Study Guide for Module 11A Solutions I

Transcription

1 Chemistry 1020, Module 11A Name Study Guide for Module 11A Solutions I Reading Assignment: Sections 4.1, 4.2, 4.3, 11.1, and 11.3 in Chemistry, 6th Edition by Zumdahl. Guide for Your Lecturer: 1. Solutions: Basic Information 2. Classification of Solutions; Classification of Common Solutes 3. Concentration Units: Molarity 4. Concentration Units: Mole Fraction 5. Concentration Units: Weight Percent 6. Concentration Units: Molality 7. Concentration Units: Converting from One to Another 8. An Overview of Factors Which Affect Solubility; Predicting Whether or Not a Given Substance Will Likely Dissolve in Water 9. More Detail about the Temperature Dependence of Solubility 10. More Detail about the Pressure Dependence of Solubility (Henry's Law) Homework Note: indicates problems to be stressed on drill quizzes and hour exams. Solutions: Basic Information 1a) A. Define solution. (p. 27) - B. Define solute. (p. 136) - C. Define solvent. (p. 136) - b) If glucose is dissolved in water,... What is the solute? What is the solvent? - c) If 6 moles of H2 and 2 moles of N2 are mixed,... What is the solute? What is the solvent? - d) Define concentration. (Amount of solute contained in a given amount of solvent or solution.) - e) Define solubility. (p. A39) - f) Distinguish between dilute and concentrated solutions. (A dilute solution has less solute per given amount of solution or solvent than does a concentrated one.) - g) Distinguish among unsaturated, saturated, and supersaturated solutions. (p. 752) (A saturated solution contains as much solute as possible at a particular temperature and pressure. In an unsaturated solution, more solute can dissolve. A supersaturated solution contains more solute than it normally can at a particular temperature due to a heating and cooling down process.) - Xavier University of Louisiana 1

2 Chemistry 1020, Module 11A Solutions: Basic Information (continued) 1h) Define electrolyte. (p. 136) - i) Distinguish among nonelectrolytes, weak electrolytes, and strong electrolytes. (Electrolytes are substances which break up into ions when dissolved in water. All molecules in a strong electrolyte break up, i.e. 100% ions; a few molecules in a weak electrolyte break up, i.e. 5-10% ions; no molecules in a nonelectrolyte break up, i.e. 0% ions.) - j) What determines the degree to which a solution conducts electricity if a low voltage is applied? (p. 136) - k) Distinguish between miscible and immiscible. ( Immiscible substances do not mix together. An example is oil and water. Miscible substances do mix together.) - l) Give an example of... a liquid which is miscible with water. a liquid which is immiscible in water. - m) S. If the solubility of NaCl in water at a temperature is 35 g/100 g of water, how much NaCl would dissolve in 40 grams of water at this temperature? 35 g NaCl 100 g H2O = x 40 g H2O so x = 40 g H 2O * 35 g NaCl 100 g H2O = 14 g NaCl A. How many grams of KNO3 would dissolve in 200 grams of water at a certain temperature if the solubility of KNO3 at this temperature is 42 g/100 g of water? B. If the solubility of NaCl in water is 39 g/100 g of water at a certain temperature, how much NaCl would dissolve in 60 grams of water at this temperature? C. How many grams of NaCl will dissolve in 150 g of water at a temperature if the solubility of NaCl at that temperature is 52 g/100 g of water? D. How many grams of water would be required to dissolve 20 g of NaCl at a temperature if the solubility NaCl at that temperature is 45 g/100 of water. E. How many grams of water would be required to dissolve 45 g of NaCl at a temperature if the solubility NaCl at that temperature is 32 g/100 of water. 2 Xavier University of Louisiana

3 Chemistry 1020, Module 11A Name Classification of Solutions; Classification of Common Solutes 2a) The following diagrams are intended to represent solutions of the given solute in water. Determine the composition of each of the following solutions and then choose the terms which correctly describe each. Solute Diagram of solution Words which correctly describe solute or solution S. X2Y3 X2Y3 Y 2- X2Y3 solution: saturated Composition X2Y3 unsaturated of solution: X 3+ X2Y3 X2Y3 No X2Y3(s) present X2Y3 3 Y 2- Y 2- X2Y3 solute: nonelectrolyte 9 X2Y3 X2Y3 X 3+ weak electrolyte 2 X 3+ Y 2- X2Y3 strong electrolyte Undissociated X2Y3 AND ions present - A. M3Q M + Q 3- M + solution: saturated Composition M + M + unsaturated of solution: M + M + M + M + Q 3- M + solute: nonelectrolyte M + Q 3- M + weak electrolyte Q 3- M + strong electrolyte - B. X2Y3 Y 2- Y 2- solution: saturated Composition Y 2- X 3+ unsaturated of solution: X 3+ X 3+ Y 2- Y 2- Y 2- solute: nonelectrolyte X 3+ weak electrolyte X2Y3(s) strong electrolyte - C. solution: saturated Composition unsaturated of solution: solute: nonelectrolyte weak electrolyte strong electrolyte - Xavier University of Louisiana 3

