solutions and concentration€¦ · for only qualitative tests, simply knowing the solution is...
TRANSCRIPT
Solutions and
Concentration
A solution is made up of two
or more substances
combined together.
The state of matter for a
solution depends upon the
state of the solvent.
The solvent is the compound
present most often
The solute is the compound(s)
present less often then the
solvent
The interactions between the solvent and the
solute determine the extent of solvation
Solutions
Ionic compounds
in water
When a solid ionic compound is dissolved in
water, first the compound must be
surrounded on the molecular level with
solvent.
Once surrounded, it will leave the ionic solid
and enter the solvent.
Next, the ionic compound will separate into
ions
Finally, the water molecules will separate the
two ions and surround them in a specific
orientation according to the ion charge
When the solvent is water and the solute is an
ionic compound, the dissolving process is
called hydration.
Expressing
Concentration
When planning a reaction, it is important to
know the concentration of the solution.
For only qualitative tests, simply knowing the solution is concentrated or dilute may be specific enough.
Usually this is not enough information.
For quantitative information there are five
methods to express concentration.
1. Mass percentage
2. Parts per million
3. Mole fraction
4. Molarity
5. Molality
Mass Percentage
Mass of component in solution * 100
Total mass of solution
A solution is made by dissolving 13.5 g of
glucose, C6H12O6, in 0.100 kg of water.
What is the mass percentage of solute in
this solution?
13.5 g * 100 =
113.5 g
11.9%
Parts per million (ppm)
ppm is used for very dilute solutions
Examples; air pollution, ozone levels and water contamination
Mass of component in solution *106
Total mass of solution
1 ppm = 1 g of solute for each million grams
of solution
Problem A 2.5 g sample of ground water was found to
contain 5.4g of Zn2+. What is the concentration of Zn2+ in ppm?
5.4*10-6 g *106
2.5 g
=2.2ppm
Problems What is the mass percentage of 25g of HCl in
0.250 kg of water?
Problem What is the ppm of 11g of As in 25 g of
water?
Mole fraction (X) Moles of component
Total moles of all components
X is the symbol for mole fraction.
A subscript is usually added to inform what
compound was evaluated for the mole
fraction of the solution (more then one
substance may be dissolved in a solution)
Problem What is the mole fraction of 5 moles of HCl in
11 moles of NaCl and 75 moles of water?
Molarity (M) Moles of solute
Liters of solution
Molality (m) Moles of solute
Kilograms of solvent
Notice the difference between molarity and
molality, these two measurements can be
easily be confused.
Problem
• What is the molality of 5
moles of HCl in 17 kg of
water?
Solutions and
Solubility
For any solution the process of solvation is
opposed by crystallization
In a saturated solution, the maximum amount
of solute has been dissolved.
If more is added, it will collect at the bottom.
An unsaturated solution means more solute
may be dissolved
A supersaturated solution contains more
solute then expected.
These solutions are produced by dissolving solute at a high temperature and slowly cooling it.
Adding one grain of solute will result in lots of solute crystallizing.
Factors affecting
solubility
The composition of the solute and the solvent
will determine if the substances will mix.
“like dissolves like”
• Pairs of liquids that will mix are miscible
• Example: water and alcohol
• Pairs of liquids that will not mix are immiscible
• Example: oil and water
Increase dissolving
Solids 1. Increase the temperature
2. Increase the surface area of the solute
3. Agitate the solution
Increase dissolving
Gas 1. Increase the surface area of the solvent
2.Decrease the temperature
3. Increase the pressure
Henry’s law The relationship between pressure and
solubility is shown using Henry’s law:
Cg = kPg
Cg = the solubility of the gas in the solution (usually in molarity M)
Pg = the partial pressure of the gas
K = Henry’s law constant
K changes for every solute-solvent pair