Properties of Solutions, Solubility,
and the
Stoichiometry of Reactions in Solutions
Objectives:
Use different ways of expressing concentration of solutions:
1 percent by mass, mole fraction, molarity, molality, percent by
volume, percent by mass, ppm
Perform stoichiometric calculations for reactions in solution
2
What is your usual breakfast?
Solute vs. Solvent
25 grams of salt dissolved in 95 mL of water
25 mL of water mixed with 75 mL of isopropyl alcohol
Tincture of iodine prepared with 0.20 gram of Iodine and 20.0 mL
of ethanol
Examples of several different solutions
and the phases of the solutes and solvents
Solutions exhibit these defining traits:
• They are homogeneous; that is, after a solution is mixed, it
has the same composition at all points throughout (its
composition is uniform).
• The physical state of a solution—solid, liquid, or gas—is
typically the same as that of the solvent, as demonstrated by
the examples in Table earlier.
• The components of a solution are dispersed on a molecular
scale; that is, they consist of a mixture of separated
molecules, atoms, and/or ions.
Solutions exhibit these defining traits:
• The dissolved solute in a solution will not settle out or
separate from the solvent.
• The composition of a solution, or the concentrations of its
components, can be varied continuously, within limits.
The solubility of a solute in a particular solvent is the maximum
concentration that may be achieved under given conditions
when the dissolution process is at equilibrium.
A solution that contains a relatively low concentration of solute
is called dilute, and one with a relatively high concentration is
called concentrated.
Solutions may be prepared in which a solute concentration
exceeds its solubility. Such solutions are said to be
supersaturated, and they are interesting examples of
non-equilibrium states.
Percent Concentration
One way to describe the concentration of a solution is by the
percent of the solution that is composed of the solute. This
percentage can be determined in one of three ways:
(1) the mass of the solute divided by the mass of solution,
(2) the volume of the solute divided by the volume of the solution,
or (3) the mass of the solute divided by the volume of the solution.
Because these methods generally result in slightly different values,
it is important to always indicate how a given percentage was
calculated.
Mass Percent
When the solute in a solution is a solid, a convenient way to
express the concentration is a mass percent (mass/mass), which is
the grams of solute per 100g of solution.
Volume Percent
The percentage of solute in a solution can more easily be
determined by volume when the solute and solvent are both
liquids. The volume of the solute divided by the volume of the
solution expressed as a percent, yields the percent by volume
(volume/volume) of the solution.
Mass-volume Percent
The mass-volume percent is also used in some cases and is
calculated in a similar way to the previous two percentages. The
mass/volume percent is calculated by dividing the mass of the
solute by the volume of the solution and expressing the result as a
percent.
Parts per Million and Parts per Billion
Two other concentration units are parts per million and parts per
billion. These units are used for very small concentrations of solute
such as the amount of lead in drinking water. Understanding these
two units is much easier if you consider a percentage as parts per
hundred.
Very low solute concentrations are often expressed using
appropriately small units such as parts per million (ppm) or parts
per billion (ppb). Like percentage (“part per hundred”) units, ppm
and ppb may be defined in terms of masses, volumes, or mixed
mass-volume units. There are also ppm and ppb units defined with
respect to numbers of atoms and molecules.
The mass-based definitions of ppm and ppb are given here:
D = m/v
Molarity (M) is a useful concentration unit for many applications
in chemistry. Molarity is defined as the number of moles of
solute in exactly 1 liter (1 L) of the solution:
Molality
Molality is also known as molal concentration. It is a measure of
solute concentration in a solution. The solution is composed of
two components; solute and solvent. The term needs to
calculate the mass of the solvent and moles of solute.
Dilution is the process whereby the concentration of a solution
is lessened by the addition of solvent.
A simple mathematical relationship can be used to relate the
volumes and concentrations of a solution before and after the
dilution process.
According to the definition of molarity, the molar amount of
solute in a solution is equal to the product of the solution’s
molarity and its volume in liters:
Equivalents
Concentration is important in healthcare because it is used in so
many ways. It's also critical to use units with any values to
ensure the correct dosage of medications or report levels of
substances in blood, to name just two.
Another way of looking at concentration such as in IV solutions
and blood is in terms of equivalents. One equivalent is equal to
one mole of charge in an ion. The value of the equivalents is
+
always positive regardless of the charge. For example, Na and
−
Cl both have 1 equivalent per mole.
SEATWORK: Answer sample problems on concentration.
1) 6.80 g of sodium chloride are added to 2750 mL of water. Find
the mole fraction of the sodium chloride and of the water in the
solution.
g NaCl ->mol
mL H2O-> g -> mol
mole solute mole solvent
mole solution mole solution
2) How many grams of magnesium cyanide are needed to make
275 mL of a 0.075 M solution?
2+ -
Mg CN Mg (CN)2
3) How many grams of magnesium cyanide would you need to
add to 275 mL of water to make a 0.075 molal solution?
Vwater 275 mL Dwater = 1.00g/mL mwater= ?(g) → kg
4) Explain how to make one liter of a 1.25 molal sodium
hydroxide solution.
5) What is the molarity of a solution made when 52 grams of
potassium sulfate are diluted to a volume of 4100 mL?
6) The density of ethylene glycol (antifreeze, HOCH2CH2OH) is
1.09 g/mL. How many grams of ethylene glycol should be mixed
with 375 mL of water to make a 7.50% (v/v) mixture?
7) Find the volume of a 0.75 M solution if it contains 39 grams of
potassium hydroxide.
8) How many grams of hydrochloric acid are present in 3.0 L of a
0.750 M solution?
9) The concentration of oxygen in water at the bottom of a lake
is 0.48 g/L and the pressure is 2.5 atm. If water from the bottom
is moved by a current upwards to a depth where the pressure is
1.3 atm, what is the concentration of the oxygen in the water at
this depth?
10) What is the molarity of a solution in which 0.850 grams of
ammonium nitrate are dissolved in 345 mL of solution?
11) Explain how you would make 675 mL of a 0.400 M barium
iodide solution.
Applications
Takeaways