4 Chemistry 1020, Module 11A Classification of Solutions; Classification of Common Solutes (continued) 2a) The following diagrams are intended to represent solutions of the given solute in water. Determine the composition of each of the following solutions and then choose the terms which correctly describe each. Solute Diagram of solution Words which correctly describe solute or solution D. - solution: saturated Composition + unsaturated of solution: solute: nonelectrolyte - weak electrolyte (s) + strong electrolyte - b) List two common nonelectrolytes. (urea NH2CONH2 and glucose C6H12O6) c) List the chemical formulas and names of the following: Six common strong acids used in thiscourse (four on page 657, hydrobromic acid, hydroiodic acid). Others are weak Seven common weak acids. (hydrofluoric acid, citric acid [C3H5O(COOH)3], acetic acid, boric acid, formic acid, hydrosulfuric acid, and H2CO3 [called carbonic acid = CO2 in water]) d) List the chemical formulas and names of nine strong bases. (Six Group IA hydroxides, strontium hydroxide, barium hydroxide, and calcium hydroxide) e) What does R stand for as in RCOOH and RNH2? (A specific combination of C and H atoms. For example: In acetic acid, CH3COOH, R = CH3, or formic acid, HCOOH, R = H.) - f) List the chemical formulas of two common weak bases. (NH3, compounds with general formula RNH2) g) Which salts are strong electrolytes? (All which dissolve.) - 4 Xavier University of Louisiana

5 Chemistry 1020, Module 11A Name Classification of Solutions; Classification of Common Solutes (continued) 2h) In the following beakers, represent solutions of the indicated solute in water and then choose the term which correctly describes each. Diagram of solution S. HF Composition of solution: 10 HF 1 H + 1 F - H F H F F solute: nonelectrolyte weak electrolyte strong electrolyte F H H F H F F H F H F - H + H H F F H - A. HBr Composition of solution: solute: nonelectrolyte weak electrolyte strong electrolyte - B. KOH Composition of solution: solute: nonelectrolyte weak electrolyte strong electrolyte - Xavier University of Louisiana 5

6 Chemistry 1020, Module 11A Classification of Solutions; Classification of Common Solutes (continued) 2h) In the following beakers, represent solutions of the indicated solute in water and then choose the term which correctly describes each. C. HNO 2 Composition of solution: solute: nonelectrolyte weak electrolyte strong electrolyte - D. NH 3 Composition of solution: solute: nonelectrolyte weak electrolyte strong electrolyte - E. LiOH Composition of solution: solute: nonelectrolyte weak electrolyte strong electrolyte 6 Xavier University of Louisiana

7 Chemistry 1020, Module 11A Name Concentration Units: Molarity 3a) Define molarity. (p. 140) - b) Define gram formula weight. (The sum of the atomic weights of the atoms in the formula of a substance expressed in grams. It has the same numerical value as the quantity we have called molecular weight. ) - c) Define formality. (Formality is a concentration unit = the number of gram formula weights of a substance per liter of solution. The value is the same as for molarity.) d) Work the following problems. (pp and Module 4 in Chemistry 1010) NOTE: Starting with this problem and continuing throughout the course, on the first line of each problem you will be requested to provide the following information about all solutes Name or formula (which ever is not given). Type of compound--i.e. "acid," "base," "salt," or "not acid, base or salt." Strength--i.e. "strong," "weak,", or "nonelectroyte." S. How many grams of sodium perchlorate are needed to prepare 155 milliliters of a 0.32 M solution of the substance? Formula: NaClO4 Type: salt Strength: strong NaClO4 M = (mol solute)/liter soln Na 23.0 = 23.0 M = (g/mw)/l Cl 35.5 = 35.5 M*L = g/mw 4"O" 4*16.0 = 64.0 mw*m*l = g g/mol g = (122.5 g/mol)(0.32 mol/l)(155 ml)(1 L/1000 ml) = g = g which rounds to 6.1 g A. How many grams of sodium chloride must be used to prepare 502 ml of a 0.25 F solution? B. How many milliliters of 0.54 M sodium chloride could be prepared from 42 grams of sodium chloride? Xavier University of Louisiana 7

