Solution Concentration
•Molarity(M):
Moles solute/1L solution
•Molality (m):
Moles solute/1kg solvent
•Mole fraction (XA):
•Mass percent:
Moles A*
total moles solution
Mass solute
x 100
total mass of solution
*In some applications, one needs the mole fraction of solvent, not solute.
Make sure you find the quantity you need!
Changing Molarity to Molality
If we know the
density of the
solution, we can
calculate the
molality from the
molarity and vice
versa.
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Converting molality to molarity
• Determine the molarity of a 0.273m
aqueous solution of KCl?
• The density is 1.011g/mL
Converting molality to molarity
• What is the molarity of a 0.273m aqueous
solution of KCl? The density if 1.011g/mL
• Ans: 0.271M
Converting molarity to molality
• Determine the molality of a 0.907M
solution of Pb(NO3)2.
• The solution density is 1.252 g/mL.
Converting molarity to molality
• An aqueous solution is 0.907M Pb(NO3)2.
What is the molaltiy of lead (II) nitrate in
this solution? The density is 1.252 g/mL.
• Ans: 0.953m
Calculating Mass Percent
• How would you prepare 425g of an
aqueous solution containing 2.40% by
mass of sodium acetate?
Calculating Mass Percent
• How would you prepare 425g of an
aqueous solution containing 2.40% by
mass of sodium acetate (NaC2H3O2)?
• Ans: 10.2 g NaC2H3O2 in 414.8g of water
Mole Fraction
• Calculate the mole fraction of NaCl in an
aqueous 5% solution of NaCl.
Mole Fraction
• Calculate the mole fraction of NaCl in an
aqueous 5% solution of NaCl
• Mole fraction NaCl = 0.016
Solution Concentration
• Commercial concentrated aqueous
ammonia is 27% NH3 by mass and has a
density of 0.90 g/mL. What is the molarity
of this solution?
Solutions
• Solutions are homogeneous mixtures of two
or more pure substances.
• In a solution, the solute is dispersed uniformly
throughout the solvent.
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Metal Alloys:
Substitutional alloys are formed when the two metallic
components have similar atomic radii and chemical-bonding
characteristics. For example, silver and gold form such an alloy
over the entire range of possible compositions.
Metal Alloys:
Interstitial alloys are form when the component present in the interstitial
positions between the solvent atoms must have a much smaller covalent radius
than the solvent atoms. Typically, an interstitial element is a nonmetal that
participates in bonding to neighboring atoms. The presence of the extra bonds
provided by the interstitial component causes the metal lattice to become
harder, stronger, and less ductile. For example, steel is an alloy of iron that
contains up to 3 percent carbon. Steel is much harder and stronger than pure
iron.
Aqueous Solutions
The intermolecular
forces between solute
and solvent particles
must be strong enough
to compete with those
between solute particles
and those between
solvent particles.
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How Does a Solution Form?
As a solution forms, the solvent pulls solute
particles apart and surrounds, or solvates,
them.
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How Does a Solution Form
If an ionic salt is
soluble in water, it is
because the iondipole interactions
are strong enough
to overcome the
lattice energy of the
salt crystal.
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The Solution Process
• Solutes dissolve in solvents by a process
called solvation.
• Polar solvent dissolve polar solutes, nonpolar solvents dissolve non-polar solutes.
(aka: “like dissolves like”.
• If two liquids mix to an appreciable extent to
form a solution, they are said to be miscible.
• In contrast, immiscible liquids do not mix to
form a solution; they exist in contact with
each other as separate layers.
Solvation of Ions
+
When a cation exists in solution, it is surrounded by the negative dipole ends of
water molecules.
When as anion exists in solution, it is surrounded by the positive dipole ends of
water molecules.
Energy Changes in Solution
The enthalpy
change of the
overall process
depends on H for
each of these steps.
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Why Do Endothermic
Processes Occur?
Things do not tend to occur
spontaneously (i.e., without
outside intervention) unless the
energy of the system is
lowered.
The reason is that increasing
the disorder or randomness
(known as entropy) of a system
tends to lower the energy of the
system.
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Types of Solutions
• Saturated
– In a saturated solution,
the solvent holds as
much solute as is
possible at that
temperature.
– Dissolved solute is in
dynamic equilibrium
with solid solute
particles.
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Types of Solutions
• Unsaturated
– If a solution is
unsaturated, less
solute than can
dissolve in the
solvent at that
temperature is
dissolved in the
solvent.
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Types of Solutions
• Supersaturated
– In supersaturated solutions, the solvent holds
more solute than is normally possible at that
temperature.
– These solutions are unstable; crystallization can
usually be stimulated by adding a “seed crystal” or
scratching the side of the flask.
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Factors Affecting Solubility
• Chemists use the axiom “like dissolves like."
– Polar substances tend to dissolve in polar solvents.
– Nonpolar substances tend to dissolve in nonpolar
solvents.
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Factors Affecting Solubility
The more similar the
intermolecular
attractions, the more
likely one substance
is to be soluble in
another.
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Gases in Solution
• In general, the
solubility of gases in
water increases with
increasing mass.
• Larger molecules
have stronger
dispersion forces.
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Gases in Solution
• The solubility of
liquids and solids
does not change
appreciably with
pressure.
• The solubility of a
gas in a liquid is
directly proportional
to its pressure.
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Henry’s Law
Sg = kPg
where
• Sg is the solubility of
the gas,
• k is the Henry’s Law
constant for that gas in
that solvent, and
• Pg is the partial
pressure of the gas
above the liquid.
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Temperature
Generally, the
solubility of solid
solutes in liquid
solvents increases
with increasing
temperature.
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Temperature
• The opposite is true
of gases.
– Carbonated soft
drinks are more
“bubbly” if stored in
the refrigerator.
– Warm lakes have
less O2 dissolved in
them than cool lakes.
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