Solution. Solutions. Solutions: Solute + Solvent. Outline

Transcription

1 Solutions Solution Homogeneous mixtures composition may vary from one sample to another appears to be one substance, though contains multiple materials Edward Wen most Homogeneous materials are actually Solutions Gas state: common air Liquid: Gasoline (dozens of compounds), Soda water (sugar or asparatame, CO2, citric acid, fructose) Solid: Alloy such as brass 3 Outline Composition of solution: solvent vs. solute Factors affecting Solubility Concentration of solution: Definition & Calculation Mass% Molarity* Solution Stoichiometry Solutions: Solute + Solvent Solute: the dissolved substance. Example: Sugar and CO 2 gas in Soda seems to disappear takes on the state of the solvent Solvent: the substance solute dissolves in. Example: Water in Soda does not appear to change state 2 Aqueous solutions: solutions in which the solvent is water. 4 1

2 Common Types of Solution Solute Solvent Solution Phase Phase Phase Example Gaseous solutions gas gas air (mostly N 2 & O 2 ) Liquid solutions gas liquid solid liquid liquid liquid soda (CO 2 in H 2 O) vodka (C 2 H 5 OH in H 2 O) seawater (NaCl in H 2 O) Solid solutions solid solid brass (Zn in Cu) Alloys: solutions that contain Metal solutes and a Metal solvent, such as Nickel (5 cents of 1$), Brass, Stainless steel 5 Will It Dissolve? Chemist s Rule of Thumb Like Dissolves Like a chemical will dissolve in a solvent if it has a similar structure to the solvent when the solvent and solute structures are similar, the solvent molecules will attract the solute particles at least as well as the solute particles to each other 7 How Soluble? Solubility Soluble: when one substance (solute) dissolves in another (solvent) Homogeneous Salt and Sugar are soluble in water: Saline and Soda Acetic acid (HC 2 H 3 O 2 ) in water: Vinegar Oxygen gas in Nitrogen gas: Air Insoluble: when one substance does not dissolve in another Heterogeneous Oil is insoluble in water: Italian salad dressing Classifying Solvents: Polar vs. Nonpolar Solvent Class Structural Feature Water, H 2 O polar O-H Ethyl Alcohol, C 2 H 5 OH polar O-H Acetone, C 3 H 6 O polar C=O Toluene, C 7 H 8 nonpolar C-C & C-H Hexane, C 6 H 14 nonpolar C-C & C-H 6 8 2

3 Solubility in Water, A Polar Solvent? Ionic compound (Yes): Ions are attracted to polar Water. Salt NaCl dissolve in water Polar molecules (Yes): attracted to polar solvents table Sugar, Alcohol, glucose Nonpolar molecules are NOT attracted to Water β-carotene, (C 40 H 56 ), is not water soluble; it dissolves in fatty (nonpolar) tissues Those molecules with both polar and nonpolar structures: depends on structural features in the molecule 9 Solubility Definition: the maximum amount of solute that can be dissolved in a given amount of solvent Example, at room temperature, 100. g water can dissolve 200 g sugar. Solubility = 200 g sugar/100g water. Usually a limit to the solubility of one substance in another Exceptions: gases are always soluble in each other two liquids that are mutually soluble are said to be miscible alcohol and water are miscible 11 Salt Dissolved in Water Descriptions of Solubility Saturated solution has the maximum amount of solute that will dissolve in that solvent at that temperature. If more solute is added, it dissolve. Unsaturated solution is holding solute than it is capable of. It can dissolve solute. Supersaturated solution is holding solute than it is capable of at that temperature Unstable If more solute is added,. 10 Will not; less; more; more; more solute will separate from the solution 12 3

4 Adding Solute to various Solutions saturated saturated Solubility of Solid Increases as Temperature increase Application: To make pure crystals, make saturated solution at high temp cooling leads to supersaturated solution, extra crystals forms at lower temp. saturated Supersaturated Solution is unstable A supersaturated solution has more dissolved solute than the solvent can hold, not stable. When disturbed, all the solute above the saturation level will separate. Online demo: A grain of sodium acetate crystal is placed in the middle of supersatured sodium acetate solution. 14 Solubility of Gases Decreases at higher Temperature Observation 1: Warm soda pop fizzes more than cold soda pop Cause: Solubility of CO 2 in water is (higher/lower) at high temperature. Observation 2: When water is heated up, gas bubbles appear even before boiling occurs. Cause: Solubility of air in water decreases as temperature increases. 16 4

5 Solubility of Gas depends on Pressure higher pressure = higher solubility CO 2 is dissolved under Pressure into bottled/canned soda Solution Concentration Descriptions Diluted solutions have low solute concentrations. Soda drink from the soda fountain. Concentrated solutions have high solute concentrations. Syrup in the storage tank in the soda fountain Under which conditions does oxygen gas have the best solubility? A. high temperature and high partial pressure (P O 2) B. high temperature and low partial pressure C. low temperature and low partial pressure D. low temperature and high partial pressure Concentrations Quantitative Descriptions of Solutions Solutions have variable composition. Salt vs. Water in Seawater To describe a solution accurately, you need to describe the components and their relative amounts Concentration = amount of solute in a given amount of solution Seawater: Salt concentration 3.4% Dead Sea: Salt concentration 30% Vinegar: Acetic acid concentration 5% 20 5