8 Chemistry 1020, Module 11A Concentration Units: Molarity (continued) 3d) Work the following problems. (Section 4.3 in text and Module 4 in Chemistry 1010) C. How many grams of potassium sulfate must be added to water to obtain 412 ml of a 0.10 F solution of the substance? D.What is the molarity of a solution prepared by adding 132 grams of rubidium chloride to enough water to obtain 355 milliliters of solution? E. What is the molarity of a solution prepared by adding 13.2 grams of lithium acetate to enough water to obtain 135 milliliters of solution? F. How many grams of sodium iodate must be added to water to obtain 187 milliliters of a 0.23 M solution of the substance? 8 Xavier University of Louisiana

9 Chemistry 1020, Module 11A Name Concentration Units: Mole Fraction 4a) Define mole fraction. (p. 208) b) Perform the calculation indicated. S moles of H2 and 6.0 moles of N2 are mixed. What is the mole fraction of nitrogen in the mixture? Formula: N2 Type: Not a, b, or s Strength: nonelectrolyte XN2 = moles N 2 total moles = XN2 = = = R XN2 = = 0.43 S2. How many grams of sodium hydroxide must be added to 75 ml of water in order to obtain a solution with a mole fraction of in sodium hydroxide? Formula: NaOH Type: base Strength: strong electrolyte XNaOH = moles NaOH total moles = XNaOH = mol NaOH = = mol NaOH mol NaOH + mol H2O mol H 2 O = (75 ml)(1g/1ml)(1 mol/18.0 g) = 4.17 mol mol NaOH = mol NaOH = mol NaOH g NaOH= (0.433 mol)(40.0 g/mol) mol NaOH R mol NaOH = mol NaOH = 17 g NaOH A. 1.2 moles of potassium chloride are dissolved in 5.0 moles of water. What is the mole fraction of potassium chloride in the solution? B. How many moles of nitrogen must be added to 12 moles of hydrogen in order to obtain a mixture whose mole fraction of nitrogen is 0.25? Xavier University of Louisiana 9

10 Chemistry 1020, Module 11A Concentration Units: Mole Fraction (continued) 4b) C. What is the mole fraction of sodium chloride in a solution prepared by adding 5.3 grams of the salt to 98 grams of water? D. How many of grams of urea (gfw=60.0) must be added to 125 milliliters of water in order to obtain a solution with a mole fraction of in urea? E. What is the mole fraction of urea (gfw = 60.0) in a solution prepared by adding 15 grams of urea to 82 grams of water? F. How many of grams of glucose (gfw=180.0) must be added to 55 milliliters of water in order to obtain a solution with a mole fraction of in glucose? 10 Xavier University of Louisiana

11 Chemistry 1020, Module 11A Name Concentration Units: Weight Percent 5a) Define weight percent. (p. 512) b) Perform the calculation indicated. S1. 12 grams of calcium chloride are dissolved in 123 grams of water. What is the weight percent of calcium chloride in the mixture? Formula: CaCl2 Type: salt Strength: strong %CaCl2 = g CaCl 2 total g * 100% = 12 g CaCl2 %CaCl2 = 123g water + 12 g CaCl2 * 100% = %CaCl2 = 12 g CaCl g soln * 100% = R %CaCl2 = 8.89% CaCl2 = 8.9%CaCl2 S2. How many grams of potassium sulfate must be added to 105 grams of water to obtain a solution which is 8.0 % potassium sulfate by weight? Formula: K 2 SO 4 Type: salt Strength: strong %K 2 SO 4 = g K 2SO 4 total g * 100% = g K 2 SO = g K 2 SO 4 %K 2 SO 4 = g K 2 SO 4 * 100 %= 8.40 = g K 2 SO 4 g K 2 SO 4 + g H 2 O R 8.0 % K 2 SO 4 = g K 2 SO 4 *100 % g K 2 SO 4 = 9.1 g K 2 SO 4 g K 2 SO g H 2 O K 2 SO 4 = g K 2 SO 4 = g K 2 SO g H 2 O A. How many grams of a solution which is 35% sodium chloride by weight must be measured out to obtain 12 grams of sodium chloride? Xavier University of Louisiana 11

