1. Expressing Solubility and Concentration
Declan Kroll

2. Solution Equilibrium and Factors Affecting Solubility
From a molecular viewpoint
Consider the dissolution of a solid solute such as sodium chloride in a liquid solvent such as water
Initially, Sodium cations and chloride anions are dissolved resulting in the decrease of Sodium Chloride in water
Saturated solution- A solution in which the dissolved solute is in a dynamic equilibrium with the solid solute
Unsaturated solution- A solution containing less than the equilibrium amount of solute
Supersaturated solution- A solution containing more than the equilibrium amount of solute
Often unstable and excess precipitates out of the solution
As the concentration of dissolved Sodium Chloride increases, the rate of recrystallization begins to increase
Eventually the rates of recrystallization and dissolution become equal, known as dynamic equilibrium
NaCl(s) Na+(aq) + Cl- (aq)

3. Temperature dependance of the Solubility of Solids
Notice how much more sugar you can dissolve in hot tea rather than cold tea?
This is because solubility of most solids solids tends to increase with an increase in temperature
Recrystallization- common way to purify solid
Enough solid is added to a solvent to create a saturated solution at an elevated temperature. As it cools slowly, the solid forms crystals as it comes out of the solution resulting in a purer solid (crystalline structure rejects imputies, Rock Candy)
On the curve is saturated
Above the curve is supersaturated
Below the curve is unsaturated

4. Factors Affecting the Solubility of Gases in Water
Gases can dissolve in water as well
Temperature
Solubility of gases in liquid decreases with increasing temperature
Higher kinetic energy break intermolecular bonds more easily and escape from the solution
Ex. CO2 dissolved in water, Oxygen dissolved in lake water
Pressure, Temperature effects on solubility of gas in liquid
A solution is saturated in both nitrogen gas and potassium bromide at 75 oC. When the solution is 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) Both a and b
d) Nothing happens

5. Solubility of gases Continued
Pressure
The higher the pressure of a gas above a liquid, the more soluble the gas is in the liquid
First cylinder- rate of molecules entering the solution exactly equals the rate of molecules leaving solution
Second cylinder- pressure of Carbon dioxide increases, causing rate of molecules entering solution to rise until dynamic equilibrium is reached again in third cylinder

6. Calculate solubility of gas with increase pressure
Henry's law
Sgas = kHPgas
S- Solubility of gas (M)
kH- constant of proportionality (M/atm)
P- Partial Pressure of gas (atm)
What pressure of carbon dioxide is required to keep the carbon dioxide concentration in a bottle of club soda at 0.12M at 25 oC? (kH for CO2 = M/atm)
Answer- 3.5 atm
Sgas = kHPgas Rearrange formula PCO2= (Sco2) / (kH, CO2)
= (0.12 M) / ( M/atm)
= 3.5 atm

7. Determine the Solubility of oxygen in water at 25 oC exposed to air at 1.0 atm. Assume a partial pressure for oxygen of .21 atm. (kH = M/atm)
Answer: S=

8. Expressing Solution Concentration
Dilute solution- contains small quantities of solute relative to the amount of solvent
Concentrated solution- contains large quantities of solute relative to the amount of solvent
Drinking a concentrated sodium chloride solution will cause dehydration
Common ways of reporting solution concentration
Molarity
Molality
Parts by mass
Parts by volume
Mole fraction
Mole percent

9. Expressing Solution Concentration
Molarity (M) = amount solute (mol) / volume solution (L)
Molarity depends on volume and varies with temperature
Molality (m) = amount solute (mol) / mass solvent (kg)
Independent of temperature, useful to compare concentrations over range of temp.
Parts by Mass and Volume
Parts by mass = mass solute / mass solution multiplication factor
Percent by mass = mass solute / mass solution %
Ppm = mass solute / mass solution
A solution with a concentration of 15 ppm contains 15g solute per 106g solution
Ppb = mass solute / mass solution
Parts by volume = Volume solute / Volume Solution multiplication factor
Usually when both solute and solvent are liquids

10. What Volume (in mL) of a soft drink that is 10
What Volume (in mL) of a soft drink that is 10.5% sucrose (C12H22O11) by mass contains 78.5 g of sucrose? (The density of the solution is 1.04 g/mL)
g sucrose g soln mL soln
Use Percent by mass as ratio: 10.5 g sucrose per 100g soln
And Density: 1 mL per 1.04g
78.5g sucrose g soln mL = 719 mL soln
10.5g sucrose g

11. Mole Fraction and Mole Percent
Xsolute = amount solute (mol) / total amount of solute and solvent (mol)
Mol % = Xsolute %
A solution is prepared by dissolving 17.2g of ethylene glycol (C2H602) in .500 kg of water. The final volume of the solution is 515 mL. For this solution, calculate the concentration in mole fraction and mole percent
Convert g H20 to mol H20 = mol H Convert g C2H602 to mol = mol C2H602
(.2271 mol) / (.2771 mol mol) =
Mole percent = .989%

12. Practice Problem
What is the molarity of a 6.56% by mass glucose (C6H1206) solution. (density of solution is 1.03 g/mL.)
Answer .375 M glucose
6.56 g glucose * (l mol / g glucose) = mol glucose
100 g soln * (1 mL / 1.03g) * (10-3 L / mL) = L soln