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Announcements & Reminders: Last day to drop without a W : This Saturday 11/23 Exam grades will be posted by TOMORROW COMPREHENSIVE FINAL EXAM: Thursday, December 19 th : 1-3:50 pm in 615 Grade Replacement
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Announcements & Reminders: Last day to drop without a W : This Saturday 11/23 Exam grades will be posted by TOMORROW COMPREHENSIVE FINAL EXAM: Thursday, December 19 th : 1-3:50 pm in 615 Grade Replacement OPTION: If you opt in, you can replace your lowest exam grade for your final exam grade: If you improve, this will help your overall grade by bringing up your lowest exam grade If you do poorly, this will hurt your overall grade Solutions & Concentration 1 Solutions A solution is a homogeneous mixture composed of a solvent and one or more solutes. A solute is a substance dissolved in the solvent. The solvent is the substance that dissolves the solute or solutes. Note: Sometimes it is not clear what is the solvent or the solute. The solvent is generally considered to be the most abundant substance. Aqueous (aq) means dissolved in water . Compounds and Ions in solution Ionic compounds often dissociate into their ions when dissolved in water. Compounds that undergo complete (100%) dissociation (like NaCl) are called strong electrolytes because their solutions are good electrical conductors. Some compounds only partially dissociate in water (like H 3 PO 4 ) and are called weak electrolytes, because their solutions are poor electrical conductors. 2 Dissolution of Sodium Chloride in Water Attraction of water molecules to ions because of the ion-dipole force. 3 Compounds and Ions in solution Covalent compounds (like glucose, C 6 H 12 O 6 ) do NOT dissociate in water and are called nonelectrolytes, because their solutions do not conduct electricity. Note: Pure water (which we rarely actually have) is a very poor electrical conductor. Sugar Cube Dissolving in Water. 4 Degrees of Saturation: Unsaturated more solute may be dissolved in a solution. Saturated the maximum amount of solute is dissolved in a solution. Supersaturated more solute is dissolved in a solution than is stable at that temperature (A precipitate is likely to form under these conditions.) 5 Comparison of Unsaturated and Saturated Solutions Molarity Concentration is the measure of the amount of solute in a solution (part / whole). Molar concentration, or molarity is a measure of the moles of a solute in one liter of solution. Molarity n V molesof solute mol solute volume of solution L solution M Brackets around a formula indicate the concentration of the substance is being discussed: [NaCl] means the molarity of NaCl 6 Calculating Concentration Example #1: What is the concentration of a solution found to contain mol of BaCl 2 in a 20.0 ml sample of solution? Example #2: What is the concentration of sodium hypochlorite solution prepared by dissolving 5.66 g of NaOCl in enough water to make ml of solution? 7 Preparing a Solution To prepare a solution of known concentration from a solid substance soluble in water: Determine the mass required to make the desired volume of the solution. Dissolve that quantity of solid in the appropriate volumetric flask. Example: An experiment calls for ml of M solution of CuSO 4. Describe how to prepare this solution starting with solid CuSO 4 5H 2 O. The calculated amount of the solid compound is massed out on an analytical balance. 8 The copper (II) sulfate pentahydrate is then transferred carefully to the volumetric flask. Water is added to bring this solution level to the mark on the neck of the 250-ml volumetric flask. 9 Dissociation of Ionic Compounds When an ionic compound like NaCl dissociates in water, one sodium ion and one chloride ion are released into solution for each formula unit: H2O NaCl NaCl (s) (aq) Na (aq) Cl (aq) What is the concentration of sodium ions and chloride ions in a solution that is M NaCl? Dissociation of Ionic Compounds When an ionic compound like MgBr 2 dissociates in water, one magnesium ion and two bromide ions are released into solution for each formula unit: H2O MgBr MgBr 2(s) 2(aq) Mg 2 (aq) 2Br (aq) What are the concentrations of Mg 2+ ions and Br ions in a 0.30 M solution of MgBr 2? What are the concentrations of iron and phosphate ions in a 0.10 M Fe 3 (PO 4 ) 2 solution? 10 Mass % Mass Percent masssolute 100% masssolution Calculate the percent by mass of magnesium chloride in a solution if 18.3 g dissolved in ml of pure water. (D H2O =1.00 g/ml) What mass of aluminum nitrate is in ml of a solution that is 7.85% aluminum nitrate? The density of the solution is g/ml. Parts per million (ppm) ppm masssolute masssolution Calculate the ppm concentration of a solution that contains 265 mg of mercury ions in 8.00 L of solution (D = 1.00 g/ml) Express the above ppm concentration as a percent by mass AND in units of molarity. 11 N-Nitrosodimethylamine (NDMA), a by-product of drinking water chlorination, is a chemical that is highly toxic and is a suspected human carcinogen. The EPA has determined that the maximum admissible concentration of NDMA in drinking water is 7 ng/l. Tap water analysis in Upper San Leandro/Oakland Area has found concentrations of NDMA as high as ppb (parts per billion). Is this within the EPA limit of 7 ng/l? Assume the density of tap water is 1.00 g/ml. Recall: nano =
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