Exam 3: Mon, Nov. 7, 6:30 7:45 pm

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1 Fall 2011 Chem 110 Exam III Monday, November 7, 6:30 pm 7:45 pm Data sheet and periodic table will be provided. Conflicts: For legitimate PSU conflicts, sign up for a conflict exam during your lecture. Work, sports, extracurriculars, etc. do not qualify for the conflict exam. Location: My Section: My TA: My Room: My PSU student ID number: Bring: #2 pencils; PSU photo ID, Non-text programmable calculator. Do NOT bring: Cell phones, PDA's, or any other electronic devices Material Covered: Lectures 22 33, ALEKS Objectives 8 11 Format: 28 Multiple choice questions. Some calculations, some concept questions. A 100% Score earns 34 points out of 200 course points (17% of final grade). Advice: Follow suggestions about how to prepare for exam (posted on Angel -> Lessons -> Learning tools). Do the sample practice exams on Angel; select handouts Do the online practice exams on Angel Do all homework problems especially the blue packet! Go to the review sessions and office hours! Week 11: Lectures Lecture 31: W 11/2 Lecture 32: F 11/4 Lecture 33: M 11/7 (Exam Review) Reading: BLB Ch Homework: BLB 13: 9, 58, 61, 69, 75; Supp 13: Reminder: Angel Quiz 10 due on Thur 11/3 ALEKS Objective 11 due on Sat 11/5 Angel Quiz 11 due on Sun 11/6 Jensen Office Hour: 501 Chemistry Building Tuesdays & Thursdays, 10:30 11:30 am Exam 3: Mon, Nov. 7, 6:30 7:45 pm Jensen Chem 110 Chap 13 Page: 2

2 Colligative Properties: Properties that depend only on the total number (concentration) of the solute particles, but not on their identity! 0.2 M aqueous solution of Pb(NO 3 ) 2 Is it an electrolyte? What ions or molecules (solute particles) are present in this solution? total concentration of solute particles =! 0.2 M aqueous solution of HF Is it an electrolyte? What ions or molecules (solute particles) are present in this solution? total concentration of solute particles =! 0.2 M solution of C 6 H 12 O 6 (glucose) Is it an electrolyte? What ions or molecules (solute particles) are present in this solution? total concentration of solute particles = Examples of Colligative Properties vapor pressure lowering Raoult!s law: P A = X A P A P A! vapor pressure of solution X A! mole fraction of solvent P A! vapor pressure of pure solvent boiling point elevation "T b = K b m K b! molal boiling point elevation constant m! total molality of all solute particles freezing point depression "T f = K f m K f! molal freezing point depression constant m! total molality of all solute particles osmotic pressure M! total molarity of all solute particles Jensen Chem 110 Chap 13 Page: 3 Jensen Chem 110 Chap 13 Page: 4

3 Vapor Pressure Lowing Addition of a nonvolatile solute lowers the vapor pressure Example: Calculate the vapor pressure at 25 C of a 40.0% by mass solution of ordinary cane sugar (C 12 H 22 O 11, a nonvolatile non-electrolyte with MW = 342g/mol) in water. The vapor pressure of pure water at 25 C is 23.8 torr. pure solvent solvent + solute Vapor pressure lowering is a colligative property that depends on total concentration of all solute particles, but not on their identity Raoult!s Law: P A = X A P A P A! vapor pressure of solution P A! vapor pressure of pure solvent X A! mole fraction of solvent Jensen Chem 110 Chap 13 Page: 5 Jensen Chem 110 Chap 13 Page: 6

