1. Properties of Solutions
AP CHEMISTRY

2. 4.1 Water, the Common Solvent
Many common chemical reactions occur in water, or aqueous solution. To understand how chemical species interact in solution, we must first understand water, the universal solvent.
Water is an excellent solvent due to:
Its shape; water is a bent molecule.
Electrons aren’t shared evenly (oxygen is more electronegative)
Electrons spend more time close to O than to H
This uneven distribution of charge makes water polar.

3. Hydration Water is held together by covalent bonds
When water surrounds an ionic crystal, the H end attracts the anion and the O end attracts the cation. This process is called hydration.
Hydration causes salts to dissolve. H2O also dissolves polar covalent substances such as C2H5OH.

4. Water and Nonpolar Molecules
H2O doesn’t dissolve nonpolar covalent substances.
The difference in a substances’ ability to dissolve is due to its interaction with itself, and the solvent solution.

5. 4.2 The Nature of Aqueous Solutions: Strong and Weak Electrolytes
Review:
Solute-the substance that dissolves
Solvent-the substance that does the dissolving
Electrical conductivity-the ability of a solution to conduct an electrical current
Strong electrolyte-a highly ionized solution that easily conducts electrical current
Weak electrolyte-a solution with few ions that does not conduct a current very well
Nonelectrolyte- a solution made of a soluble compound that does not ionize and thus does not conduct an electrical current.

7. Arrhenius determined that the extent to which a solution can conduct an electrical current depends directly on the number of ions present.
A solute that ionizes completely conducts very well.
He said that the best conductors are:
Soluble salts (like sodium chloride or magnesium nitrate)
Strong acids (Memorize these: HCl, HBr, HI, HClO4, HNO3, H2SO4)
Strong bases that contain OH- (have a bitter taste, slippery feel; hydroxides of Group 1 & 2 metals)

8. Ex. Write an equation showing what happens when the salt sodium acetate dissolves in water.
NaC2H3O2  Na+ + C2H3O2-

9. Arrhenius went on to describe weak electrolytes
Arrhenius went on to describe weak electrolytes. He said they only ionize slightly and include weak acids and bases.
HC2H3O2  H+ + C2H3O2-
99% %
Ammonia (NH3) is a weak base NH3 + H2O  NH4+ + OH-

10. Solubility- g/given volume solvent or moles/given volume solution
Complete solubility practice problems for home work!

11. 4.3 The Composition of Solutions
In order to perform stoichiometric calculations with solutions, we must know the nature of the solution and the amounts of chemical present.
M = moles of solute
liters of solution

12. Ex. Calculate the molarity of a solution made by dissolving 23
Ex. Calculate the molarity of a solution made by dissolving 23.4g of sodium sulfate in enough water to form 125 mL of solution.
23.4 g Na2SO4 1 mol Na2SO4 = mol Na2SO g Na2SO mol = 1.32 M L

13. Ex. How many grams of Na2SO4 are required to make 350 mL of 0
Ex. How many grams of Na2SO4 are required to make 350 mL of 0.50 M Na2SO4?
0.50 M = x/0.350 L
0.175 mol Na2SO g Na2SO4 = 25 g
1 mol Na2SO4

14. Dilution problem (M1V1 = M2V2) (1.000M)(V1) = (0.250M)(500.0mL)
Ex. What volume of M KNO3 must be diluted with water to prepare mL of M KNO3?
Dilution problem (M1V1 = M2V2)
(1.000M)(V1) = (0.250M)(500.0mL)
V1 = 125 mL

15. A standard solution is one where concentration is accurately known
A standard solution is one where concentration is accurately known. Read procedure for using volumetric flasks and types of pipets. We will be using both in several labs this year.

16. 11.1 Solution Composition
Solutions can generally be described as dilute (very little solute per volume of solvent) of concentrated (a lot of solute per volume of solvent). Another way to describe a solution is by looking at the mass of the solute in terms of the mass of the entire solution, or mass percent.
We can also examine the percent solute to solvent using the mole fraction of the solution, or χ.
Finally, we can look at the amount of solute in moles per amount of solvent in kg to find molality.

17. Formulas for Solutions
Mass % = Mass solute x 100 Mass of solution Mole Fraction χA = nA . nA + nB Molality = moles solute Kg solvent

18. Ex. A solution is made using 10. 5g ethanol (C2H5OH) in 1,200
Ex. A solution is made using 10.5g ethanol (C2H5OH) in 1,200.0 g of water to make a final volume of ml. Find the molarity, molality and mole fraction of this solution.
10.5g C2H5OH 1 mol C2H5OH = 0.228mol C2H5OH
46.08g C2H5OH
1,210.5 g soln = L soln
1,200.0g H2O/18.02g = mol H2O
M = mol / L = 0.188M C2H5OH
m = mol/ kg = 0.19 m C2H5OH
χC2H5OH = (0.228) =
( )

19. Lab: Molarity & Dilution
Correctly Making Solutions:
Weigh out the solid as accurately as possible, place in a volumetric flask, add only enough distilled water to dissolve the solid and swirl to completely dissolve the solid THEN add a bit more water, filling to the mark on the flask. If you dump solid into 1.00 L of water you are neglecting the space the solid will occupy and your molar concentration will NOT be correct!

20. Cont. Lab
Dilution: One can also dilute a more concentrated solution by
Using a pipet to deliver a volume of the solution to a new volumetric flask, and add solvent to the line on the neck of the new flask.