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Home , Molality, Molar concentration, Osmotic pressure, Solvation

Chapter 13

Solutions Types of • A is a homogenous of 2 or more substances • The solute is(are) the substance(s) present in the smaller amount(s) • The is the substance present in the larger amount A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific . An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature. A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature. Crystallization is the process in which dissolved solute comes out of solution and forms crystals

Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate. A Molecular View of the Solution Process Three types of interactions in the solution process: • solvent-solvent interaction • solute-solute interaction • solvent-solute interaction

DHsoln = DH1 + DH2 + DH3 A Molecular View of the Solution Process

DHsoln = DH1 + DH2 + DH3 • If the solute-solvent attraction is > the solute-solute and solvent-solvent attractions, then the solution process is favorable, or EXOTHERMIC (DHsoln < 0). • If the solute-solvent interaction is weaker than the solvent-solvent and solute-solute interactions, then the solution process is ENDOTHERMIC (DHsoln > 0). • Solution process is governed by 2 main factors: 1. Energy – determines if a solution process is endothermic or exothermic 2. Inherent tendency toward disorder in all natural events () – increase in randomness or disorder when solute and solvent mix to form a solution * It is the increase in disorder of the system that favors the of any substance, even if the solution process is endothermic Solubility – “Like Dissolves Like”

 Solubility is a measure of how much solute will dissolve in a solvent at a specific temperature. Two substances with similar intermolecular forces are likely to be soluble in each other. • non-polar molecules are soluble in non-polar

CCl4 in C6H6 • polar molecules are soluble in polar solvents

C2H5OH in H2O • ionic compounds are more soluble in polar solvents

NaCl in H2O or NH3 (l) • Two liquids are said to be Miscible if they are completely soluble in each other in all proportions. • is the process in which an or a is surrounded by solvent molecules arranged in a specific manner. Is molecular iodine more soluble in or in carbon disulfide?

Carbon disulfide – a nonpolar solvent

“like dissolves like”: nonpolar iodine will be soluble in nonpolar carbon disulfide Units The concentration of a solution is the amount of solute present in a given quantity of solvent or solution.

Percent by Independent of temperature mass of solute % by mass = x 100% mass of solute + mass of solvent mass of solute = x 100% mass of solution

Mole Fraction (X) moles of A X = A sum of moles of all components Concentration Units Molarity (M)

moles of solute M = liters of solution

Molality (m) Independent of temperature

moles of solute m = mass of solvent (kg) What is the of a 5.86 M (C2H5OH) solution whose is 0.927 g/mL? moles of solute moles of solute m = M = mass of solvent (kg) liters of solution

Assume 1 L of solution: 5.86 moles ethanol = 270 g ethanol 927 g of solution (1000 mL x 0.927 g/mL) mass of solvent = mass of solution – mass of solute = 927 g – 270 g = 657 g = 0.657 kg

moles of solute 5.86 moles C2H5OH m = = = 8.92 m mass of solvent (kg) 0.657 kg solvent The Effect of Temperature on Solubility solubility and temperature

solubility increases with increasing temperature solubility decreases with increasing temperature Fractional crystallization is the separation of a mixture of substances into pure components on the basis of their differing .

Suppose you have 90 g KNO3 contaminated with 10 g NaCl.

Fractional crystallization: 1. Dissolve sample in 100 mL of water at 600C 2. Cool solution to 00C 3. All NaCl will stay in solution (s = 34.2g/100g)

4. 78 g of PURE KNO3 will precipitate (s = 12 g/100g). 90 g – 12 g = 78 g Temperature and Solubility

O2 gas solubility and temperature

solubility usually decreases with increasing temperature The Effect of on the Solubility of Gases The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law).

