Notes: Concentration of Solutions,
Percent Concentration, and
Molarity
Name_______________________________
Date___________
Block____
Solutions are composed of two parts: the solute and the solvent.
The solute is the part in the smaller amount; the solvent is the part in the larger amount.
Assuming the solute is soluble in the particular solvent, the solute is said “to dissolve” in the
solvent. During the solvation process with water (i.e., as the solute is dissolving in the solvent),
water molecules surround each particle of solute. (Diagram of this will be drawn on the board.
Be sure to copy it!)
Remember that water is a polar molecule; the two O-H bonds are adjacent to each other and
there is not an even distribution of electrons around the oxygen atom and the two hydrogen
atoms – more electrons are around the oxygen atom than there are around either hydrogen atom.
This uneven distribution of negative charge results in the oxygen “end” being more negative and
the hydrogen “ends” being more positive.
..
:O : H
..
H
If the solute is ionic, the ions that form the formula unit separate from each other: the ions in the
ionic compound become dissociated from one another and are kept apart from each other by the
water molecules surrounding each ion. Each ion now behaves independently of all other ions in
the solution. This will also occur if the solute is a polar molecule. This will not occur of the
molecule is nonpolar; there are no regions of slight charge on a nonpolar molecule.
Some molecular compounds form polar molecules: opposite “ends” of the molecule are slightly
charged – one end is slightly positive, the other end is slightly negative.
If the two components are fluids (gases and / or liquids) mix with or dissolve in each other in all
proportions, the fluids are said to be miscible. If the two components do NOT mix but form
distinct layers (based on their densities), the fluids are said to be immiscible.
Solutions may exist in any of the three states of matter: gas, liquid, or solid.
1
Gaseous solutions are composed of different nonreactive gases in all proportions.
Most liquid solutions are obtained by dissolving a gas, a liquid, or a solid in some liquid.
Carbonated water (seltzer) consists of a solution of carbon dioxide gas and water. Sea water
contains both dissolved gases and solids (mostly sodium chloride).
It is also possible to make a liquid solution by mixing certain solids together: potassium and
sodium are both solids at room temperature but a liquid solution forms when the mixture
contains 10% - 50% sodium.
Solid solutions are commonly called alloys. When gold is used to make jewelry, the gold is
mixed with a small amount of silver to make the metal harder and easier to cast (into a shape)
because the alloy has a lower melting point than pure gold. “Silver” dental filling is a solution of
mercury (a liquid at room temperature) in silver and small amounts of other metals.
Solubility
Solubility is defined as the amount of solute that dissolves in a given amount of solvent at a
given temperature to produce a saturated solution.
The solubility of one substance in another is determined by two factors:
1) the natural inclination toward disorder (“order” being the tendency of the particles of a
substance to stay separate from all other substances; “disorder” being the tendency for the
particles of a substance to spread out in all directions);
2) the strength of the forces of attraction between particles (the two types of particles found in
solutions are molecules and ions);
It is the balance between these two factors that determines the solubility of the solute in a
particular solvent. For example, in a previous lab, Properties of Ionic and Covalent Substances,
we observed that polar and ionic solutes dissolve in polar solvents but not in nonpolar solvents
and that nonpolar solutes dissolve only in nonpolar solvents. (“Like dissolves like.”)
However, there was one molecular nonpolar substance (benzoic acid) that did not dissolve in the
nonpolar alcohol as expected. This is because the alcohol used has one region of the molecule
that is very slightly polar – just enough to prevent the benzoic acid from dissolving.
Ionic substances differ in their solubility in water. In most cases, this difference in solubility can
be explained in terms of the energies of attraction between ions in the crystal (known as lattice
energy, the attractive force holding the ions together in a lattice structure) and between the
individual ions and the water molecules. As lattice energy increases, the solubility of ionic
solids decreases. Therefore an ionic solid with relatively large lattice energy is usually insoluble
in water.
2
Lattice energies depend on the charges of the ions as well as the distance between the centers of
the ions): the greater the size of the ionic charge, the greater the lattice energy. Therefore we
expect substances containing only ions having a single charge (+1 or -1: H+1, Li+1, NO3-1, CN-1,
F-1, Cl-1, etc.) to be comparatively soluble in water. Those substances that contain multiple
charges (+2. +3, -2, -3: Be+2, Mg+2, S-2, P-3, PO4-3, etc.) are expected to be less soluble.
This is observed using the Solubility Table (used in the units on Chemical Reactions).
Compounds of the alkali metals and ammonium ions (all have a +1 charge) are generally soluble.
Those compounds with phosphate ions (-3 charge) are generally insoluble.
Effects of Temperature and Pressure on Solubility
Effect of Temperature Changes
The solubility of solutes usually varies with temperature: solids and liquids tend to have an
increase in solubility with an increase of temperature; gases have a decrease in solubility with an
increase in temperature.
