Week 1, Lesson 1
Chapter 11 – Measuring
Solubility
Solubility
• Solubility of a substance refers to the maximum
amount of that substance that can be dissolved in
a given quantity of solvent at a certain
temperature.
• A solution in which no more solute can be
dissolved at that temperature is described as a
saturated solution.
• One way of measuring solubility is to determine
the maximum mass of solute that can be
dissolved in 100 grams of solvent at a particular
temperature.
• Solubility values allow us to compare the extent
to which different solutes dissolve.
Solubility Curves
• The relationship between solubility and
temperature can be represented by a solubility
curve.
• Each point on the solubility curve represents a
saturated solution – no more solute can be
dissolved at the particular temperature.
• Any point below the curve represents an
unsaturated solution for that solute.
• Any unsaturated solution contains less solute
than is needed to make a solution saturated at
that temperature.
Solubility Curve cont…
Crystallisation
• If a hot saturated solution of potassium nitrate
is cooled, crystals of the solute will appear.
• This happens because potassium nitrate also
becomes less soluble as the temperature falls.
• The potassium nitrate crystallises from
solution.
• This process is known as crystallisation.
Supersaturation
• With some substances, it is possible to produce an unstable
solution that contains more dissolved solute than in a
saturated solution.
• Such a solution is said to be supersaturated.
• In a solubility graph, any point above a solubility curve
represents a supersaturated solution for that solute.
• A supersaturated solution of potassium nitrate can be
prepared by cooling a saturated solution very carefully so
that crystallisation does not occur.
• Adding a small seed crystal or knocking a supersaturated
solution can cause the solute to crystallise or settle out
until a saturated solution remains.
• Crystallisation is often accompanied by the evolution of
much heat energy.
Solubility of Gases
• Gases such as oxygen and carbon dioxide are much less
soluble in water than solids such as sodium chloride and
sucrose.
• The presence of oxygen and carbon dioxide is essential for
the survival of aquatic life.
• Aquatic plants and animals require a source of oxygen for
respiration and although oxygen is a part of all water
molecules, they cannot extract it from these molecules.
• They must rely on the small amount of dissolved oxygen in
the water.
• Similarly, aquatic plants rely on dissolved carbon dioxide for
photosynthesis.
• The solubility of a particular gas in a liquid depends on the
temperature of the liquid and the pressure of the gas.
Temperature and Gas Solubility
• Unlike most solids, gases become less soluble as the
temperature increases.
• Thermal pollution: a power station located next to a
river or lake can release hot water as a waste material.
• As the temperature of the water increases, less oxygen
remains dissolved.
• In addition, at increased temperatures, aquatic life
becomes more active and so the demand for oxygen
increases.
• Competition for oxygen can become intense and can
lead to large decreases in certain animal populations.
Pressure and Gas Solubility
• The solubility of gases increases with increasing
pressure.
• Carbon dioxide is forced into soft drink under high
pressure to increase the amount that can be dissolved.
• When the bottle top is removed, the carbon dioxide in
the space at the top of the bottle escapes.
• The gas pressure above the drink falls and so the
solubility of carbon dioxide in the drink is lowered.
• Dissolved carbon dioxide escapes from the liquid until
only enough remains to saturate the solution at this
pressure.
Solubility Question Examples…
Use the data in Table 11.2 to calculate the percentage of oxygen that would be lost from a pond,
saturated with oxygen, if the temperature of water in the pond rose from 0°C to 20°C
Mass of oxygen, in 1kg saturated solution of oxygen;
= 0.069g at 0°C
= 0.043g at 20°C
So, mass of oxygen lost:
0.069 – 0.043 = 0.026g
% of oxygen lost = 0.026 100
0.069
= 38%
Week 1, Lesson 2
Concentration of Solutions
• The concentration of a solution describes the
relative amounts of solute and solvent
present.
• A solution in which the ratio of the solute to
solvent is high is said to be concentrated.
• A solution in which the ratio of solute to
solvent is low is said to be dilute.
Concentrations of Solutions cont…
• Chemists use different measures of concentration
depending on the particular situation.
• Earlier, units of grams of solute per 100 grams of
solvent were used to describe the concentration
of saturated solutions.
• Other ways of expressing concentration describe
the amount of solute in a given solution.
• They vary only in the units used to measure the
amount of solute and the amount of solution.
Units of Measurement
• The most commonly used units of
concentration are:
– Mass of solute per litre of solution
– Amount, in mol, of solute per litre of solution
Mass of Solute per Litre of Solution
• This unit expresses concentration in terms of the
mass of solute present in 1 litre of solution.
• Example: A 250mL glass of orange-flavoured mineral
water contains 4.0mg of sulfate ions. What is the
concentration (in mgL-1) of sulfate ions in the mineral
water?
Concentration = mass of sulfate ions(mg)
volume of mineral water (L)
= 4.0
0.250
= 16mg/L or 16mg L-1
Conversions…
Amount, in mol, of solute per
solution
• Expressing concentration in moles per litre of solution allows
chemists to compare relative numbers of atoms, molecules or
ions present in a given volume of solution.
• This measure of concentration is known as molarity or molar
concentration.
• Molarity is defined as the number of moles of solute particles
per litre of solution.
• A one molar (1 M) solution contains one mole of solute
dissolved in each litre of solution.
• A concentration of such a solution is said to be one mole per
litre, 1 mol L-1 or 1 M.
• The term molarity can be used to mean concentration
measured in moles per litre.
Amount of Solute Equation
• The amount of solute is linked to the
concentration (molarity) and volume of the
solution by the relationship:
n=cxV
Where n = amount, mol
c = concentration, mol L-1
v = volume, L
Example
• Calculate the molar concentration of a
solution that contains 0.105 mol of potassium
nitrate dissolved in 200mL of solution.
STEP 1: Convert volume into L
200mL = 200 x 10-3L
STEP 2: Calculate the molar concentration
c=n/v
c = 0.105 / 200 x 10-3
c = 0.525M
Unit Conversion
grams
litre
M
xM
moles
litre
 Molar Concentration and Solubility
Dilution
• The process adding more solvent to a solution
is known as dilution.
• When a solution is diluted, the solute particles
are more widely spaced.
• A dilution formula exists, which can be used to
find the concentration or volume changes of a
solution.
Dilution Formula
C1V1 = C2V2
C1 = Initial concentration
V1 = Initial volume
C2 = Final concentration
V2 = Final volume
In this equation, the concentration unit is g L-1
Example
• The concentration of a seaweed extract in a
bottle of seaweed fertiliser solution is 9.0gL-1.
When used to fertilise plants, the seaweed
fertiliser must be diluted. If 10mL of seaweed
fertiliser is diluted with water to fill a 2.0L
container, what is the new concentration?
C1V1 = C2V2
C1 = 9.0
V1 = 0.010L
C2 = ?
V2 = 2.0
C1V1 = C2V2
9.0 x 0.010 = C2 x 2.0
0.09 = C2 x 2.0
C2 = 0.09 / 2.0
C2 = 0.045g L-1