# Solutions and Their Properties by 242CpTmR

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```									  CHAPTER 11

SOLUTIONS AND THEIR PROPERTIES

Dr Ayesha Mohy-ud-din
Colligative Properties
of Nonvolatile Solutes

•   Raoult’s Law: Psoln = P°solv Xsolv
•   For a single solute solution, Xsolv= 1 – Xsolute ,
•   We can obtain an expression for the change in vapor
pressure of the solvent (the vapor pressure lowering).
Psoln = P°solv – Psoln
= P°solv – Xsolv P°solv
= P°solv – (1 – Xsolute ) P°solv
∆P = Xsolute P°solv
Where superscript o is for pure substance.
Chapter 11                     Slide 2
Van’t Hoff Factor

•   For incompletely dissociating ionic solids

• Van’t Hoff Factor i = moles of particles in solution
•                       moles of solute dissolved
•

Chapter 11                    Slide 3
Colligative Properties
of Nonvolatile Solutes

•   The vapor pressure of a glucose (C6H12O6) solution
is 17.01 mm Hg at 20°C, while that of pure water is
17.25 mm Hg at the same temperature. Estimate the
molality of the solution.

•   How many grams of NaBr must be added to 250 g
of water to lower the vapor pressure by 1.30 mm Hg
at 40°C? The vapor pressure of water at 40°C is
55.3 mm Hg.

Chapter 11                 Slide 4
Colligative Properties of a
Mixture of Two Volatile Liquids

•   What happens if both components are volatile
(have measurable vapor pressures)?

•   The vapor pressure has a value intermediate
between the vapor pressures of the two liquids.
PT = PA + PB
= X A P ° A + X B P °B
= XAP°A + (1 – XA)P°B
PT = P°B + (P°A – P°B)XA

Chapter 11                  Slide 5
Boiling-Point Elevation and
Freezing-Point Depression

•   Boiling-Point Elevation (∆Tb): The boiling point of
the solution (Tb) minus the boiling point of the pure
solvent (T°b):
∆Tb = Tb – T°b
∆Tb is proportional to concentration:
∆Tb = Kb m
Kb = molal boiling-point elevation constant.
Also for incompletely dissociating ionic solids
∆Tb = Kb m i
Chapter 11               Slide 6
Boiling-Point Elevation and
Freezing-Point Depression
•   Freezing-Point Depression (∆Tf): The freezing point
of the pure solvent (T°f) minus the freezing point of
the solution (Tf).
∆Tf = T°f – Tf
∆Tf is proportional to concentration:

∆Tf = Kf m
Kf = molal freezing-point depression constant.
∆Tb = Kb m i
Chapter 11               Slide 7
Boiling-Point Elevation and
Freezing-Point Depression

Chapter 11       Slide 8
Boiling-Point Elevation and
Freezing-Point Depression

•   van’t Hoff Factor, i: This factor equals the number
of ions produced from each molecule of a
compound upon dissolving.
i = 1 for CH3OH                     i = 3 for CaCl2
i = 2 for NaCl                      i = 5 for Ca3(PO4)2

•   For compounds that dissociate on dissolving, use:
∆Tb = iKb m     ∆Tf = iKf m           ∆P = ix2 P°1
Chapter 11                         Slide 9
Boiling-Point Elevation and
Freezing-Point Depression

•   How many grams of ethylene glycol antifreeze,
CH2(OH)CH2(OH), must you dissolve in one liter of
water to get a freezing point of –20.0°C. The molar
mass of ethylene glycol is 62.01 g. For water, Kf =
1.86 (°C·kg)/mol. What will be the boiling point?

Chapter 11                     Slide 10
Boiling-Point Elevation and
Freezing-Point Depression

•   What is the molality of an aqueous solution of KBr
whose freezing point is –2.95°C? Kf for water is
1.86 (°C·kg)/mol.

•   What is the freezing point (in °C) of a solution
prepared by dissolving 7.40 g of K2SO4 in 110 g of
water? The value of Kf for water is 1.86
(°C·kg)/mol.            Chapter 11                   Slide 11
Osmosis and Osmotic Pressure

Chapter 11         Slide 12
Osmosis and Osmotic Pressure

•   Osmosis: The selective passage of solvent
molecules through a porous membrane from a
dilute solution to a more concentrated one.

•   Osmotic pressure (π or ∏): The pressure
required to stop osmosis.
π = iMRT
R = 0.08206 (Latm)/(molK)

Chapter 11               Slide 13
Osmosis and Osmotic Pressure

Chapter 11         Slide 14
Osmosis and Osmotic Pressure

Chapter 11         Slide 15
Osmosis and Osmotic Pressure

•   Isotonic: Solutions have equal concentration of
solute, and so equal osmotic pressure.

•   Hypertonic: Solution with higher concentration of
solute.

•   Hypotonic: Solution with lower concentration of
solute.

Chapter 11                    Slide 16
Osmosis and Osmotic Pressure

•   The average osmotic pressure of seawater is about
30.0 atm at 25°C. Calculate the molar
concentration of an aqueous solution of urea
[(NH2)2CO] that is isotonic with seawater.

•   What is the osmotic pressure (in atm) of a 0.884 M
sucrose solution at 16°C?

Chapter 11                Slide 17
•   Desalination:

Chapter 11   Slide 18

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