As a glass of cold water slowly warms to room temperature, bubbles of gas are observed on the
inside of the glass. These bubbles are bubbles of air released from solution as the increasing
temperature decreases the solubility of the air in water. A glass of soda aleft out at room
temperature will lose its carbonation (“go flat”) over time as the carbon dioxide gas is released
from solution as the soda warms to room temperature. Also, soda in unopened plastic bottles
will lose its carbonation (CO2(aq)) over time as the CO2(g) diffuses out of the bottle through the
plastic. This does not occur for sodas in unopened glass bottles; gases cannot pass through the
glass bottle.
There are a few ionic solids that decrease in solubility with increasing temperatures: calcium
sulfate and calcium hydroxide are two common examples.
Heat can be released or absorbed when ionic substances are dissolved in water. In some cases,
this heat of solution is very noticeable. For example, sodium hydroxide dissolves in water with
the release of a large amount of heat (the solution process is exothermic). On the other hand,
ammonium nitrate dissolves in water and the solution becomes very cold (the solution process is
endothermic). This cooling effect from the dissolving of ammonium nitrate in water is the basis
for “instant cold packs.” Heat is absorbed by the solution and so the pack feels cold.
Effect of Pressure Changes
In general pressure changes have very little effect on the solubility of a solid or a liquid in water.
However, the solubility of a gas increases when the partial pressure of the gas over the solution
increases.
For example: when you first open a bottle of soda, the soda fizzes as some carbon dioxide comes
out of solution when the partial pressure of the carbon dioxide above the soda decreases when
the cap is removed.
3
Colligative Properties of Solutions
Colligative properties of a solution are those properties that depend on the concentration of
solute molecules or ions in solution, but not on the nature of the solute (ionic or molecular).
In general, as the concentration of solute particles increases, the boiling point of the solvent will
increase and the freezing point of the solvent will decrease.
This is why salt water freezes at a lower temperature than fresh water. Adding salt to cooking
water will increase the boiling point of the water.
Concentration of Solutions
The concentration of a solution is measured by the concentration of the solute in the solution.
Concentration = amount of solute dissolved in a given quantity of solvent or solution.
The concentration of a solution may be described as “dilute” or “concentrated.” Dilute means a
little solute in the solvent whereas concentrated means a lot of solute in the solvent If we dilute
a solution, we add more solvent to existing solution (such as adding more hot water to a cup of
strong coffee or tea). If we increase the concentration of a solution, we add more solute to the
existing solution (such adding more iced tea mix to a glass of iced tea solution).
The words “dilute” and “concentrated” give qualitative information about the concentration of
the solution; the examples given, the terms dilute and concentrated are strongly influenced by
personal taste. The words do not give quantitative information about the concentration – we do
not know how much solute is in either solution.
In science, we must use terms that give quantitative information about the concentration of
solutions: molarity, mass percentage of solute, and molality.
Molarity of a solution is the number of moles of solute dissolved in one liter of solution.
( for sig. figs. purposes, the “one liter” has an infinite number of sig. figs.)
Molarity = moles of solute
liters of solution
M = moles
liter
For example: 0.20 mole of ethylene glycol dissolved in enough water to make 2.0 L of solution
has a molarity of:
M = moles = 0.20 moles = 0.10 M ethylene glycol
L
2.0 L
4
This solution is said to 0.10 molar (denoted with the M) meaning 0.10 moles solute per one liter
of solution.
This solution would be prepared by adding enough water to 0.10 moles ethylene glycol to make
2.0 liter of solution. Such solutions are prepared in a 1.00-L or 2.00L graduated cylinder for
accurate measurement of the volume of the solution.
Mass Percentage of Solute
Sometimes concentration of a solution is expressed as the percent solution by mass (or mass
percentage of solute.)
mass percentage of solution = mass of solute (g) x 100
mass of solution
For example, an aqueous solution that is 3.5% sodium chloride by mass contains 3.5 g NaCl in
100.0 g of solution. This solution would be prepared by dissolving 3.5 g NaCl(s) in 96.5 g
water.
Molality
The molality of a solution is the number of moles of solute dissolved in one kilogram of solvent.
(Again, the “one” has an indefinite number of sig. sigs.)
Molality =
moles of solute
kg of solvent
=
moles
kg
Molality is denoted as m.
For example: 0.20 mole of ethylene glycol are dissolved in 2.0 kilograms of water.
The molality of the solution is
m = moles of solute = 0.20 moles = 0.10 molal (denoted m)
kg of solvent
2.0 kg
The units of molality and molarity are sometimes confused by the student. Molality is defined in
terms of mass of solvent while molarity is in terms of volume of solution.
NOTE: It is very important that all quantities be labeled
completely and accurately and are in the correct sig. figs.
5