c is the concentration (M) of the dissolved gas c = kP P is the pressure of the gas over the solution k is a constant for each gas (mol/L•atm) that depends only on temperature

low P high P

low c high c Pressure & Solubility of Gases

 Each gas has a different k value at a given temperature  The amount of gas that will dissolve in a solvent depends on how frequently the gas molecules collide with the liquid surface and become trapped by the condensed phase  Practical application: effervescence of a soft drink when the bottle cap is removed Calculate the of oxygen in water at 25C for a of 0.22 atm. The Henry’s law constant for oxygen is 1.3 x 10-3 mol/Latm. c=kP c = (1.3 x 10-3 mol/Latm)(0.22 atm) c = 2.9 x 10-4 mol/L

How is oxygen highly soluble in blood when only sparingly soluble in water? of Nonelectrolyte Solutions

• Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles. 1. Vapor-pressure lowering 2. Boiling-point elevation 3. Freezing-point depression 4. • Dealing with Dilute solutions (< or = 0.2 M)

Colligative Properties 1. Vapor-Pressure Lowering • If a solute is nonvolatile (does not have a measurable vapor pressure), then the vapor pressure of its solution is always less than that of the pure solvent • Concentration of solute in solution determines relationship between vapor pressure of solution and vapor pressure of solvent

0 P 0 = vapor pressure of pure solvent P1 = X1 P 1 1

Raoult’s law X1 = fraction of the solvent If the solution contains only one solute:

X1 = 1 – X2 0 0 P 1 - P1 = DP = X2 P 1 X2 = of the solute

Vapor-Pressure Lowering

 Vapor pressure of a solution is lower than that of pure solvent…Why? – Increasing disorder in a solution – Solvent molecules have less of a tendency to leave a solution than to leave the pure solvent to become vapor  Volatile = has measurable vapor pressure – If both components of a solution are volatile, then the vapor pressure of the solution is the sum of the individual partial = any solution that obeys Raoult’s Law

– Characteristics: Heat of solution, DHsoln = 0; are additive

An Ideal Solution

0 PA = XA P A 0 PB = XB P B

PT = PA + PB 0 0 PT = XA P A + XB P B PT is greater than  Intermolecular forces predicted by Raoults’s law between A & B molecules are weaker than those between A molecules & B molecules  Greater tendency for these molecules to leave the solution  Vapor pressure of solution > sum of vapor pressures predicted by Raoult’s Law

Force Force Force  Positive deviation < & A-B A-A B-B  Heat of solution is positive (endothermic)  A molecules attract B PT is less than molecules more predicted by Raoults’s law strongly than they do their own  Vapor pressure of solution is < sum of vapor pressures predicted by Raoult’s Law  Negative deviation  Heat of solution is Force Force Force negative > & (exothermic) A-B A-A B-B Fractional Distillation  Solution vapor pressure has direct bearing on Apparatus fractional distillation (procedure for separating liquid components of a solution based on different boiling points)  At each step, the composition of the vapor in the column will be richer in the more volatile (lower bp) component  The vapor that rises to the top of the column condenses & is collected in a receiving flask Colligative Properties 2. Boiling-Point Elevation DT = T – T 0 • Boiling-point elevation = boiling b b b point of the solution – boiling point of T 0 is the boiling point of pure solvent b the pure solvent •Solute must be nonvolatile T b is the boiling point of the solution 0 Tb > T b DTb > 0

DTb = Kb m

m is the molality of the solution

Kb is the molal boiling-point elevation constant (0C/m) for a given solvent (Table 12.2) Colligative Properties 3. Freezing-Point Depression 0 DTf = T f – Tf • Freezing-point depression = freezing T 0 is the freezing point of point of the pure solvent – freezing point f of the solution the pure solvent T f is the freezing point of the solution 0 T f > Tf DTf > 0

DTf = Kf m

m is the molality of the solution

Kf is the molal freezing-point depression constant (0C/m) for a given solvent (Table 12.2)

Calculate the freezing point and boiling point of a solution containing 478 g of ethylene glycol in 3202 g water.

