What are Solutions?
•
•
Homogeneous mixtures
– composition will not vary from one sample of the mixture to another
sample of the same mixture but will result in a uniform,
homogeneous mixture: ethanol (CH3CH2OH mixed into H2O)
– it appears to be one substance, though it really contains multiple
materials
– The definition of a solution is that it is a homogeneous mixture
Most homogeneous materials we encounter are actually solutions
– e.g. pure air
The solute is the dissolved substance
•
The solvent is the substance the solute is dissolved in
•
When both the solute and solvent appear to have the same state, the
solvent is the component present in the highest percentage.
•
Solutions in which the solvent is water are called aqueous solutions.
•
Common Types of Solutions
Solution Phase
gaseous solutions
Solute
Phase
Solvent
Phase
Example
gas
gas
air (mostly N2 & O2)
liquid solutions
gas
liquid
solid
liquid
liquid
liquid
soda (CO2 in H2O)
vodka (C2H5OH in H2O)
Table salt in water
solid solutions
solid
solid
brass (Zn in Cu)
Solubility
•
•
•
•
Solutions that contain Hg and some other metal are called amalgams.
Solutions that contain metal solutes and a metal solvent are called alloys.
When one substance (solute) dissolves in another (solvent) it is said to be
soluble:
– salt is soluble in water,
– bromine is soluble in methylene chloride.
When one substance does not dissolve in another it is said to be
insoluble:
– oil is insoluble in water.
Will It Dissolve?
•
Chemist’s Rule of Thumb –
Like Dissolves Like
•
A chemical will dissolve in a solvent if it has a
similar structure to the solvent.
•
When the solvent and solute structures are
similar, the solvent molecules will attract the
solute particles at least as well as the solute
particles to each other.
Classifying Solvents
Solvent
Class
Structural
Feature
Water, H2O
polar
O-H
Ethyl Alcohol, C2H5OH
polar
O-H
Acetone, C3H6O
polar
C=O
Benzene, C6H6
nonpolar
C-C & C-H
Hexane, C6H14
nonpolar
C-C & C-H
Diethyl Ether, C4H10O
nonpolar
C-C, C-H &
C-O
Will It Dissolve In Water?
•
Ions are attracted to polar solvents
– many ionic compounds dissolve in water.
•
Polar molecules and ones with a lot of –OH groups are attracted to polar
solvents
– table sugar, ethyl alcohol and glucose all dissolve well in water.
•
Nonpolar molecules are attracted to nonpolar solvents
– b-carotene, (C40H56), is not water soluble; it dissolves in fatty (nonpolar)
tissues.
•
Many molecules have both polar and nonpolar structures – whether they will
dissolve in water depends on the kind, number and location of polar and
nonpolar structural features in the molecule.
Salt Dissolving in Water
When materials dissolve, the solvent
molecules surround the solvent particles
which is due to the solvent’s attraction for the
solute particles.
This process is called solvation. Solvated
ions are effectively isolated from each other.
Solubility
•
There is usually a limit to the solubility of one substance in another:
– gases are always soluble in each other
– two liquids that are mutually soluble are said to be miscible
• alcohol and water are miscible
• oil and water are immiscible.
•
The maximum amount of solute that can be dissolved in a given
amount of solvent is called the solubility:
•
Saturated solutions have the maximum amount of solute that will dissolve in
that solvent at that temperature; i.e., colder solvents dissolve less than warm
or hot solvents.
•
Unsaturated solutions can dissolve more solute.
•
Supersaturated solutions are holding more solute than they should be able
to at that temperature and are unstable. When disturbed the amount of
solute above the solubility limit comes out of solution.
Electrolytes
•
Electrolytes are substances whose aqueous solution
is a conductor of electricity.
•
For strong electrolytes, all the electrolyte molecules
are dissociated into ions.
•
For nonelectrolytes, none of the molecules are
dissociated into ions.
•
For weak electrolytes, a small percentage of the
molecules are dissociated into ions, the rest are
molecules.
Solubility and Temperature
•
the solubility of the solute in the solvent depends on the temperature
– higher temp = higher solubility of solid in liquid
– lower temp = higher solubility of gas in liquid
Warm soda pop fizzes more than cold soda
pop because the solubility of CO2 in water
decreases as temperature increases.
Solubility and Pressure
•
•
the solubility of gases in water depends on the pressure of the gas
higher pressure = higher solubility
When soda pop is sealed, the CO2 is under
pressure.
Opening the container lowers the pressure,
which decreases the solubility of CO2 and
causes bubbles to form.
Solution Concentration = the amount of solute in a given
amount of solution
•
Dilute solutions have low solute concentrations.
•
Concentrated solutions have high solute concentrations.
•
Concentration can be in units of Mass % = # parts of solute/100 parts
of solution, Molarity (M) = moles of solute/Liter of solution,
Molality = moles of solute/kg of solvent, and Volume % = mL of
solute/100mL of solution.