12 Chemistry 1020, Module 11A Concentration Units: Weight Percent (continued) 5b) B. What is the weight percent of sodium chloride in 62 grams of a mixture which contains 35 grams of sodium chloride? C. How many grams of a solution which is 2.5% in glucose (gfw=180.0) must be taken in order to obtain 15 grams of glucose? D. What is the weight percent of sulfuric acid in a solution prepared by adding 14 grams of sulfuric acid to 432 milliliters of water? E. What is the weight percent of sodium chloride in a solution prepared by dissolving 32 grams of the substance in 72 grams of water? 12 Xavier University of Louisiana

13 Chemistry 1020, Module 11A Name Concentration Units: Weight Percent (continued) 5b) F. How many grams of glucose (gfw = 180.0) must be added to 35 grams of water to obtain a solution which is 41% glucose by weight? Concentration Units: Molality 6a) Define molality. (p. 512) b) Perform the indicated calculation. S. How many grams of sodium sulfide must be combined with 50.0 grams of water to obtain a 0.20 molal solution of the substance? Formula: Na2S Type: salt Strength: strong Na2S m = mol solute/kg solvent 2Na 2*23.0 = 46.0 m = mol Na2S/kg water S 32.1 = 32.1 m = (g/mw)/kg solvent 78.1 g/mol g = (kg solvent)(mw)(m) g = ( kg)(78.1 g/mol)(0.20 molal) = kg water = (50.0 g H2O)(1 kg/1000 g) = R kg water = kg H2O g = g Na2S = 0.78 g Na2S A. Calculate the molality of a solution prepared by dissolving 2.5 grams of a potassium chlorate in 512 grams of water. B. How many grams of sodium nitrate must be combined with 213 grams of water to obtain a 0.40 molal solution of the substance? Xavier University of Louisiana 13

14 Chemistry 1020, Module 11A Concentration Units: Molality (continued) 6b) C. Calculate the molality of a solution prepared by dissolving 4.5 grams of naphthalene (C10H8) in 50.0 grams of benzene (C6H6) D. How many grams of iodine must be combined with 159 grams of ethanol (C2H5OH) in order to obtain a 0.30 m solution of the substance? E. What is the molality of a solution prepared by adding 13 grams of sodium iodide to 835 grams of water? F. How many grams of iodine must be added to 52 grams of carbon tetrachloride to produce a solution which is 0.47 m in iodine? 14 Xavier University of Louisiana

15 Chemistry 1020, Module 11A Name Concentration Units: Converting from One to Another 7. Perform the conversion between concentration units indicated. Algorithm for solving problems which convert from one type of concentration unit to another: ❶Divide your paper into two columns. ❷Write the given concentration value and its definition in the left column under the heading Given. ❸If the given concentration unit is %, assume you have 100 grams of solution. If the given concentration unit is anything else, assume you have 1 of the unit in the denominator of the concentration unit. In either case, write your assumption on paper. (Note: You will get the same answer if you assume ANY amount of the substance in the denominator of the concentration unit. The assumptions here are merely the easiest ones to use.) ❹Write down the amount of solute associated with the amount of solvent/solution you assumed in step 3. (Note: This equals the value of the given concentration. The units are the same as those in the numerator of the given concentration.) ❺Write the desired concentration units and its definition on the right under the heading Desired. ❻Write any other information you are given (such as density of the solution) in the Given column. ❼Use the data in your Given column to calculate the quantities needed to obtain the concentration unit you desire. S1. What is the mole fraction of sodium chloride in a solution which is 25% sodium chloride by weight? Formula: NaCl Type: salt Strength: strong Given: Desired: ❷ Concentration = 25% NaCl ❺ mole fraction NaCl =? % NaCl = g NaCl mol NaCl total g * 100% mole fraction NaCl = total mol ❸ Assume have 100 grams soln ❼ Need moles Na and moles H2O ❹ Therefore, have 1 mol NaCl mol NaCl = 25 g NaCl * 58.5 g NaCl = 25 grams NaCl = mole NaCl 75 grams H2O mole H2O = 75 g H2O * 1 mol H 2O 18.0 g H2O = and know that = 4.17 mol H2O gfw NaCl = = 58.5 g/mol mol NaCl XNaCl = mol total = R gfw H2O = 2* = 18.0 g/mol XNaCl = = S2. What is the molarity of a solution of H 2 SO 4 that is 4.3% H 2 SO 4 if the specific gravity of the solution is 1.21? Formula: H 2 SO 4 Type: acid Strength: strong Given: Desired: ❷ Concentration = 4.3% H 2 SO 4 ❺ molarity of H 2 SO 4 =? % H 2 SO 4 = g H 2SO 4 total g * 100% molarity of H 2 SO 4 = mol H2SO4 L sol n ❸ Assume have 100 grams soln ❼ Need moles H 2 SO 4 and liters sol n ❹ Therefore, have mol H 2 SO 4 = 4.3 g H 2 SO 4 *1 mol H2SO4 = 4.3 grams H 2 SO g = mol H 2 SO 4 and know that L sol n = 100 g * 1 ml * 1 L = L l 1.21 g 1000mL gfw H 2 SO 4 = 2* * l = 98.1 g/mol M of H 2 SO 4 = mol H 2 SO L sol n ❻ specific gravity = 1.21 R therefore, density = 1.21 g/ml = 0.53 M Xavier University of Louisiana 15

