solutions --don’t worry—we’ll have problems to go with them
TRANSCRIPT
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Solutions
--don’t worry—we’ll have problems to go with them.
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How does a solute dissolve in a solvent?
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How does a solute dissolve in a solvent?
• IM forces in mixture are strong enough overcome IM forces in solute
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How much of a solute will dissolve in a solvent?
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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”
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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)
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How can you speed up dissolving?
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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
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Why do these methods help?
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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
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How can you express concentration?
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How can you express concentration?
1) Mass % composition
2) Molarity
3) Molality
4) Mole fraction
5) Density
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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
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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.
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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?
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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
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Colligative properties include…
• Freezing point depression
• Boiling point elevation
• Vapor pressure depression
• Osmotic pressure
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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
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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)
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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?
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Boiling point elevation
• Same issues
Tb=iKbm
Kb for water =.51 oC/m
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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?
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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.
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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)
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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
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• What is the osmotic pressure of .15M NaCl?
• What is the osmotic pressure of pure water?
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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