Chem 110 06 SOLUTIONS

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Chem 110 06 SOLUTIONS


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Chapter 06 Solutions

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Properties of Solutions Types of Solutions based on their capacity to dissolve a given solvent Saturated – a solution that contains the maximum amount of a solute in a given solvent, at a specific temperature. Unsaturated – a solution that contains less solute that it has the capacity to dissolve. Supersaturated – a solution that contains more solute than is present in a saturated solution. Crystallization – the process in which dissolved solute comes out of solution and forms crystals.

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Properties of Solutions Solubility – is a measure of the amount of a solute that will dissolve in a solvent at a specific temperature. “like dissolves like” CCl4 (nonpolar) is miscible with C6H6 (nonpolar) CH3COOH (polar) is miscible with H2O (polar) NaCl (ionic) is soluble with H2O (polar) but not in oil (nonpolar) Solvation – is the process in which an ion or a molecule is surrounded by solvent molecules arranged in a specific manner. - When the solvent is H2O, the process is called Hydration.

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Example 13.1 Predict the relative solubility's in the following cases: (a) Bromine (Br2) in benzene (C6H6, μ = 0 D ) and in water (μ = 1.87 D) Br2 is a nonpolar molecule and therefore should be more soluble in C6H6 which is also nonpolar, than in water. The only intermolecular forces between Br2 and C6H6 are dispersion forces. Predicting Relative Solubilities

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Example 13.1 Predict the relative solubility's in the following cases: (b) KCl in carbon tetrachloride (CCl4, μ= 0 D) and in liquid ammonia (NH3, μ = 1.46 D) (b) KCl is an ionic compound. For it to dissolve, the individucal K+ and Cl- ions must be stabilized by ion-dipole interaction. Because CCl4 has no dipole moment, KCl should be more soluble in liquid NH3, a polar molecule with a large dipole moment. Predicting Relative Solubilities

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Example 13.1 Predict the relative solubility's in the following cases: (c) formaldehyde (CH2O) in carbon disulfide (CS2, μ = 0) and in water. (c) Because CH2O is a polar molecule and CS2 (a linear molecule) is nonpolar, the forces between molecules of CH2O and CS2 are dipole-induced dipole and dispersion. On the other hand, CH2O can form hydrogen bonds with water, so it should be more soluble in that solvent. Predicting Relative Solubilities

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Example 13.2 Calculate the molality of a sulfuric acid solution containing 24.4 g of sulfuric acid in 198 g of water. The molar mass of sulfuric acid is 98.08 g/mol. 1.26 m Calculating molality

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Concentration Units Percent by mass (Percent by weight or weight percent) Molarity Molarity

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Example 13.3 The density of a 2.45 M aqueous solution of methanol (CH3OH) is 0.976 g/mL. What is the molality of the solution? The molar mass of methanol is 32.04 g/mol. 2.73 m Molality and density

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Example 13.4 Calculate the molality of a 35.4 percent (by mass) aqueous solution of phosphoric acid (H3PO4). The molar mass of phosphoric acid is 98.00 g/mol. 5.59 m Percent by mass calculations

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Effect of Temperature on Solubility Solid Solubility and Temperature In most cases, solubility of solids increases as temperature also increases Gas Solubility and Temperature Solubility of gases decreases as temperature increases - Thermal pollution - Fishing

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Henry’s Law states that “… the solubility of a gas in a liquid is proportional to the pressure of the gas over the solution..” The solubility of gases increases as the pressure increases c = molar concentration (moles per liter) of the dissolved gas P = is the pressure (in atmospheres) k = is a constant that depends only on temperature Effect of Pressure on Solubility of Gases

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Example 13.5 The solubility of nitrogen gas at 25°C and 1 atm is 6.8 x 10 -4 mol/L. What is the concentration of nitrogen dissolved in water under atmospheric conditions? The partial pressure of nitrogen gas in the atmosphere is 0.78 atm. 5.3 x 10 -4 M Henry’s Law

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Colligative Properties Colligative Properties – properties of solutions which depend on the number of solutes and not the nature of the solute. Vapor-Pressure Lowering Roult’s Law states “…The vapor pressure of an ideal solution is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution”

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Colligative Properties Colligative Properties – are solution properties that depend on the concentration of the solute particles, rather than the identity of the solute. Colligative Properties of Solution Vapor Pressure Lowering Freezing Point Depression Boiling Point Elevation Osmotic Pressure

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Colligative Properties Vapor Pressure Lowering Raoult’s Law states that “when a nonvolatile solute is added to a solvent, the vapor pressure of the solvent decreases in proportion to the concentration of the solute.”