4 Phase Diagrams for a pure solvent and for a solution of a nonvolatile solute Boiling Point Elevation & Freezing Point Depression Boiling point increase: "T b = K b m Boiling point of solution = T b,solvent + "T b K b : molal boiling point elevation constant Freezing point decrease: "T f = K f m Freezing point of solution = T b,solvent - "T f K f : molal freezing point depression constant! m! total molality of all solute particles in the solution How is melting point affected by the solute particles? How is boiling point affected by the solute particles?! K b and K f are tabulated for different solvents Eg. water K b = 0.51 C/m; K f = 1.86 C/m; benzene K b = 2.53 C/m K f = 5.12 C/m Jensen Chem 110 Chap 13 Page: 7 Jensen Chem 110 Chap 13 Page: 8

5 Daniel Fahrenheit Fahrenheit prepared a saturated ammonium chloride solution called a Frigorific Mixture (a solution that stabilizes at a fixed temperature). The mixture was 20.4% NH 4 Cl by weight. What was the freezing point of the mixture? The freezing point depression constant for H 2 O is 1.86 C/m. Practice Example: Ethanol normally boils at 78.4 C. The boiling point elevation constant for ethanol is 1.22 C/m. What is the boiling point of a 1.00 m solution of CaCl 2 in ethanol? A C B C C C D C E C Jensen Chem 110 Chap 13 Page: 9 Jensen Chem 110 Chap 13 Page: 10

6 Practice Example: A solution contains 10.0 g of an unknown liquid and 90.0 g water, and has a freezing point of 3.33 C. Assuming that the solute is a non-electrolyte, what is the molar mass of the unknown liquid? The freezing point depression constant for water is 1.86 C/m. A g/mol B. 333 g/mol C. 619 g/mol D. 161 g/mol E g/mol Osmosis: Flow of molecules through a semipermeable membrane NET movement of solvent is toward solution with higher solute concentration; movement continues until osmotic pressure builds up to stop it. Osmotic pressure (#): pressure needed to stop osmotic flow of a molecule through a membrane # is osmotic pressure (units of atm) R is gas constant in (L atm)/(mol K) T is temperature in K M is concentration in molarity (mol/l) Jensen Chem 110 Chap 13 Page: 11 Jensen Chem 110 Chap 13 Page: 12

7 Example: What is the osmotic pressure of a M solution of a non-electrolyte at 20 C? A atm B atm C atm D atm E atm Practice Example: Calculate the molecular weight of a small protein (a non-electrolyte) if a 200 mg sample dissolved in 100 ml of water has an osmotic pressure of 9.8 mmhg at 25 C. A. 3.5 x 10 3 g/mol B. 3.8 x 10 3 g/mol C. 4.0 x 10 4 g/mol D. 4.5 x 10 4 g/mol E. 5.5 x 10 3 g/mol Jensen Chem 110 Chap 13 Page: 13 Jensen Chem 110 Chap 13 Page: 14

8 Colligative properties Review: Arrange the aqueous solutions according to increasing boiling point. Colloids particle size i m Na 2 SO 4 ii m AlCl 3 iii m KNO 3 Small true solution colloidal dispersion Large heterogeneous mixture A. i < ii < iii B. i < iii < ii uniform molecules uniform Å particles non-uniform sedimentary particles C. ii < iii < i D. iii < ii < i E. iii < i < ii NaCl in H 2 O milk (fat particles) fog (water droplets) silt in river According to increasing freezing point? According to increasing vapor pressure? Jensen Chem 110 Chap 13 Page: 15 Jensen Chem 110 Chap 13 Page: 16

9 Light Scattering Hydrophilic vs. hydrophobic colloids Tyndall effect: light is scattered when $ of light % particle size For molecules (~5 Å), $ in x-ray region: visible light passes through solutions without scattering hydrophilic: water-loving hydrophobic: water-fearing (water-soluble) proteins: hydrophobic core with hydrophilic surface For colloids (~200 nm), $ in visible region: visible light passes through colloidal suspensions with scattering (milk, fog) dust in atmosphere scatters smaller $ light, explains why the sky is blue detergents: hydrophobic tail with hydrophilic head Jensen Chem 110 Chap 13 Page: 17 Jensen Chem 110 Chap 13 Page: 18