Chapter 15 Solutions
15-1 The Nature of Solutions
Mixture- blend of two or more pure substances that are chemically combined
o Very few things are
pure substances in nature
o Usually heterogeneous
(visibly different parts) or homogeneous mixtures (not visibly different parts)
Solution- a homogenous mixture of two ore more substances in a single physical state.
Properties of Solutions
o Small particle size-
not visibly seen
o Particles evenly distributed-
different samples of the same solution will have the same concentration
o The particles in a solution
will not separate no matter how long it is left standing under constant conditions.
Solute- the substance dissolved or broken down (lesser quantity)
Solvent- the substance doing the dissolving (greater quantity)
Soluble- the ability to be dissolved in another substance
Insoluble- the inability to be dissolved in another substance
Types of Solutions
Solid Solutions- contain two or more solids
o Alloys- solid solutions
of two or more metals
Properties are often different than the individual elements
May improve melting points, durability, resistance to corrosion.
Gaseous solutions- mixture of two gases
o Gas particles are far
apart- mix readily
o If the gases do not
react with each other, they become a solution readily.
o Properties of gas solution
depend on the properties of the gasses added
Liquid solutions- a mixture that results in the liquid state of matter.
o The solvent and the
solution are liquids
o Solute can be either
solid, liquid or gas (soda, vinegar, sugar-water)
o Miscible- able to mix
in any amounts
o Immiscible- liquids
that cannot mix in any proportions
Aqueous Solutions- solutions in which water is the solvent
o Since water can dissolve
so many things- it is called the universal solvent
o Electrolyte- a substance
that when dissolved in water will conduct an electric current – usually contains some number of ionic bonds.
o Non-electrolyte- a substance
that when dissolved in water will not conduct an electric current- usually does not contain any ionic bonds
15-2 Concentration of Solutions
Concentration- the amount of solute in a given amount of solvent or solution
Molarity- the number of moles of a solute dissolved in each liter of solution
Molarity = moles
of solute/liters of solution
o 10.0 grams of NaOH in
enough solvent to make .100 liters of solution.
(10.0 g NaOH/.100L solution
)(1 mol NaOH/ 40.0 g NaOH)
mol NaOH/ 1 L solution
2.50 M NaOH
Volumetric Flasks are the best to use to make solutions
o Use balance to get proper
amount of solute
o Then add enough liquid
to flask to attain a liter.
Molality- number of moles of a solute dissolved in each kilogram of solvent.
Molality = moles of
solute/kilograms of solvent
Mole Fraction (X)
Mole fraction- number of moles of one component divided by the total number of moles in the solution.
Mole fraction = moles of component/total moles of solution
Can figure out the mole fraction of the solute or the solvent
= moles of solute/total moles of solution
= moles of solvent/total moles of solution
The total mole fraction must equal 1
Is there a limit to how concentrated a solution can be?
Saturated-a solution in which no more solute can be dissolved- under the existing conditions of temperature and pressure.
o Saturated and concentrated
are not the same thing
May take only a very little solute to cause a solution to be saturated.
Unsaturated solutions- a solution that has less than the maximum amount of solute
Supersaturated- a solution that contains a greater amount of solute that that needed to form a saturated solutions
o Very unstable
o Will often form precipitate
15-3 The Formation of Solutions
How a Solution Forms
It deals with the ability of the solvent to break intermolecular and intramolecular forces.
Dissolving takes place at the surface of the solvent- in the case of NaCl and water
o Water molecules orient
themselves so that they can separate the ions and pull them into solution- called solvation (hydration when the solvent is
o The solvent and solute
particles are intermingled
So intermolecular forces in the water must break, attractive forces in the salt must break- and new forces between
the salt and water must form
Any time attractions are broken- energy is required (endothermic)
The formation of new attractions releases energy (exothermic)
Whether or not heat is given off or taken in depends on which process requires more
Why are some substances soluble and some not?
Why do different solutes dissolve to different rates in the same solvent?
Solubility- the amount of a solute that will dissolve in a specific solvent under given conditions- or the amount of
solute required to saturate a solution.
o Must be determined experimentally
o Usually given in grams
solute per 100 grams solution.
Factors that affect solubility:
o Nature of the solute
o Pressure (for gasses)
Nature of the Solute and Solvent
Polar solvent- a liquid made up of polar molecules (large difference in electronegativity)
o Polar molecules tend
to dissolve in polar solvents
o Ionic substances tend
to dissolve more readily in polar solvents than they due in non-polar solvents due to the charged nature of the ions.
o Many ionic substances
are only slightly soluble in water
Non-polar solvent – a liquid made up of non-polar molecules (not a large difference in electronegativity)
o Non-polar molecules
tend to dissolve in non-polar solvents
Soda may go flat when left out on a table due to the solubility difference of carbon dioxide in warm temperatures
o Carbon dioxide is more
soluble at colder temperatures
Solutions of gases dissolved in liquids are greatly affected by changes in temperatures
o As temperature increases,
the gas gains more kinetic energy and is more likely to escape from the surface of the liquid
o As temperature increases,
solubility of a gas decreases.
