Solutions

Review of Properties of Water Is a polar molecule Has extensive hydrogen bonding Many of water’s unusual properties are due to hydrogen bonding

How aqueous solutions are formed Solvation The process by which cations and anions become surrounded by water molecules Occurs at the surface of the solute Solvation Process

16.1 Solution Formation The nature of the solvent and solute determine solution formations “Like dissolves like” ◦Polar dissolves polar/ionic ◦Non-polar dissolves non-polar

Rate of solution formation Temperature ◦Kinetic energy of solvent & solute Amount of surface area exposed to solvent ◦Particle size ◦Stirring

Surface Area and Particle Size 6 m 2 x 8 = 48 m 2 24 m 2

Solubility Expresses the degree to which a substance dissolves Usually expressed in g solute/100 g solvent Examples ◦Solubility of KNO 3 is 85 g/100 g water at 20°C

Important Factors Affecting Solubility Temperature ◦For most solid solutes, solubility increases with increasing temperature ◦ ↑ T → ↑ Solubility ◦For gases, solubility decreases with increasing temperature ◦ ↑ T → ↓ Solubility Pressure ◦No effect on solubility of solids ◦For gases, ↑ P → ↑ Solubility

Solubility and Temperature

Types of solutions Saturated ◦Contain maximum amount of solute at a given temperature ◦Dissolved solute in equilibrium with undissolved ◦All solutions are saturated solutions on a solubility curve Unsaturated ◦Contain less than maximum solute ◦No undissolved solute present Supersaturated ◦Contain more than the maximum amount of solute at a given temperature ◦How could this happen?

Henry’s Law and Solubility of Gases At a given temperature, solubility of a gas is directly proportional to the partial pressure of the gas

Saturated, Unsaturated and Supersaturated solutions

16.2 Concentrations of Solutions Concentration tells how much solute is dissolved in an amount of solution or solvent Many ways of expressing concentration Most common is molarity (M) Concentrated solutions have much solute Dilute solutions have little solute

Molarity Is the moles of solute per liter of solution What is the molarity of a solution containing 0.9 g NaCl dissolved in 100 mL of solution?

Molarity  To make a 0.5-molar (0.5M) solution, first add 0.5 mol of solute to a 1-L volumetric flask half filled with distilled water. 16.2

Molarity  Swirl the flask carefully to dissolve the solute. 16.2

Molarity  Fill the flask with water exactly to the 1-L mark. 16.2

Molarity Problem What mass of CaCl 2 is needed to make 0.50 L of 2.5 M solution? g Explain how to prepare the solution.

Making Dilutions  The total number of moles of solute remains unchanged upon dilution, so you can write this equation.  M 1 and V 1 are the molarity and volume of the initial solution, and M 2 and V 2 are the molarity and volume of the diluted solution.

Making Dilutions  Making a Dilute Solution

Percent Concentration Sometimes concentration is expressed as a percent Example: the label on a bottle of bleach reads 8.25% sodium hypchlorite (NaClO) What does this mean?

Percent Concentration Volume percent means What is the V% when 15.0 mL ethanol is dissolved in 65.0 mL water? 18.8%, why not 23.1%

Percent Concentration Mass percent means What is the m% when 2.50 g of KCl is dissolved in 75.0 mL of water? 3.23%, why not 3.33%

Colligative Properties of Solutions Solutes change the properties of solvents Colligative properties depend upon the concentration of solute particles in solution

Important colligative properties Vapor pressure lowering Boiling point elevation Freezing point depression

Vapor Pressure Lowering

Colligative Properties and Number of Solute Particles Colligative properties depend on the total moles of solute particles and not upon their identity 1 mol glucose  1 mol glucose ◦(= 1 mol particles) 1 mol NaCl  1 mol Na mol Cl - ◦(= 2 mol of particles) 1 CaCl 2  1 Ca Cl - ◦(= 3 mol particles)

Colligative Properties and Number of Solute Particles

Changes in Pvap, bp, fp are proportional to the number of solute particles dissolved in the solvent