Solutions --don’t worry—we’ll have problems to go with them.

How does a solute dissolve in a solvent?

IM forces in mixture are strong enough overcome IM forces in solute

How much of a solute will dissolve in a solvent?

It depends on the solute and the solvent --some things, not much (e.g. Ag 2 S/H 2 O) --others, any proportions, recall: “Miscible” (N 2 /O 2,for example) Answer: Its “solubility”

How much of a solute will dissolve in a solvent? A solid in water us. increases at higher T o --expressed as g solute /100 g solvent Gasses have very low solubilities in water --solubilities decrease with T o --solubilities increase directly with P (Henry’s Law)

How can you speed up dissolving?

1)Raise the temperature of the solvent 2)Stir or shake 3) Grind or crush a solid 4)Add more solvent

Why do these methods help?

Grind or crush a solid —increases surface area—more solute is in contact with solvent Raise the temperature —causes particles to move faster—more collisions & more effective collisions Stir or shake, or add more solvent —brings fresh solvent into contact with solute

How can you express concentration?

1)Mass % composition 2)Molarity 3)Molality 4)Mole fraction 5)Density

Dilution and titration problems M x V = Moles Moles of solute in new solution = moles in the stock solution. Molarity goes down, volume goes up Moles of H + and OH - are equal. The more concentrated solution requires less volume

The relationship between M and m We measured masses and volumes and were able to calculate M, m, and D. We can use two of these to calculate the third.

For example: 1.What is the molality of a.980 M AlCl 3 solution? (D=1.089 g/ml) 2.What is the molarity of a 1.00 m HC 2 H 3 O 2 solution? (D=1.064 g/ml) 3.What is the density of a 1.19 M CaCl 2 solution, if the molality is 1.23 m?

Colligative properties Solutes affect the physical properties of a solution. If the only effect is caused by the number of solute particles, it’s a colligative property

Colligative properties include… Freezing point depression Boiling point elevation Vapor pressure depression Osmotic pressure

Freezing point depression  T f =iK f m If you dissolve a solute, the solution has a lower freezing point than the pure solvent The effect is directly related to the number of particles K f for water is 1.86 o C/m

Freezing point depression  T f =iK f m The constant, K f, is specific to the solvent, K f =1.86 o C/m, for water The new freezing point is FP’=FP-  T f i is the van’t Hoff factor —it represents the moles of particles per mole of solute. Ionic compounds dissociate (i=2,3,etc), non-electrolytes do not (i=1) m is molality (moles solute/ kg solvent)

FP depression problems: 1.What is the freezing point of a.86 m glucose solution (i=1)? 2.How about a.86 m NaCl solution (i=?) ? 3.What is the freezing point of 1.83 g C 2 H 4 (OH) 2 dissolved in 15 g H 2 O? (i=1) 4.What is the freezing point of a solution of 4.9 g CCl 4 dissolved in 33 g benzene? 5.What is the formula mass of a non-electrolyte solute if 12 g dissolved in 48 g H 2 O gives a FP of -1.2 o C?

Boiling point elevation Same issues  T b =iK b m K b for water =.51 o C/m

BP elevation problems: 1.What is the boiling point of a.86 m glucose solution (i=1) ? 2.How about a.86 m NaCl solution (i=?) ? 3.What is the boiling point of 1.83 g C 2 H 4 (OH) 2 dissolved in 15 g H 2 O? (i=1) 4.What is the boiling point of a solution of 1.9 g CCl 4 dissolved in 33 g benzene? 5.What is the formula mass of a non-electrolyte solute if 12 g dissolved in 48 g H 2 O gives a BP of o C?

Vapor pressure depression Nonvolatile solute  vapor pressure decreases. This causes the boiling point effect. We use a mole fraction for a direct relationship here. If the solute can vaporize—both components add to the vapor pressure.

Vapor pressure problems 1.What is the VP, at 100 o C, of the solution of 12g C 2 H 4 (OH) 2 dissolved in 150 g H 2 O? 2.What is the VP, at 100 o C, of the solution of 12g NaCl dissolved in 150 g H 2 O? 3.What is the VP, at 35 o C, of the solution of 120 g C 2 H 5 OH dissolved in 150 g H 2 O? (VP’s at 35 o C: C 2 H 5 OH:101 mmHg H 2 O: 42 mmHg)

Osmotic pressure Used in biology, osmosis is the diffusion of water across a membrane  =-iMRT  is the osmotic pressure i is the van’t Hoff factor M is the molarity of the solution R is the ideal gas constant T is the absolute temperature

What is the osmotic pressure of.15M NaCl? What is the osmotic pressure of pure water?

Non-Electrolytes Electrolytes Molecular Do not dissociate i=1 Don’t conduct electricity in solution Raise BP Lower FP, VP Ionic (&strong acids) Dissociate into ions i>1 Conduct electricity in solution Raise BP more Lower FP, VP more