12.4-12.5 & 12.8 Solution Equilibrium and Factors affecting
Solubility – Expressing Solution Concentration & Colloids
The Process of Solvation (Dissolving)
• As sodium chloride is added to water the rate of dissolution is
rapid and the ions dissolve in the water. As more salt is added the
rate of recrystallization of ions begins to increase until both rates
are equal. Here a dynamic equilibrium exists. Now, the solution is
said to be saturated.
NaCl(aq) ↔ Na+(aq) + Cl-(aq)
• An unsaturated solution: A solution containing less than the
equilibrium amount of solute.
• A supersatured solution: A solution containing more than the
equilibrium amount of solute. Here the solution is unstable, and a
disturbance to the system will cause recrystallization to occur.
Factors that Affect Solubility
What factor does temperature have on the solubility
of most solids? What about gases?
As temperature increases, the solubility of most solids
will increase as well, and the solubility of gases
decrease with increasing temperature.
Let’s Try a Practice Problem!
A solution is saturated in both nitrogen gas and
potassium bromide at 75oC. When the solution cooled to
room temperature, what is most likely to happen?
a.) Some nitrogen gas bubbles out of solution.
b.) Some potassium bromide precipitates out of solution.
c.) Some nitrogen gas bubbles out of solution and some
potassium bromide precipitates out of solution.
d.) Nothing happens.
b.) Some potassium bromide precipitates out of solution.
Pressures Affect on Gases
• An increase in pressure, causes an increase in gas
solubility, and a new equilibrium to be established.
Here, we can use Henry’s Law:
Sgas = kHPgas (where Sgas = solubility of a gas in M,
and KH is Henry’s law constant that
depends both on a specific
solute/solvent relationship (in M/atm),
Pgas is the partial pressure of a gas
(usually in atm))
Let’s Try a Practice Problem!
Determine the solubility of oxygen in water at 25oC
exposed to air at 1.0 atm. Assume the partial
pressure for oxygen of 0.21 atm. (KH for O2 in water at
25oC = 1.3X10-3 M/atm).
Sgas = kHPgas
Sgas = (1.3X10-3 M/atm)(0.21 atm) = 2.7X10-4 M
Expressing Solution Concentration
• A dilute solution – contains a small quantities of solute
compared to the quantity of solvent.
• Concentrated solution – Contains large quantities of solute
compared to the quantity of solvent.
If you are stranded on an island, and you are thirsty, should you
drink the sea water? Why?
No. Since sea water is highly concentrated with salt, the salt
absorbs the water from the cells that line your digestive tract
causing dehydration, and also draws extra water into your
intestines causing diarrhea. No Good!!!
Reporting Solution Concentration
• Here are ways to report solution concentration:
– Molarity (M) = moles of solute / volume of solution (in Liters)
(note: this is not per L of solvent!)
– Molality (m) = moles of solute / mass of solvent (in kg)
– Percent by mass = (mass of solute /mass of solution) X 100
– Percent by volume = (volume of solute / volume of solution) X 100
– For my dilute solutions the following two equations may be used:
• Parts per million (ppm) = (mass of solute / mass of solution) X 106
• Parts per billion (ppb) = (mass of solute / mass of solution) X 109
Let’s Try a Practice Problem!
A solution is prepared by dissolving 50.4 g sucrose
(C12H22O11) in 0.322 kg of water. The final volume of
the solution is 355 mL. Calculate the concentration of
the solution in molarity.
M = mol solute / L solution
M = ((50.5 g C12H22O11) X (1.00 mol / 342.34 g)) / 0.355 L = 4.16X10-1 M
Colloids
• Colloid (or colloid dispersion) – a mixture in which a
dispersed substance (which is solute-like) is finely
divided in a dispersing medium (which is solvent-like).
– Examples: fog, smoke, whipped cream, milk and opal.
• Whether or not a mixture is a colloid is determined by
the size of the particles it contains.
– If the particles are between 1 nm and 1000 nm, the mixture
is a colloid.
– Colloid particles scatter light; this is called the Tyndall effect.
12.4-12.5 & 12.8 pgs. 589-590 #’s 42, 44, 52 (molarity only), 54,
and 62 & 64 (a only).
Study for Quiz Chapter 12