Solutions with solids dissolved in liquids is quite different
o As temperature increases,
The relationship between solubility and temperature depends on the energy change during solution formation
o If the temperature drops
when the solute and solvent are mixed- raising the temperature will raise the solubility
o If the temperature stays
the same- changing the temperature will have little effect
o If the temperature increases
when the solute and solvent are mixed- raising the temperature will lower the solubility.
Supersaturated solutions are made by increasing the temperature so that more solute can be added to solution and then
allowing it to cool back down to room temperature.
Solubility of a solid in a solvent is not significantly affected by pressure
Solubility of a gas in a liquid is significantly affected.
o The solubility of any
gas in a solution is increased when pressure above the solution is increased
The rate at which gas molecules hit the surface of the liquid increase
William Henry- solubility of a gas was proportional to the partial pressure of the gas above the liquid
Factors Affecting the Rate of Dissolving
Whether a solute dissolves quickly or slowly does not alter or depend on its solubility
Dissolving takes place at the surface of the solid
o To speed dissolving-
the surface area from which dissolving occurs needs to be increased
Grinding the solid tends to increase surface area- and thus dissolving
Since dissolving occurs at the surface of the solid, dissolved solids tends to build up near the surface of the solid
o Stirring moves those
dissolved solids away from the surface to allow for quicker attack on the underlying solid.
o Increases contact between
solvent and solute
As temperature increases solvent particles move faster
o When particles move
faster- more come in contact with the solute
o Also the solute particles
are moving faster and require less energy to be removed.
15-4 Colligative Properties
Some properties of liquid solutions differ from those of pure solvent
Colligative property- a property that depends on concentration of solute particles but is independent of their nature
o Depend on the collective
effect, not their chemical identity
Vapor Pressure Reduction
Vapor pressure- arises since some molecules of a pure liquid leave the liquid surface and enter the gaseous state (vaporization)
o Some return (condensation)
o At some point, in a
closed container- these two will be in equilibrium
When a non-volatile (will not vaporize) solute is added to a liquid, the vapor pressure of the solution becomes lower.
o The solute takes up
some of the space at the liquid’s surface- preventing some solvent molecules from vaporizing
o Since more molecules
leave the gas than enter it, the pressure of the gas is reduced
o Does not depend on the
chemical nature of the solute added.
o Raoult’s law-
the magnitude of vapor pressure reduction is proportional to solute concentration
Boiling Point Elevation
Boiling point is the point at which the vapor pressure of a liquid is equal to the external pressure on its surface-
generally atmospheric pressure
Since addition of solute lowers the vapor pressure, a higher temperature is required to get the vapor pressure of the
solution up to atmospheric pressure so the solution boils.
Boiling point elevation- the amount the boiling temperature rises.
o It is the difference
between the boiling point of the pure solvent and the solution
o Directly propotional
to the number of solute particles per mole of solvent particles- also known as molality
o DTb = Kbm
Kb is called the molal boiling elevation constant, and depends on the solvent. Each solvent has its own value.
Freezing Point Depression
Freezing point depression- the ability of a solute to lower the freezing point of its solution
The freezing point is the temperature at which the vapor pressures of the solid and liquid phases are the same.
o If the solute is non-volatile-
the vapor pressure of the solution is lowered in proportion to the mole fraction of the solute.
o DTf = Kfm
Kf is the specific effect of a solute on a given solvent. Each
solvent has its own value.
Osmosis- the movement of solvent particles from less concentrated solutions to more concentrated solutions through
a semipermeable membrane.
Osmotic pressure (P) is the pressure required to prevent osmosis
o When solvent flows to
opposite sides of a membrane- the levels become unequal
o When the levels are
so unequal that the pressure difference prevents more net movement, osmotic pressure is reached.
Isotonic- when two solutions have the same osmotic potential- results in no net movement of water and no osmosis
Hypotonic solutions- water enters cells and may rupture cell
Hypertonic solution- water exits cells and cells crenate
Determining Molar Mass
Colligative properties of solutions provide a useful means of experimentally determining the molar mass of an unknown
Any of the colligative properties can be use to determine molar mass by manipulating the basic equations.
a 10.0 grams sample of an unknown compound is dissolved in a .100 kilogram of water.
The boiling point of the solution is elevated to .433 C above the normal boiling point of pure water. What is the molar mass of the unknown sample?
o DTb = Kbm
m = DTb
The molality can also be used to determine the number of moles of solute.
solute/ kg solvent
mol solute = m x
Once you have determined the moles of solute you can solve for molar mass
Mol solute = mass solute/
molar mass of solute
Molar mass = mass solute/