6 Mass Percent (%) mass of solute (gram) in every 100 gram of solution if a solution is 0.9% by mass, then there are 0.9 grams of solute in every 100 grams of solution Mass of solution = Mass of solute + Mass of solvent Mass% = x Using Concentrations as Conversion Factors concentrations show the relationship between the amount of solute and the amount of solvent 12% by mass sugar(aq) means 12 g sugar 100 g solution The concentration can then be used to convert the amount of solute into the amount of solution, or visa versa Example: Calculate the mass percent of a solution containing 27.5 g of ethanol (C 2 H 6 O) and 175 ml of H 2 O (assume the density of H 2 O is 1.00 g/ml) Information Given: 27.5 g C 2 H 6 O; 175 ml H 2 O Find: % by mass Example: A soft drink contains 11.5% sucrose (C 12 H 22 O 11 ) by mass. What volume of soft drink in milliliters contains 85.2 g of sucrose? (assume the density is 1.00 g/ml) Information Given: 85.2 g C 12 H 22 O 11 Find: ml sol n Mass of Solvent = 175 g Mass of Solute = 27.5 g Mass of Solution = g Mass% = V = 741 ml 24 6

7 Preparing a Solution What we need to know: Amount of solution & Concentration of solution Calculate the mass of solute needed start with amount of solution use concentration as a conversion factor 5% by mass solute 5 g solute 100 g solution Solution Concentration: Molarity Definition: Moles of solute per 1 liter of solution Purpose: describing how many molecules of solute in each liter of solution Unit: mole/l, abbreviated as M. molarity = moles of solute liters of solution If a sugar solution concentration is 2.0 M, 1 liter of solution contains 2.0 moles of sugar Preparing a Solution by Mass% Example - How would you prepare g of 5.00% by mass glucose solution (normal glucose)? Why Molarity? Many reagents used in chemistry, even many biology labs, are in the form of solution. Molarity concentration of solution is particularly important and useful because: Easy to use: To obtain given amount (mole) of reagent, just calculate the volume of solution to be used: 12.5 g of glucose g of water Easy to prepare a solution to a given molarity

8 Calculations involving Molarity Example: How to prepare 500 ml of M NaCl solution? Part B. Preparation Molarity = mole Volume (L) Solve for mole: Mole = Solve for volume of solution in liters: Volume (L) = Weigh out 0.58 g NaCl and Add it to a 500mL Volumetric flask. Add water to dissolve the NaCl, then add water to the mark. Put the lid on, Invert the flask to homogenize the solution 29 Step 1 Step 2 Step 3 Example How to prepare 500 ml of M NaCl solution? Part A. Calculate the mass NaCl needed: Example: Calculate the molarity of a solution made by putting 15.5 g of NaCl into a beaker and adding water to make 1.50 L of NaCl solution. Information Given: 15.5 g NaCl; 1.50 L sol n Find: molarity, M CF: g = 1 mol NaCl; mol NaCl 0.58 g NaCl mol NaCl M NaCl 32 8

9 Example: How many liters of a M NaOH solution contains 1.24 mol of NaOH? Information Given: 1.24 mol NaOH Find: L solution CF: mol = 1 L SM: mol L Molarity & Dissociation CaCl 2 (aq) = Ca 2+ (aq) + Cl - (aq) 1 mole molecules = mole ions + mole ions 1 M CaCl 2 = M Ca 2+ ions + M Cl - ions 0.25 M CaCl 2 = M Ca 2+ + M Cl L Molarity of Ions: Dissociation of Ionic Compound Making a Solution by Dilution When strong electrolytes dissolve, virtually all the solute particles dissociate into ions From the formula of the compound and the molarity of the solution Determine the Molarity of the dissociated Ions by simply multiplying the salt Concentration by the Number of Ions 34 Example: A student added 1.00 L water to 2.00 L 1.00 M HCl. The final volume became 3.00 L. mole HCl before mixing = M 1 x V 1 = 2.00 mole = mole HCl after mixing more water. When mixing more solvent into a solution, the volume of final solution is greater than the original solution The mole of solute remains the same before and after mixing more solvent For dilution, mole solute = M 1 x V 1 = M 2 x V 2 9

10 Example What Volume of 12.0 M KCl Is Needed to Make 5.00 L of 1.50 M KCl Solution? Given: M1 = ; V2 = L, M2 = M Find: V1 Equation: M 1 x V 1 = M 2 x V 2 Solution Stoichiometry Stoichiometry: Balanced chemical equation shows the mole ratio among reactants and products in a reaction. 2 H 2 (g) + O 2 (g) 2 H 2 O(l) L 37 Solution molarity moles of solute and liters of solution Measure the moles of a material in a reaction in solution by knowing its molarity and volume. 39 Example What is the final concentration (molarity) if 10.0 ml 12.0 M HCl is diluted to a final volume of 5.00 L? Given: M1 = ; V1 = L, V2 = Find: V1 Equation: M 1 x V 1 = M 2 x V 2 Example: How much M KI solution, in liters, is required to completely precipitate all the Pb 2+ in L of M Pb(NO 3 ) 2? 2 KI(aq) + Pb(NO 3 ) 2 (aq) PbI 2 (s) + 2 KNO 3 (aq) mol KI 1 L solution mol Pb(NO 3 ) 2 1 L solution 2 mol KI 1 mol Pb(NO 3 ) 2 Solution Map: L Pb(NO 3 ) 2 mol Pb(NO 3 ) 2 mol KI L KI M 38 = L 40 10