DTf = Kf m 1 mol EG 478 g EG   7.70 mol EG 62.07g EG moles of solute 7.70 mol EG m    2.41 m kg solvent 3.202 kg water o o DTf  K f m  (1.86 C / m)(2.41m)  4.48 C Because pure water freezes at 0o C, the solution will freeze at - 4.48o C

DTb = Kb m

o o DTb  Kbm  (0.52 C / m)(2.41m) 1.25 C The solution will boil at (100  1.25)o C or 101.25o C. Colligative Properties

4. Osmotic Pressure (p) is the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one. A allows the passage of solvent molecules but blocks the passage of solute molecules. Osmotic pressure (p) is the pressure required to stop osmosis.

more dilute concentrated Osmotic Pressure (p)

High Low P P

p = MRT

M is the molarity of the solution R is the (0.0821 Latm/molK) T is the temperature (in K) Osmotic Pressure

 Directly proportional to the concentration of a solution  Solution 1. Isotonic – if 2 solutions are of equal concentration & have same osmotic pressure 2. Hypertonic – if 2 solutions are of unequal osmotic pressures, the more concentrated solution is hypertonic 3. Hypotonic – the more dilute solution if unequal osmotic pressures A in an:

Crenation

isotonic hypotonic hypertonic solution solution solution Colligative Properties of Nonelectrolyte Solutions

Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles. 0 Vapor-Pressure Lowering P1 = X1 P 1

Boiling-Point Elevation DTb = Kb m

Freezing-Point Depression DTf = Kf m

Osmotic Pressure (p) p = MRT What is the osmotic pressure (in atm) of a 0.884 M solution at 16C?

p= MRT p= (0.884M)(0.0821 Latm/molK)(289 K) p= 21.0 atm A solution of 0.85 g of an organic compound in 100.0 g of benzene has a freezing point of 5.16C. What are the molality of the solution and of the solute?

DTf (5.5  5.16) molality   o  0.066m Kf 5.12 C / m Moles of solute in 100.0 g benzene: 0.066 mol 0.1 kg benzene   0.0066 mol 1 kg benzene grams of compound 0.85 g molar mass   1.3102 g/mol moles of compound 0.0066 mol A 202-mL benzene solution containing 2.47 g of an organic polymer has an osmotic pressure of 8.63 mmHg at 21C. Calculate the molar mass of the polymer. p= MRT 1 atm 8.63 mmHg p 760 mmHg M    4.70104 M RT (0.0821 Latm/mol K)(294 K)

Moles polymer  M  V  (4.7010-4 mol/L)(0.202 L)  9.50 10-5 mol

grams of polymer 2.47 g Molar mass    2.60104 g/mol moles of polymer 9.5010-5 mol Colligative Properties of Solutions

0.1 m NaCl solution 0.1 m Na+ & 0.1 m Cl- ions Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.

0.1 m NaCl solution 0.2 m ions in solution

actual number of particles in soln after van’t Hoff factor (i) = number of formula units initially dissolved in soln

i should be nonelectrolytes 1 NaCl 2

CaCl2 3 Colligative Properties of Electrolyte Solutions

• An Ion pair is made up of 1 or more cations and 1 or more anions held together by electrostatic forces. • The presence of an ion pair reduces the # of particles in solution, causing a reduction in colligative properties • containing multicharged ions are more likely to form ion pairs

Boiling-Point Elevation DTb = i Kb m

Freezing-Point Depression DTf = i Kf m Osmotic Pressure (p) p = iMRT The freezing-point depression of a 0.100m magnesium sulfate solution is 0.225C.

Calculate the van’t Hoff factor of MgSO4 at this concentration.

DTf = i Kf m

o DTf 0.225 C i   o 1.21 K f m (1.86 C / m)(0.100m) A is a dispersion of particles of one substance throughout a dispersing medium of another substance.

Colloid versus solution • collodial particles are much larger than solute molecules • collodial is not as homogeneous as a solution Colloids

 Tyndall effect – way to distinguish a solution from a colloid – Dispersed phase of a colloid scatters light; solute molecules in solution do not (too small)  Hydrophilic & Hydrophobic Colloids – Hydrophilic = water-loving  Usually contain extremely large molecules (i.e. proteins) – Hydrophobic = water-fearing  Normally, not stable in water but can be stabilized by of ions on their surface  Adsorbed ions can interact with water, thus stabilizing the colloid

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