16 Chemistry 1020, Module 11A Concentration Units: Converting from One to Another (continued) 7. A. What is the weight percent of calcium acetate in a solution whose mole fraction of calcium acetate is 0.40? B. What is the weight % of sodium sulfate in a M solution whose specific gravity 1 is 1.12? C. What is the molarity of a solution whose concentration is m and whose specific gravity 1 is 1.10? GFW = Specific gravity of X = density of X density of H2O. Therefore, at room temperature where density of H 2O = 1.00 g/ml, the specific gravity of a substance has the same value as its density. 16 Xavier University of Louisiana

17 Chemistry 1020, Module 11A Name Concentration Units: Converting from One to Another (continued) 7. D. What is the molality of a 25% solution of ethanol (C2H5OH)? E. What is the molality of a solution of methanol (CH3OH) in water in which the mole fraction of methanol is 0.23? F. What is the mole fraction of sodium chloride in a 1.8 M solution with a specific gravity 1 of 1.17? 1 Specific gravity of X = density of X density of H2O. Therefore, at room temperature where density of H 2O = 1.00 g/ml, the specific gravity of a substance has the same value as its density. Xavier University of Louisiana 17

18 Chemistry 1020, Module 11A Concentration Units: Converting from One to Another (continued) 7. G. What is the molarity of a solution of lithium chloride in water in which the mole fraction of lithium chloride is 0.036? The specific gravity of the solution is H. What is the mole fraction of urea (gfw = 60.0) in a solution that is 0.83m? I. What is the molality of 0.95 M solution of potassium chloride with a specific gravity 1 of 1.02? 1 Specific gravity of X = density of X density of H2O. Therefore, at room temperature where density of H 2O = 1.00 g/ml, the specific gravity of a substance has the same value as its density. 18 Xavier University of Louisiana

19 Chemistry 1020, Module 11A Name Concentration Units: Converting from One to Another (continued) 7. J. What is the weight percent of sodium bromide in a 0.56 m solution of sodium bromide? K. What is the molarity of a solution of HCl that is 3.6% HCl if the specific gravity 1 of the solution is 1.13? 1 Specific gravity of X = density of X density of H2O. Therefore, at room temperature where density of H 2O = 1.00 g/ml, the specific gravity of a substance has the same value as its density. Xavier University of Louisiana 19

20 Chemistry 1020, Module 11A An Overview of Factors Which Affect Solubility; Predicting Whether or Not a Given Substance Will Likely Dissolve in Water 8a) List three factors which affect solubility and state how each generally does so. (Section 11.3 in text) Factor How it generally affects solubility For solids in water: Temperature For gases in water: b) Variation of solubility as the nature of the solvent and solute varies: Indicate the extent to which each of the substances will dissolve in water and explain your answer. (Section 11.3 in text) S. CH3CH2CH2OH The "CH3CH2CH2" part of the molecule is nonpolar because "C" and "H" have about the same electronegativity. On the other hand, the "OH" part is polar because "O" has a mucher higher electronegativity than the "C" or the "H" it is bonded to. The nonpolar "CH3CH2CH2" part would tend NOT to dissolve in H2O whereas the polar "OH" WOULD tend to dissolve in H2O. Whether or not the molecule overall is soluble in water depends on the relative size of the nonpolar and polar parts. In this case, the nonpolar part is relatively small so this substance is probably moderately soluble in water. A. CH3CH2CH3 B. CH3CH2CH2CH2CH2CH2CH2CH2CH2OH 20 Xavier University of Louisiana

