Notes - Solutions
Mr. Forte
Atascadero High School
Chemistry
Definitions
• Solute: Substance that is dissolved.
• Solvent: Substance doing the dissolving.
• Solution: A homogeneous mixture of a
solute and solvent.
• Saturated: Solution holding maximum
amount of solute at current conditions.
• Concentration: Measure of the amount of
a solute dispersed in a given amount of
solvent.
Factors Affecting Solubility
• Polarity: Like dissolves like.
• Temperature: May increase or decrease
amount of solute that will dissolve.
• Concentration: Once a solution is
saturated, no more solute dissolves. A solid
may precipitate or a gas may bubble out.
• Pressure: The solubility of a gas increases
with an increase of gas pressure above
solution.
Increasing Rate of Dissolving
• Increase Surface Area – Crushing a
solid helps it dissolve faster.
• Stirring – Allows particles to leave
surface of solid faster.
• Increase Temperature - Increased
kinetic energy allow particles to leave
solid surface faster.
Expressing Concentration
• Molarity
• Molality
• Percent by Mass
• Parts per Million (or Billion, etc)
Molarity (M)
• Expresses concentration by indicating
the moles of solute per liter of solution.
(moles of solute/liters of solution)
• Most common way to express solution
concentrations in chemistry.
• A 3.0 molar sodium chloride solution
indicates that there are three moles of
sodium chloride per liter of solution.
Example
What is the molarity of a solution that has
3.0 moles of solute in 2.0 liters of solution?
Molarity = (3.0 mol solute/2.0 L solution)
= 1.5 M
Molality (m)
• Expresses concentration by indicating
the moles of solute per kilogram of
solvent.
(molality = moles of solute/kg of solvent)
• Unlike molarity, molality is not
temperature dependent.
Example
What is the molality of a solution
containing 1.5 moles of sugar in 300 grams
of water?
Molality = 1.5 mol solute/0.300 kg solvent
= 5.0 m
Percent by Mass
• Expresses concentration by indicating
the solute’s mass as a percentage of the
mass of the solution.
• A 5% salt solution means that of the
total mass of the sample, 5% of the mass
is from salt.
• Most common way to express
concentrations of household chemicals.
• To calculate, divide mass of solute by
mass of solution, then multiply by 100.
Example
What is the percent by mass of ammonia in a
solution that contains 12 grams of ammonia
dissolved in 150 grams of water?
(Note: mass of solution = solute + solvent)
Percent by Mass (aka, parts per hundred)
= (12 g solute/162 g solution) x 100
= 7.4%
Parts per Million (ppm)
• Indicates mass of solute per one million
mass units of solution.
• For example, 3 ppm indicates that there
are 3 grams of solute in every one
million grams of solution.
• To calculate, divide mass of solute by
mass of solution, then multiply by one
million.
Example
What is the parts per million of lead in
drinking water that contains 0.03 grams of
lead in a 3800 gram sample?
Note: Parts per billion (ppb) or Parts per
trillion (ppt) work the same way!
Parts per million:
= (0.03 g Pb /3800 g solution) x 1000000
= 7.9 ppm
Solution Process
• Forming a solution requires three steps:
• Separating the solute (requires energy)
• Separating the solvent (requires energy)
• Mixing of solute-solvent particles
(releases energy)
• The amount of energy involved in each
step determines the Heat of Solution.
Heat of Solution
• When a solute dissolves in a solvent, there is
always an absorption or release of energy.
• The Heat of Solution can be calculated by
determine the enthalpy change (H) per gram
or mole of solute dissolved.
• The amount of heat released or absorbed can
be determined using Q = m x s x T.
• Heat of Solution = Q/grams or moles of
solute.
Diluting Solutions
• M1V1 = M2V2
– Where ones represent before and the twos
represent after.
• This formula is very versatile and allows
you to determine new concentrations or
volumes given changing conditions.
• See board for example.
Preparing Solutions
• Determine amount and concentration needed.
• From scratch:
• Weigh out solute/solvent
• Completely dissolve solute in solvent
• Added solvent (water) to desired volume
Electrolyte Solutions
• Solutions that conduct an electrical current are
called electrolytic solutions. Since they contain
electrolytes. (charged particles, like ions.)
• A conductivity meter allows for a relative
determination of the number of electrolytes
present.
• Solution that do not conduct electricity contain
nonelectrolytes. (Most molecular solutes, like
sugars, fall into this category.)
Colligative Properties
• Properties of a solution that depend only on the
number of solute particles present, not their
identity, are called colligative properties.
• Examples include freezing-point depression,
boiling point elevation, and vapor pressure.
– Solutions always boil at a higher temperature and
freeze at a lower temperature relative to the pure
solvent.