Chapter 13. Solutions Solutions are homogeneous mixtures of two or more substances. Each substance retains its chemical identity. The particles are of atomic or molecular size. Solutions Solutions can be solids, liquids, or gases. Solid solutions: Metal alloys Liquid solutions: Numerous Gaseous solutions: Air! Is it a solution? Liquid and gaseous solutions are transparent (not necessarily colorless). Solutions Definitions: Solvent: the major component, in which others are dissolved. Solute(s): minor component(s), active ingredients in a solution. Solution: Solvent + Solute(s) Solubility Solubility is the maximum amount of solute that will dissolve in a solvent. What dissolves in what? Like Dissolves Like Polar substances dissolve in polar solvents. Nonpolar substances dissolve in nonpolar solvents. Solubility Water is a very polar solvent, and will dissolve some, but not all, ionic compounds. Ionic compounds are less likely to dissolve in water if: a. the ions have charges other than +/- 1. b. The bonds are really polar covalent. Rate of Solution Formation Small particles of solute dissolve faster than large particles. Stirring makes the process go faster. Getting the solvent hot makes the process go faster. How Much Solute Will Dissolve? Increasing the temperature usually increases the solubility of a compound. How Much Solute Will Dissolve? High pressure increases the solubility of gases. (Henry’s Law) There is usually an optimal temperature for solubility of gases. How Much Solute Will Dissolve? When dissolved gases come out of solution, their volume expands drastically! This occurs when the pressure is released. D:\ExtremeDietPepsiandM.gvp How Much Solute Will Dissolve? How Much Solute Will Dissolve? A saturated solution contains the maximum amount solute that can dissolve in the solvent under specified conditions. Undissolved solute may be present. An unsaturated solution does not contain the maximum amount of solute that can dissolve in the solvent under specified conditions. How Much Solute Will Dissolve? The concentration of a solution is a ratio of the amount of solute per amount of solvent, or per amount of solution. Solute Solution Solute Solvent Concentration: Percent of Solute Percent by mass (mass %, % w/w) % by mass = Mass of Solute x 100% Mass of Solution Percent by volume (volume %, % v/v) % by volume = Volume of Solute x 100% Volume of Solution Concentration: Percent of Solute Problems: What is the concentration of a solution containing 0.290 g of H2S in 75.00 g of water? (V-1) How many grams of glucose are present in 75 g of a solution that is 10.% (m/m) glucose? How many grams of iodine (I2) must be added to 25.0 g of alcohol to make a solution that is 5.00% by mass? Concentration: Percent of Solute Problems: How many milliters of isopropanol (C3H8O) are present in 500. mL of a 70.0% (v/v) solution in water? What volume of water must be added to 25.0 mL of H2SO4 to make 100.0 mL of 25.0% (v/v) solution? Concentration: Percent of Solute When two different liquids are combined, volumes are not additive. Volumetric flasks are filled to the 50.0 mL mark with ethanol and with water. The liquids are combined in a 100 mL volumetric flask, and the volume is 96.5 mL. Concentration: Percent of Solute Percent by mass-volume (% m/v) % m/v = Mass of Solute (g) x 100% Volume of Solution (mL) Since units do not cancel, they must be specified. % m/v is convenient for preparing solutions of solids in liquids, but not as versatile as other percent concentrations. Concentration: ppm and ppb ppm = Parts per Million ppb = Parts per Billion (m/m), (v/v) or (m/v) ppm (m/m) = Mass of Solute x 106 Mass of Solution ppb (v/v) = Volume of Solute x 109 Volume of Solution Concentration: mg/dL mg/dL = milligrams per deciliter 1 mg = 0.001 g 1 dL = 0.1 L = 100 mL 1 mg/dL = 1 mg solute . 100 mL of solution Frequent in medical reports; it's unambiguous about units. Concentration: Molarity Molarity = Moles of Solute Liters of Solution Abbreviated M, or mol/L [solute], molarity of solute, in equations The concentration used by chemists. It allows one to convert from moles to volume. mol L L mol Concentration: Molarity Volume x Molarity = Moles How many moles of NaCl are present in 100. mL of 0.500 M solution? Concentration: Molarity Problems: What is the molarity of 1.00 L of solution that contains 25.0 g of NaCl? (V-1) What is the molarity of a solution if 2.0 g of glucose is dissolved to give 15 mL of solution? How many moles of glucose are present in 3.0 mL of the solution? How many milliters of solution would be needed to obtain 0.0065 moles of glucose? Concentration Conversions Sometimes one needs to know the concentration of a solution in units other than those on the container. Often, conversions can be made if one knows the density of the solution. Concentration Conversions A saturated solution of NaOH is 50.% by mass. Its density is 1.52 g/mL What is the molarity of the solution? A what is the mass percent of glucose in a 0.10 M solution? The density of the solution is 1.0 g/mL, and the molar mass of glucose is 180. g/mol. What is the molarity of pure water? Dilution of Solutions Dilution is the process of adding solvent to a known volume of solution to reduce its concentration. It is often useful to keep a stock (concentrated) solution in the laboratory. It can be diluted to lower concentrations as needed. Dilution calculations are simple: C1V1 = C2V2 Dilution of Solutions If I take 15 mL of that CuSO4 solution I made, and dilute it to 100. mL with water… Can I tell by looking that I made a difference? What is the concentration of the dilute solution, in molarity? in mass percent? (V-2) Dilution of Solutions Problems: The most concentrated form of HCl(aq) is 12.0 M. How much of this solution is required to prepare 500. mL of 3.0 M HCl? What is the concentration of a solution made by diluting 25.0 mL of 50.