21 Chemistry 1020, Module 11A Name An Overview of Factors Which Affect Solubility; Predicting Whether or Not a Given Substance Will Likely Dissolve in Water (continued) 8b) Variation of solubility as the nature of the solvent and solute varies: Indicate the extent to which each of the substances will dissolve in water and explain your answer. C. CH3CH2-O-CH2CH3 D. CH3CH2NH2 E. CH2 CH CH CH2 OH OH OH OH F. CH3CH2OH Xavier University of Louisiana 21

22 Chemistry 1020, Module 11A More Detail About the Temperature Dependence of Solubility 9. Answer the following regarding the solubility curve below. (Section 11.3 in text) 60_ S (grams o 50_ l of u 40_ b solute i 30_ l per i 20_ t 100 g y 10_ of water) 0_ temp ( o C) --> S. a) What is the solubility of NaCl at 20 o C? 40 g/100 g H2O b) What happens to a solution with a concentration of KNO3 of 30 grams/100 grams of water if it is cooled from 70 o C to 30 o C and no supersaturation occurs? All is dissolved at 70 o C since the solubility at that temperature is >60 g/100 H2O. At 30 o C, only 24 g dissolves in 100 g H2O. Therefore, 30 g - 24 g = 6 grams KNO3 must precipitate when the substance is cooled. A. a) What is the solubility of KNO3 at 10 o C? b) What is the lowest T at which 45g of KNO3 dissolves in 100g of water? B. a) Which salt is least soluble at 60 o C? b) What happens if 1 gram of KNO3 is added to a solution at 55 o C whose concentration is 25 g/100ml of water? C. a) What is the lowest T at which 40 grams of KNO3 will dissolve in 100 g of water? b) Describe what happens if a solution prepared by adding 25 g of NaCl and 25 g of KNO3 to 100 ml of water at 40 o C is cooled to 5 o C? 22 Xavier University of Louisiana

23 Chemistry 1020, Module 11A Name Temperature Dependence of Solubility (continued) 9. D. a) What is the solubility of KNO3 at 40 o C? b) How many grams of KNO3 must be added to 100 grams of water at 50 o C to obtain a saturated solution? E. a) What is the lowest T at which 55 grams of KNO3 will dissolve in 100 g of water? b) How many grams of NaCl must be added to 100 grams of water at 65 o C to obtain a saturated solution? More Detail about Pressure Dependence of Solubility (Henry's Law) 10a) State Henry's Law indicating the relationship between the pressure of a gas and its solubility. (p. 521) b) S. If the solubility of carbon dioxide in water at 25 o C and 402 torr of carbon dioxide is 5.69 M, what is the solubility of carbon dioxide in water at 25 o C if the pressure of carbon dioxide is doubled to 804 torr? Since the change in pressure is an easy multiple of the original pressure, you can find the answer easily by remembering that the solubility of a gas in a liquid is directly proportional to the pressure of that gas above the liquid. Therefore, if the pressure of carbon dioxide above water doubles, the solubility of carbon dioxide in water will also double. That is, solubility = 2*5.69 M = M which rounds to 11.4 M. A. If the solubility of carbon dioxide in water at 25 o C and 402 torr of carbon dioxide is 5.69 M, what is the solubility of carbon dioxide in water at 25 o C if the pressure of carbon dioxide is halved to 201 torr? B. If the solubility of carbon dioxide in water at 25 o C and 402 torr of carbon dioxide is 5.69 M, what pressure of carbon dioxide would be required to cause the solubility of carbon dioxide to triple? C. If the solubility of nitrogen in water at 25 o C and 1.5 atm nitrogen is 1.1*10-2 M, what pressure of nitrogen is required to cause the solubility of nitrogen to decrease to 1.1*10-3 M? Xavier University of Louisiana 23