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Colligative Properties Freezing Point Depression When a nonvolatile solute is added to a solvent, the freezing point of the resulting solution decreases ( a lower temperature is required to convert the liquid to a solid) Boiling Point Elevation When a nonvolatile solute is added to a solvent, the boiling point of the resulting solution increases (a higher temperature is required to convert the liquid to a gas)

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Colligative Properties Osmotic Pressure Semipermeable membranes – allow the solvent, but not the solute, to diffuse from one side of the membrane to the other. (cellophane, cell membranes ,etc..) Osmosis – movement of solvent from a less concentrated to a more concentrated solution through a semipermeable membrane. Osmotic Pressure – is the amount of pressure required to stop the flow of solvent from a lesser concentration to a higher concentration.

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Osmosis Diffusion of water (solvent) through a semipermeable membrane from less conc’d to a more conc’d sol’n. Prunes (more conc’d) put in water (less conc’d) will swell. Cucumber (less conc’d) placed in a concentrated salt solution (more conc’d), Cucumber shrinks and becomes pickle. Why do sailors (less conc’d) die of dehydration when they drink salt water (more conc’d)? Osmosis

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Isotonic Solutions 2 solutions that have the same solute conc’n Solutions isotonic with blood 0.9% NaCl sol’n (physiologic saline solution) 5.5 % glucose solution (dextrose) PSS can be administered under the ff conditions: Dehydrated Lost considerable fluid (hemorrhage) To prevent postoperative shock Isotonic Solutions

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Hypotonic Solutions A sol’n that contains a lower solute concentration than that of another sol’n Solutions hypotonic with blood Distilled water Tap water The rbc will burst (hemolysis) Not usually used for blood transfusions Hypotonic Solutions

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Hypertonic Solutions A sol’n that contains a higher solute concentration than that of another sol’n Solutions hypertonic with blood 5 % NaCl solution 10% C6H12O6 solution The rbc will shrink (plasmolysis) Saline cathartics (a) MgSO4, (b) milk of magnesia, and (c) magnesium citrate  laxatives solute conc’n in (hypertonic), water will diffuse  (watery stool) Caution: dehydration Used to rid the body of excess fluid  Diuretics Hypertonic Solutions

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Example 13.6 Calculate the vapor pressure of a solution made by dissolving 218 g of glucose (molar mass = 180.2 g/mol) in 460 mL of water at 30°C. What is the vapor-pressure lowering? The vapor pressure of pure water at 30°C is given table 5.2. Assume the density of the solution is 1.00 g/mL 1.4 mmHg Vapor Pressure Calculation

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Example 13.7 Ethylene glycol (EG), CH2(OH)CH2(OH), is a common automobile antifreeze. It is water soluble and fairly nonvolatile (b.p. 197°C). Calculate the freezing point of a solution containing 651 g of this substance in 2505 g of water. Would you keep this substance in your car radiator during the summer? The molar mass of ethylene glycol is 62.01 g/mol. (Kf H2O = 1.86°C/m; kbH2O = 0.513°C/m) Because the solution will boil at 102.2°C it would be preferable to leave the antifreeze in your car radiator in summer in summer to prevent the solution from boiling. Freezing point depression calculations

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Example 13.8 A 7.85-g sample of a compound with the empirical formula C5H4 is dissolved in 301 g of benzene. The freezing point of the solution is 1.05°C below that of pure benzene. What are the molar mass and molecular formula of this compound? (Kf = 5.07°C/m) Molar mass = 127 g/mol Molecular formula = C10H8 Molar Mass from Freezing Point Depression

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Example 13.9 A solution is prepared by dissolving 35.0 g of hemoglobin (Hb) in enough water to make up 1 L in volume. If the osmotic pressure of the solution is found to be 10.0 mmHg at 25°C, calculate the molar mass of hemoglobin. 6.51 x 104 g/mol Calculating Molar Mass from Osmotic Pressure

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Colligative properties depend on the amount of solute particles in the solution. 1 particle of the non-electrolyte sugar molecule will produce 1 sugar particle dissolved 1 particle of the electrolyle sodium chloride molecule will produce 2 particles (1 sodium ion + 1 chloride ion) Van’t Hoff Factor, i - Ratio of moles of solute particles to moles of formula units dissolved Sugar, i = 1 Sodium chloride, i = 2 - Measured van’t Hoff factors are often lower than you might expect due to ion pairing in solution Colligative Properties of Electrolytes

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Example 13.10 The osmotic pressure of a 0.010 M potassium iodide (KI) solution at 25°C is 0.465 atm. Calculate the Van’t Hoff factor for KI at this concentration. i = 1.90 Van’t Hoff Factor

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