% (w/w) NaOH to 200 mL? How many milliters of 3.0 M HNO3 can be prepared from 15.0 mL of 17 M HNO3? Solution Stoichiometry One can use molarities in theoretical yield type calculations like molar masses. If all solutions have the same molarity, combining volumes will match reaction coefficients! 2 AgNO3(aq) + K2CrO4(aq) Ag2CrO4(s) + 2 KNO3(aq) If we start with 5.0 mL of 0.10 M AgNO3, what volume of 0.10 M K2CrO4 will be required to complete the reaction? Solution Stoichiometry If molarities don't match, a theoretical yield calculation sorts things out: 2 AgNO3(aq) + K2CrO4(aq) Ag2CrO4(s) + 2 KNO3(aq) If we start with 5.0 mL of 0.10 M AgNO3, what volume of 0.50 M K2CrO4 will be required to complete the reaction? Solution Stoichiometry If we're very careful with our measurements, we can use a solution of known molarity to determine the molarity of another solution. AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) When all the NaCl is used up, AgNO3 reacts with K2CrO4 and there is a color change! 2 AgNO3(aq) + K2CrO4(aq) Ag2CrO4(s) + 2 KNO3(aq) Solution Stoichiometry I carefully measure 10.00 mL of a solution that contains an unknown amount of NaCl. It takes 23.45 mL of the AgNO3 solution to get the reddish endpoint that shows all the NaCl has reacted. The concentration of the AgNO3 solution is 0.09468 M. What is the concentration of the NaCl solution? How many grams of NaCl are present in the sample? Titrations Experiments of the type we just worked through are called titrations, and they are very important in analytical chemistry. A titration is a lab procedure in which two solutions, one of which has a known concentration, are mixed in carefully measured amounts just until a chemical reaction is completed. Titrations Careful measurement of the volumes is critical to good results. We use special glassware that is usually accurate to 4 sig figs. Pipets are glass tubes designed to deliver a known volume of liquid. They are filled by suction. Burets are calibrated tubes designed to measure liquid as it is dispensed. Proper use of these devices will be demonstrated in the laboratory. Titrations The analyte is the solution in the flask, and is usually delivered by pipet. The analyte contains the limiting reagent in the titration reaction. The titrant is the solution in the buret. It is added until chemical equivalence is reached. At chemical equivalence, the analyte is completely reacted. Titrations The endpoint of a titration occurs when chemical equivalence is reached. There are lots of ways to determine endpoints. Often, we use indicators like K2CrO4 that change color. We can also follow electrical changes in the solution. It is critical to stop adding solution exactly at the endpoint (color change). Otherwise, the volume of titrant added will be too large, and the results inaccurate. Colligative Properties of Solutions Nonvolatile solutes: Lower the vapor pressure of the solution relative to that of the pure solvent Raise the boiling point of the solution relative to that of the pure solvent Lower the freezing point of the solution relative to that of the pure solvent. Osmosis and Osmotic Pressure Osmosis is the passage of solvent through a semipermeable membrane from a dilute solution or pure liquid to a concentrated solution. Semipermeable membranes have very small pores, and allow only certain small molecules to pass from one side to the other. A semipermeable membrane separating (a) pure water and solution (b) dilute solution and contrated solution (a) Osmosis, the flow of solvent through a semipermeable membrane from a dilute to a concentrated solution. (b) At equilibrium, solvent molecules move back and forth at equal rates. Osmotic pressure is the amount of pressure needed to prevent the solution in the tube from rising as a result of the process of osmosis. Isotonic, Hypertonic, and Hypotonic Solutions Isotonic solutions have the same osmotic pressure as intracellular fluids Red blood cells are stable Hypertonic solutions have higher osmotic pressure that intracellular fluids Red blood cells undergo crenation Hypotonic solutions have lower osmotic pressure than intracellular fluids Red blood cells undergo hemolysis Isotonic, Hypertonic, and Hypotonic Solutions Dialysis Dialysis is the process in which a semipermeable membrane permits the passage of solvent and small solute particles (ions and molecules) but blocks the passage of large molecules. Cell membranes permit dialysis. In dialysis, there is a net movement of ions from a region of higher concentration to a region of lower concentration. (a) Before dialysis. (b) After dialysis. Impurities (ions) can be removed from a solution by using a dialysis procedure. Reverse Osmosis Reverse osmosis is a process in which water (or other solvent) is purified by forcing it through a semipermeable membrane under pressure. It’s used for desalination of water. Reverse Osmosis A diagram of a reverse osmosis filter. The membrane is delicate and needs reinforcement against the solvent pressure.