Solutions

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

How does a solute dissolve in a solvent?

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?

How much of a solute will dissolve in a solvent?

• It depends on the solute and the solvent

--some things, not much (e.g. Ag2S/H2O)

--others, any proportions, recall: “Miscible” (N2/O2 ,for example)

Answer: Its “solubility”

How much of a solute will dissolve in a solvent?

• A solid in water us. increases at higher To

• --expressed as gsolute/100 gsolvent

• Gasses have very low solubilities in water

--solubilities decrease with To

--solubilities increase directly with P (Henry’s Law)

How can you speed up dissolving?

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?

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?

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 AlCl3 solution? (D=1.089 g/ml)

2. What is the molarity of a 1.00 m HC2H3O2 solution? (D=1.064 g/ml)

3. What is the density of a 1.19 M CaCl2

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

Tf=iKfm

• 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

• Kf for water is 1.86oC/m

Freezing point depression

Tf=iKfm• The constant, Kf, is specific to the solvent,

Kf =1.86oC/m , for water • The new freezing point is FP’=FP-Tf

• 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 C2H4(OH)2 dissolved in 15 g H2O? (i=1)

4. What is the freezing point of a solution of 4.9 g CCl4 dissolved in 33 g benzene?

5. What is the formula mass of a non-electrolyte solute if 12 g dissolved in 48 g H2O gives a FP of -1.2oC?

Boiling point elevation

• Same issues

Tb=iKbm

Kb for water =.51 oC/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 C2H4(OH)2 dissolved in 15 g H2O? (i=1)

4. What is the boiling point of a solution of 1.9 g CCl4 dissolved in 33 g benzene?

5. What is the formula mass of a non-electrolyte solute if 12 g dissolved in 48 g H2O gives a BP of 103.2oC?

Vapor pressure depression

• Nonvolatile solutevapor 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 100oC, of the solution of 12g C2H4(OH)2 dissolved in 150 g H2O?

2. What is the VP, at 100oC, of the solution of 12g NaCl dissolved in 150 g H2O?

3. What is the VP, at 35oC, of the solution of 120 g C2H5OH dissolved in 150 g H2O?

(VP’s at 35oC: C2H5OH:101 mmHg

H2O: 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