Chem 128 SampEx S04

CHEM 128 SAMPLE EXAM 1

[These are representative questions. A few questions will be short answer in form (such as fill-in-the-blank, multiple choice, or explain briefly). Others will involve calculations. For problems involving calculations, you are expected to show all your work. Answers should include the units for the numbers. Numbers without units are not correct answers. You are responsible for all topics covered in class and homework assignments. A list of equations and useful constants will be provided. This list will not include an indication of which equation goes with which order of reaction. It also will not include the definitions of molarity, molality, mole fraction and mass percent. You are expected to know these definitions, as well as density and molar mass. Do not forget to bring a calculator to the exam.]

1. A bottle of commercial concentrated aqueous ammonia is labeled "29.89% NH₃ by mass; density = 0.8960 g/mL." What is the molarity of the ammonia solution?

2. A solution is prepared by mixing 1.00 g of ethanol (C₂H₅OH) with 100 g of water to give a final volume of 101 mL. Calculate the molality, mass percent and mole fraction of ethanol in this solution.

3. Which of the following would be more soluble in ethanol (C₂H₅OH): acetic acid (CH₃COOH) or stearic acid (C₁₇H₃₅COOH)? Explain your answer.

4. Raising the temperature of a solution generally causes the solubility of a solid solute to________________ and the solubility of a gas solute to __________________ .

5. Lowering the pressure of carbon dioxide gas ________________ its solubility in water.

6. A solution is made by dissolving 4.9 g of sucrose (C₁₂H₂₂O₁₁) in 175 mL of water. Calculate the boiling point of this solution. [k_b of water = 0.52°C/m]

7. Dextran is a polymeric carbohydrate produced by certain bacteria. It is used as a blood plasma substitute. An aqueous solution contains 0.582 g of dextran in 106 mL of solution at 21°C. It has an osmotic pressure of 1.47 mm Hg. What is the average molecular weight of the dextran?

8. The freezing point depression of a 0.091 m aqueous solution of CsCl is 0.302°C. Is CsCl mostly ionized or mostly nonionized in this solution? [k_f of water = 1.86°C/m.] Show supporting calculations.

9. Be able to do Problem 25 in the Assignment 1.

10. For the reaction A + B --> C + D, what is the order of the reaction with respect to each reactant and the overall order if the rate expression is: rate = k[A][B]² ?

11. Be able to do Problem 3 in Assignment 3.

12. A graph of the decomposition of ethane gives a straight line plot with a negative slope when ln [ethane] is plotted as a function of time (s).

a. What is the order of the reaction?

b. If the rate constant for the reaction is 4.0 x 10^-4 s^-1, how long will it take for an initial concentration of 0.24 M ethane to decrease to 0.12 M?

c. Write the rate expression for the decomposition of ethane.

13. Be able to do Problem 55 in Assignment 5.

14. Be able to do Problem 61 in Assignment 5.

15. The rate of a reaction increases as temperature is raised because ____________________

16. The rate of a reaction increases when a catalyst is used because _____________________

[Note: for problems 15 and 16, the form of the problem may be different on the exam. You should understand in what manner each of these factors causes an increase in reaction rate.]

CHEM 128 EXAM 2 STUDY GUIDE (for chapters 12-14)

Instead of sample exam questions, here are some specific suggestions for what to study. Working the suggested problems at the ends of Chapters 12 and 13, including assigned homework problems, is an effective way to study the indicated material.

FORMAT: A few questions will be short answer in form (fill-in-the-blank, multiple choice, explain briefly). Others will involve calculations. For problems involving calculations, you are expected to show all your work, including set-up. Answers should include the units for the numbers, except for equilibrium constants. Unitless numbers are not correct answers. You are responsible for all topics covered in class and homework assignments.

INFORMATION: A list of equations and useful constants will be provided. This list will not include the definitions of molarity, molar mass, and density. You should know how to write the equilibrium constant expression for a balanced reaction equation. Since this expression depends on the reaction, a general equation will not be given. A periodic table will be provided if necessary.

NOTE: the summary problems at the ends of the chapters are good practice.

Do not forget to bring a functioning calculator to the exam!

HERE IS A CHECKLIST:

1. Be able to write equilibrium constant expressions for reaction equations. Try Problems 5b, 6a, 7a in Chapter 12 and 28a, 28a and 49e in Chapter 13.

2. Be able to calculate K for a reaction, given equilibrium pressures (or molarities) of reactants and products. Try Problem 20 in Chapter 12.

3. Understand and be able to apply the reciprocal rule, the coefficient rule, and the rule of multiple equilibria. Problems 16-18 in Chapter 12 will give you practice.

4. Be able to compute Q for a reaction, given certain pressures (or concentrations) of reactants and products. Then be able to compare Q to K and predict the direction in which the reaction will shift to reach equilibrium. Problems 27b and 28 in Chapter 12 are good practice.

5. Given initial pressures of reactants and products and an equilibrium constant, be able to calculate the equilibrium pressures of reactants and products for a reaction at a given temperature. Try Problem 36 in Chapter 12.

6. Understand and be able to apply Le Chatelier's Principle. Problems 43-46 and 49 and 50 in Chapter 12 are good practice.

7. Know definitions of the following: Brønsted-Lowry acids and bases; conjugate acid-base pairs; amphiprotic species. Be ble to identify a Brønsted-Lowry acid or base; give the conjugate acid or base of a chemical species; recognize an amphiprotic species. Problems 2, 4, and 6 in Chapter 13 will give you practice. Problem 5 is also a good one.

