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Aqueous solutions - (Water solutions)

1. A solution consists of a "solute" dissolved in "solvent"  - the solute and solvent are intimately mixed on the molecular level.  Solutions are clear (not cloudy, can still be colored)

the solute is the minor component which is dissolved in the solvent (water)

Ionic compounds dissolve to form ions in solution

2. Solvation - interaction of a solute with a solvent

remember our picture of a salt dissolving in water.  Water is said to "solvate" the ions of the salt.

dissolution of a salt

water (because it is polar) is a good solvent for ions and polar molecules

nonpolar molecules are insoluble in water (oil and water don't mix)

3.  Solutions can be

-saturated - the solution contains maximum amount of solute that can be dissolved

- unsaturated - more solute can be dissolved

-supersaturated - unstable to the "precipitation" of solute.  contains more solute than it can "theoretically" hold.

4. Gases dissolve in water (carbonated beverages)

CO2 in soda

Demo: sugar in CO2

O2 in water for fish

Henry's law - at a given temperature the solubility is directly proportional to the pressure above it.  Increase pressure of gas above a liquid, more of that gas will dissolve. 

5. Reporting the concentrations of solutions

a. Molarity

moles of solute

molarity =

----------------------

= ____ M

liters of solution

example: 3M NaCl = 3 moles of NaCl / 1 liter of solution

Problem:

What is the molarity of a solution of table sugar (sucrose, molar mass = 342 g / mol) which contains 10 grams of sugar in 0.25 L (about one cup).  This is the amount of sugar in you typical soft drink.

molarity is amount over volume so we will start with that and the convert to the proper units

10 g sugar

1 mol sugar

= 0.117 M sugar

 

0.25 L solution

342 g sugar

 

 

 

b. volume / volume percent (used for solutions of liquids)

volume solute (mL)

v/v percent=

---------------------------

x100%

volume solution (mL)

Problem: how much acetic acid (in mL) is in 50 mL of vinegar?

vinegar is a 5% (v/v) solution of acetic acid in water

5 % (v/v) = 0.05 (v/v) tell gives us to conversion factors

0.05 mL acetic acid

1 mL vinegar

---------------------------

   or  

---------------------------

1 mL vinegar

0.05 mL acetic acid

 

50 mL vinegar

0.05 mL acetic acid

 = 2.5 mL vinegar

 

1 mL vinegar

 

 

c. mass / volume percent

 mass / volume percent (used for solutions of liquids)

mass of solute (g)

m/v percent=

---------------------------

x100%

volume solution (mL)

based on the fact that 1 ml solution approximately = 1 gram.

Sugar example above:

10 g sugar

1 L solution

= 0.04 g sugar

= 4 % (w/v)

0.25 L solution

1000mL solution

mL solution

 

 

d. For very low concentrations parts per million (ppm) or parts per billion (ppb) often used

ppm =

1 part in 1 million parts solution

1 gram in 1 million grams solution

1 drop in 1 million drops solution

1 ppm = 1 mg / L = 1x10-3 g / L = 1x10-3g / 1000mL = 1x10-6 g / mL / 0.0001% (m/v)

ppb =

 1 part in 1 million parts solution

1 gram in 1 million grams solution

1 drop in 1 million drops solution

1ppb = 0.001 mg / L = 1x10-6 g / L = 1x10-6g / 1000mL = 1 x 10-9 g / mL = 0.0000001% (m/v) = 1x10-7 % (m/v)

Sugar example above:

4 %(m/v) sugar

1 ppb sugar

= 4x107 ppb sugar

1x10-7 % (m/v) sugar

Maximum contaminant levels in water systems allowable by law:

arsenic

50 ppb

lead

50 ppb

mercury

2 ppb

nickel

100 ppb

benzene

5 ppb

6. How do the properties of a solution differ from that of pure solvent?

Colligative properties - properties that depend on the number of dissolved particles in a given mass of solvent

does not depend on the chemical properties

1.0M NaCl and 1.0M KBr have the same colligative properties because there solutions contain the same number of ions.

a. Vapor pressure lowering - solute added to a solvent makes the v.p. of a solution less than solvent

decrease in vapor pressure is directly proportional to the number of solute particles in the solution

b. boiling point elevation - because vapor pressure is lowered, must go to higher temperature to get solution boiling.

Covalent compounds:

1M ethanol(aq) boils at 100.7oC

Ionic compounds:

dissolve with the formation of ions, number of ions is important

1M NaCl(aq) boils at 101.4oC  (NaCl forms Na+ and Cl- when it dissolves in solution so the affect is twice as much as with the molecule ethanol)

1M KBr(aq) boils at 101.4oC (colligative properties do not depend on what the material is, just the number of particals)

c. freezing point depression - the addition of solute lowers the freezing point of water. 

1M NaCl(aq) freezes at -3.72oC

Demo: string on ice cube

d. Osmotic pressure

Water seeks solute

consider two solutions of different solute concentrations separated by a osmotic membrane.

osmotic membranes allow only water molecules to pass through (no solute passes through)

 As if a pressure was pushing down on the side with the pure water.  The amount of pressure on the left tube it would require to bring the two water levels equal is the osmotic pressure.

e. dialysis

solute seeks water

dialyzing membranes - semipermeable membranes that permit the passage of small molecules and ions, large molecules are retained.

 

Hemodialysis is where a dialyzing membrane Is used to remove toxic waste products from the blood.