solutions suroviec spring 2014 chapter 12. i. types of solution most chemical reaction take place...
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Solutionssuroviec
Spring 2014
Chapter 12
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I. Types of Solution
Most chemical reaction take place between ions/molecules dissolved in water or a solvent.
Solution: homogenous mixture of 2 or more substances in a single phase
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II. Molecular view of solution process
Liquids and solids held together by intermolecular attractions
Looking at specifically Solvent-solvent Solvent - solute Solute – solute
3 steps Separate solvent molecules
Separate solute molecules
Mix together
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Ex. Dissolve CaCl2 (s) in H2O
What happens to the concentration of Ca2+ and Cl-?
Saturated: stable solution in which the maximum amount of the solute has been dissolved
Equilibrium in solution
Solubility: concentration of solute in equilibrium with undissolved solute in a saturated solution
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Examples:
CCl4 and CH4NaCl and CCl4
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III. Concentration
A. Molarity: (M) Moles of solute / liter of solution
Does not tell us the EXACT amount of volume to use
B. Molality: (m)Mole of solute / mass of solvent (kg)
Cannot be the same measure of a solution
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Example
The concentration of a K2CrO4 solution is undetermined, but it was prepared from 38.3 g of K2CrO4 (MW 19.4 g/mol) and 1.00 kg of water. What is the molality?
Calculate the molality of 1 L of 1.22M sucrose (MW 342.3 g/mol) solution. The density of the solution is 1.12 g/mL.
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C. Mole Fraction
Amount of the component we are interested divided by the total amount of all the components of the mixture
Mole fraction of A (XA) =
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example
Consider a solution that has 1.00 mole ethanol (46.1 g) dissolved in 9.00 moles of water (162 g). Find the mole fraction of each.
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D. Mass Percent
Common unit of measurement for consumer products
Mass of one component divided by the total mass of the mixture multiplied by 100%
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Example
Consider an alcohol-water mixture that is prepared by dissolving 46.1 g of ethanol in 116.2 g of water. What is the mass percent of ethanol in the mixture?
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IV. Pressure, Temperature and Solubility
A. Solid solubility and Temperature When a solid is dissolved an
equilibrium is established An increase in temp will shift the
equilibrium
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B. Gas Solubility
Solubility of gasses decreases with increasing temperature
Why do fish go to bottom of lake in the summer?
Gases that dissolve to an appreciable extent in water do so in an exothermic process
So what do you need to do to get the gas out of the water?
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V. Pressure and Solubility
A. Henry’s Law• Solubility of a gas in a liquid is
directly proportional to the gas pressure• Henry’s Law• cg = kHPg
cg = molar concentration (mol/L)
kH = Henry’s law constant (mol/L · atm)
Pg =pressure of a gaseous solute (atm)
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Example Deep-sea diving
Solubility of gases is the concentration of gases in equilibrium with substances in gaseous state
What would happen if we increased the pressure?
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VI. Colligative Properties
Properties of a solution that depend only in the number of solute particles per solvent molecule and not the nature of the solute or solvent
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A. Raoult’s Law
Equilibrium vapor pressure: pressure of the vapor when the liquid and the vapor are in equilibrium
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Example
Solution contains 82.0 g of ethylene glycol in 100g of water at 22oC. (where is vapor pressure of water is 19.88 mm Hg) what is the new vapor pressure?
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B. Boiling Point Elevation
You have a solution of 0.200 mole of sugar in 100.0 g of benzene
The normal vapor pressure of pure benzene is 400 mmHg, what is the new vapor pressure of this solution?
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This vapor pressure lowering causes a BP elevation!
Normal BP is the temp at which vapor pressure =760 mm Hg
BP elevation
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B. BP elevation
Leads to general equation:
The melting point of water is 0oC at 1atm. If 12.15 g of ZnSO4 (MW 161.5 g/mol) are dissolved into 161.2 g of water: What is the molality of the solution What is the new melting point of the solution?
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C. Freezing Point Depression
Freezing point of a solution will be lower than of a pure solvent
General equation
Why add antifreeze to your car?
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D. Osmosis
Movement of solvent molecules through a semi-permeable membrane from a region of lower concentration to a region of higher concentration
Movement continues until equilibrium is reached
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D. Osmosis Over time water flows from region of low solute concentration to region of high solute concentration.
Flow continues until pressure exerted by column of solution in tube above water level is great enough to equal flow in and out of bag
This is measure of osmotic pressure