dissolve, they
dissociate into solvated ions
• Each ion counts as a particle for
colligative properties