24 Chemistry 1020, Module 11A More Detail about Pressure Dependence of Solubility (Henry's Law, continued) 10b) D. If the solubility of nitrogen in water at 25 o C and 1.5 atm nitrogen is 1.1*10-2 M, what is the solubility of nitrogen in water at 25 o C if the pressure of nitrogen is tripled to 4.5 atm? E. If the solubility of nitrogen in water at 25 o C and 1.5 atm nitrogen is 1.1*10-2 M, what is the solubility of nitrogen in water at 25 o C if the pressure of nitrogen is quadrupled to to 6.0 atm? c) The solubility of carbon dioxide in water at 25 o C and 402 torr of carbon dioxide is 5.69 M while that of nitrogen at the same temperature and 1.5 atm nitrogen is 1.1*10-2 M. Use this information to answer the following. Explain all answers in detail. S. What is the solubility of CO2 at 25 o C when the pressure of CO2 is 567 torr? Henry s Law (Cg = k*pg ) tells us that the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid. Therefore, the problem can be worked using ratio-andproportion as follows: 5.69 M 402 torr = x 567 torr x = ( 5.69M 402 torr ) * 567 torr = M which rounds to 8.03 M A. What is the solubility of N2 at 25 o C when the pressure of N2 is 2.9 atm? B. What pressure of CO2 must be maintained at 25 o C to raise the concentration of CO2 to 8.21 M? C. What is the solubility of carbon dioxide at 25 o C if the pressure is 872 torr? D. What pressure of N2 must be maintained at 25 o C to obtain a solubility of N2 of 8.5*10-3 M? 24 Xavier University of Louisiana

25 Chemistry 1020, Module 11A Name More Detail about Pressure Dependence of Solubility (Henry's Law, continued) 10c) E. What pressure of N2 must be maintained at 25 o C to obtain a solubility of N2 of 5.3*10-4 M? F. What is the solubility of carbon dioxide at 25 o C under a carbon dioxide pressure of 4.3 atm? Bonding/Model Activity to Improve Ability to Visualize in 3-D Algorithm to write the electron-dot (Lewis) structure of covalent species (from Mod 8A, Example: HNO2 formal charged reviewed on pages just before this module in Handbook.) ❶ Draw the skeleton of the chemical species showing a single bond between each atom. If the species is a polyatomic ion, enclose it in brackets [] and put the charge on it. ❷ Add up the valence electrons, then add or subtract electrons to compensate for the charge on the species (if any). ❸ Subtract two electrons for each bond in the skeleton. ❹ Add the other electrons so that each atom has a filled outer shell (8 electrons around all atoms except H). If this requires more electrons than are left in the valence e- pool, put in multiple bonds, one for each pair of e-'s you are short. If you have electrons left over after you fill all outer shells, put them around the largest atom (which is usually the central one). ❺ Check your structure by adding up all of the electrons shown to make certain the total number electrons showing in the structure = total number of valence electrons (plus adjustment for charge if any). ❻Write resonance structures if needed. ❼Calculate the formula charge on each atom (# electrons on a neutral atom - # electrons in lone pairs on atom - 0.5*# electrons in bonds attached to atom). If not zero, write the formal charge above the atom enclosed in a small circle. Then, if resonance exists, STOP. ❽If there is no resonance, look to see if the central atom can have an expanded octet (i.e. if it is an element in the 3rd or higher rows). If not, STOP. ❾If an atom can have an expanded octet, see if the number and size of the formal charges can be reduced by expanding the octet, if so shift them and recalculate formal charges. IF NOT, STOP. ❿If Lewis structure was changed in ❾ above, check again for resonance structures. H - O - N - O Note: In most oxyacids, the acidic H is attached to an O atom in the same manner show in this skeleton. Valence electrons on H = 1 Valence electrons on N = 5 Valence electrons on 2 O s = There are two bonds in the skeleton and each bond contains 2 electrons so subtract 6 electrons from the 18 to get (18-6) = 12. Now put the 12 electrons in to get: H - O - N - O N atom does not have 8 electrons so shift to get _ H - O - N = O All except H now have 8 electrons. There are 10 pairs of electrons, 20 total, in the structure above. Since this is the same number of valence electrons as obtained in #2 above and everything has a filled shell, the structure is okay. No resonance possible for this structure since "X=O-H" structures are not stable. All formal charges are zero for this example so you have _ H - O - N = O NA NA NA Xavier University of Louisiana 25