Percentage by Mass
A convenient, but not as useful, method to measure solution
concentration is percentage by mass
% mass = grams solute x 100% =
grams solute
x 100%
grams solution
g solute + g solvent
Why is this often more convenient?
Why don’t we use this more often?
Percentage by Mass
1720 grams of water is added to 236 grams of sugar to produce a
tasty drink. Calculate the percentage by mass of sugar in the drink.
% mass = grams solute x 100% =
grams solute
x 100%
grams solution
(g solute + g solvent)
% mass =
236 grams
x 100 % =
( 236 g + 1720 g)
The drink is 12.1 % sugar by mass
12.1 %
Molarity (Concentration)
Concentration is defined as the amount of solute in a
given volume of solution.
The scientific unit for concentration is molarity, M.
Molarity describes the amount of solute in moles and
the volume of solution in liters.
Molarity is:
M = Molarity = moles of solute = mol
liters of solution
L
Beware!!!
Joe adds 30 grams of NaCl to 1.00 L of water to make 1.03 L of a
salt water solution. What is the molarity of the solution?
1. Convert 30 g NaCl to moles
30 g NaCl x (1 mole NaCl)
58.44 g NaCl
=
0.513 moles NaCl
2. moles of solute = Molarity = 0.513 moles NaCl = 0.498 M
liters of solution
1.03 L
Note that liters of solution is not necessarily the liters of solvent
(water) that is added
Moles, Liters, and Molarity
Example: The concentration of an aqueous sodium chloride solution
is 0.498M. How many moles of NaCl are in 0.025 L of this
solution?
(0.025 Liter) x (0.498 moles NaCl) =
(Liter)
0.0125 moles NaCl
Example: Using the above solution, a student needs 0.200 moles of
NaCl. How many liters of solution should the student take?
(0.200 moles NaCl) x
(Liter)
=
(0.498 moles NaCl)
0.402 L of the
0.498M solution
Now Your Turn
# Moles Solute # Liters Solution Molarity
3.00 moles
6.00 L
0.100 moles
0.200 L
0.050 L
6.0 M
0.250 L
0.100 M
0.010 moles
0.500 M
0.200 moles
0.050 M
Notes
• Standard solution: is a solution whose
concentration is known very accurately.
• Ionic compounds separate into component
ions when they dissolve in water.
Example: The concentration of the ions of 1M
CaCl2 solution is: 1M Ca2+ and 2M Cl1- since
CaCl2(s)
H2O(l)
Ca2+(aq) + 2Cl1-(aq)
Dilution
Molarity =
# moles
volume
Solving for # moles…
# moles = (Molarity) (Volume) = constant
(for a dilution)
So…if the volume goes up by a factor of two…
then molarity (concentration) goes down by a factor of two
Calculations
# moles = (Molarity) (Volume) = constant
(for a dilution)
M1 V1 = constant = M2 V2
So…
M1 V1 = M2 V2
Given any three, we can solve for the fourth
Practical Example
You have 0.050 L of a 4.0 M solution of NaOH. If you
diluted this solution to 1.0 L, then what is the resulting
concentration of the NaOH solution?
M1 V1 = M2 V2
Solving for M2 (final molarity)…
M2 = M1 V1
V2
M2 = (4.0M) (0.050 L) =
(1.0 L)
0.20 M
Another Example
We need to fill a 0.50 L bottle with 0.10 M aqueous HCl. In the
stockroom, there is a 1.0 L bottle filled with 6.0 M HCl. How
can we make our desired solution?
M1 V1 = M2 V2
M1 = 6.0 M
V1 = ???
M2 = 0.10 M
V2 = 0.500 L
V1 = M2 V2 = (0.10 M) (0.500L) = 0.0083 L = 8.3 mL
M1
(6.0 M)
So, 8.3 mL of the 6.0 M HCl is put in the bottle and water is
added until the 0.50 L bottle is filled.
Making a Solution by Dilution
M1 x V1 = M2 x V2
M1 = 12.0 M V1 = ? L
M2 = 1.50 M V2 = 5.00 L
M 1  V1  M 2  V2
M 2  V2
V1 
M1

1.50 M   5.00 L 
V1 
 0.625 L
12.0 M 
dilute 0.625 L of 12.0 M solution to 5.00 L
Now Your Turn
How many liters of concentrated H2SO4 (18 M) should
be used to prepare 2.0 L of 1.0 M H2SO4?
(Hint: M1, M2, and V2 are given, solve for V1)
How many liters of concentrated HCl (12 M) should be
used to prepare 1.0 L of 2.0 M HCl?
If 0.20 L of concentrated H2SO4 (18 M) is diluted with
water to 4 L, then what is the concentration (molarity) of
the resulting solution?