8. Know how to find pH and pOH of an aqueous solution, given the hydrogen ion concentration (or hydroxide ion concentration), and know how to find the hydrogen ion concentration and hydroxide ion concentration, given a pH or pOH. Understand how K_w fits into the picture. Be sure you know how to do these operations on your calculator! The exam is not a good time to learn how. Good practice problems are 8 and 10 in Chapter 13.

9. Be able to find the pH of

(a) solutions containing strong acids or bases (Problems 20 and 22a)

(b) solutions containing weak acids or bases (Problems 42 and 56)

Be able to calculate the percent ionization for a weak acid or base, and determine whether successive approximation is necessary to determine an accurate pH.

10. Be able to rank the relative strength of weak acids or weak bases based on their ionization (equilibrium) constants or on their pK_as or pK_bs. (Problems 31, 32, 51, 52) Understand and be able to use the relationship between K_a, K_b and K_w, or pK_a, pK_b and pK_w. (Problems 55ab, 56ab)

11. Be able to figure out whether an aqueous solution of a salt will be acidic, basic, or neutral, given a Table of information. (Try Problems 59, 60, 66).

12. Be able to identify whether a solution is a buffer solution. See specifically Problems 31 and 32, Ch. 14, for practice. Be able to determine the capacity of a buffer solution to absorb H+ or OH-. See Problems 11 and 12, Chapter 14, for practice. Be able to determine the pH of a buffer solution. See Problems 13, 19 and 20, Ch. 14. Be able to determine the ratio of acid to conjugate base needed to make a buffer of a given pH. See Problem 18, Ch. 14. Be able to determine the pH of a buffer after acid or base has been added to the buffer. See the exercise sheet we did in class and your class notes. Be able to calculate the pH of a buffer, given appropriate information. Be able to determine what conjugate acid-base pair would be suitable to buffer a solution at a certain pH.

13. Know how to calculate the pH at the beginning, during, and at the equivalence point of a titration of a strong acid with a strong base. See Problems 43 and 44, Ch. 14. Know how to calculate the pH at the beginning, during, and at the equivalence point of a titration of a weak acid with a strong base, or a weak base with a strong acid. See Problems 45 and 46, Ch. 14. Be able to pick an appropriate indicator for a titration, given sufficient information. See Problems 39 and 40.

CHEM 128 EXAM 3 STUDY GUIDE (for Chapters 16, 17 and 18)

Here are some specific suggestions for what to study. Working the suggested problems at the ends of Chapters 16-18, including assigned homework problems, is an effective way to study the indicated material. Reviewing in-class exercises and class notes will also be helpful.

A list of equations and useful constants will be provided. This list will not include the definitions of molarity, molar mass, and density. You should know how to write the equilibrium constant expression for a balanced reaction equation. Since this expression depends on the reaction, a general equation will not be given.

Note that the summary problems at the ends of the chapters will give you a good review of the basic concepts and computational skills. Here is a list of things you should know or be able to do. The numbers in brackets are references to problems at the end of the chapter that have answers given in the back of the book.

1. Be able to write the reaction equation and the equilibrium constant expression (K_sp expression) for the dissolving of ionic compounds in water. See Ch. 16, Problems 2 and 4.

2. Be able to calculate the concentrations of ions in solution at equilibrium. See Chapter 16, Problems 6 and 8.

3. Be able to calculate a P value and compare it to a K_sp value to determine whether a precipitate will form. See Problems 12 , 14a.

4. Be able to calculate the solubility of an ionic compound in pure water. See Problem 18a.

5. Be able to calculate the solubility of an ionic compound in an aqueous solution containing an ion common to the compound. See Problem 18bc.

6. Know the definition of entropy and be able to (a) predict the spontaneity of a reaction with respect to entropy and (b) calculate the standard entropy change for a reaction. [4, 10, 14]

7. Know the definition of free energy and be able to (a) predict the spontaneity of a reaction from its standard free energy change, and (b) calculate standard free energy change from standard free energies of formation at 25°C or from the Gibbs-Helmholtz equation at other temperatures. [22, 18]

8. Understand the Gibbs-Helmholtz equation. Be able to calculate thermodynamic quantities using this equation. Be able to interpret whether a reaction will be spontaneous or not and under what conditions, if you are given the enthalpy change and the entropy change. Be able to calculate the temperature at which the spontaneity of a reaction will be reversed, if the reaction is affected by temperature. [18, 32, 34, 36, 38]

9. Be able to calculate the free energy change for a reaction under nonstandard conditions. [52]

10. Be able to relate the free energy change for a reaction to the equilibrium constant K for the reaction. [62, 66]

11. Know the basic components of a voltaic cell: anode, cathode, where oxidation and reduction occur in the cell, salt bridge. Understand the notation for a voltaic cell. [4a]

12. Be able to break a balanced redox equation into half-reactions and determine how many electrons are transferred in the reaction.

13. Be able to calculate the standard cell potential E° for a redox reaction, given appropriate information. This includes being able to figure out which species will be oxidized and which will be reduced, based on the reduction potentials for the two species. [16, 18, 20]

14. Understand how the standard reduction potentials are related to the strength of a species as an oxidizing agent or as a reducing agent. [12, 14a]

15. Be able to decide from E° for a reaction whether it will be spontaneous. [24]

16. Understand the relationships between E°, K, and DG° for a reaction. Be able to use the equations (that will be provided) to find out one of these quantities if given another. [36, 42a]

17. Be able to use the Nernst equation to determine (a) E for a cell under nonstandard conditions, and (b) the concentration of a reagent of product under nonstandard conditions. [48, class notes]

18. Understand what an electrolytic cell is. Be able to use information given to calculate grams of a substance produced by electrolysis, time needed to produce a certain amount, etc [58]. [see class notes and in-class exercise sheet]

SAMPLE EXAMS

EXAM 1

EXAM 2

EXAM 3