26 Chemistry 1020, Module 11A II. Formal Charges: Using Formal Charges to Predict Most Stable Lewis Structures(Review from Module 8A) a) Define formal charge. (p. 385) The difference between the number of valence electrons on a free atom and the number of electrons assigned to the atom in the molecule or ion. - b) What two things does one need in order to calculate formal charges? (p. 386) number of valence electrons on the free atom number of electrons belonging to atom in a molecule or ion - c) What are the two rules needed to determine which electrons belong to a given atom in a species? (p. 386) (Each lone pair of electrons contributes 2 electrons and each shared pair contributes 1 electron. Therefore, the number of electrons belonging to an atom for formal charges = 2*# lone pairs + # shared pairs on the atom.) - d) What is the formula used to calculate the formal charge on a given atom in a species? (p. 386) formal charge = number of valence electrons on free atom - 2*# lone pairs - # shared pairs on the atom) - e) What is the relationship between formal charges and the charge on a chemical species? (p. 378) sum of formal charges of all atoms in a chemical species = overall charge on chemical species - g) What is the overall rule for using formal charges? (p. 381, The most stable Lewis structures are those with a) the formal charges closest to zero and b) negative formal charges on the the more electronegative atoms.) - h) Calculate the formal charge on each atom in the following Lewis structure. f.c. H = 1 val e lp e - s - ( 1 shared pair) = 0 f.c. C = 4 val e lp e - s - (4 shared pairs) = 0 f.c. O = 6 val e - - 2*2 lp - (2 shared pairs ) = 0 0 H 0 H C 0 0 O H H Sum of formal charges = 0, the charge on the species 0 0 i) When writing Lewis structures, it is best to go through the following scheme: Draw the Lewis structure so that every atom has an octet (except H). If there is resonance at this point, draw the resonance structures and stop. If there is no resonance, see if any atom can have an expanded octet. If not, stop. If an atom can have an expanded octet, see if the number and size of the formal charges can be reduced by expanding the octet. If not, stop. If the answer to the above is yes, do it and then check again for resonance structures. Write the BEST Lewis structure for the following species using formal charges to evaluate various possibilities. Hint: Start by writing it like we would have done above and then try expanded shells. (pp ) S. ClO2 - This is the Lewis structure you would get without considering 1Cl = 1*7 = 7 1O = 1*6 = 6 1O = 1*6 = 6 charge = expanded shells [ O Cl O ] - It probably isn't as good as the following which uses the "d" orbitals on Cl because the following has smaller formal charges. Both the following and that above have negative formal charges on the more electronegative element "O" [ O Cl O ] O Cl O [ ] - The second of the structures (the one with expanded shells and resonance) is more likely because it has smaller charges overall. 26 Xavier University of Louisiana

27 Chemistry 1020, Module 11A Name Bonding/Model Activity to Improve Ability to Visualize in 3-D (continued) A. a) Draw the Lewis structure of nitric acid. B. a) Draw the Lewis structure of chloric acid. C. a) Draw the Lewis structure of sulfuric acid. D. a) Draw the Lewis structure of sulfurous acid. E. a) Draw the Lewis structure of acetic acid. Xavier University of Louisiana 27

28 Chemistry 1020, Module 11A Bonding/Model Activity to Improve Ability to Visualize in 3-D (continued) F. a) Draw the Lewis structure of phosphoric acid. A-F b) Then use your model set to assemble a model of the species. Challenge Questions A. A saturated solution of potassium nitrate at 50 o C is cooled to 30 o C. How much KNO3 precipitates? Refer to the diagram below. 60_ S (grams o 50_ l of u 40_ b solute i 30_ l per i 20_ t 100 g y 10_ of water) 0_ temp ( o C) --> B. Thirty one grams of KNO3 is placed in ml of water at 10 o C and the mixture is stirred until solute stops dissolving. How many grams of solute remain undissolved? Use the diagram above. C. 42 grams of KNO3 are placed in grams of water at 20 o C and allowed to come to equilibrium. What percent of the KNO3 dissolves? Use the diagram above. D. The solubility of CuS in water is 9.3*10-23 M. Calculate the solubility of CuS in grams CuS/100 grams of water. Make any reasonable assumptions. E. The solubility of PbSO4 is 1.0*10-4 M. Calculate the solubility of PbSO4/100 grams of water. Make any reasonable assumptions. F grams of KNO3 is added to 100 grams of water and the mixture is heated to 60 o C so that all solute dissolves. The solution is then cooled to 10 o C. How much KNO3 precipitates from the solution as it is cooled? Refer to the diagram above. Note: All parts of this module must be filled out when you arrive at drill. In addition, you should also complete the following skills module (logarithms) and be ready to take a quiz on both the regular and skills modules. Revised by MA, 5/7/2002, 5/5/ Xavier University of Louisiana