Name: __________________________Page 1 of 10Chemistry 142 Exam #2 Preparation KEYPart IDirections. You will work out the problems on this page then enter them on the computer. Once you enter them into thecomputer you can submit your work for grading. You can re-enter (and re-score) the problems as many times as you wish. Question 1. A weak acid, HA, has a pKa of 4.05. What is the pH of a 0.149 M solution of HA?Question 2. A weak base, B–, has a pKb of 5.52. A 25.00 mL aliquot of 0.137M B– is titrated with 0.082M HCl.Calculate the pH after 18.05 mL of acid are added.Name: __________________________Page 2 of 10Question 3. A weak base, B–, has a pKb of 5.52. A 25.00 mL aliquot of 0.100M B– is titrated with 0.100M HCl.Calculate the pH after 25.00 mL of acid are added.Question 4. A weak base, B–, has a pKb of 5.52. A 6.63 mL aliquot of 0.148M B– is titrated with 0.154M HCl. Calculatethe pH after 10.17 mL of acid are added.Name: __________________________Page 3 of 10Question 5. 25.00 mL of an acetic acid solution are titrated to the endpoint with 36.00 mL of 0.1234 M KOH solution.The Ka for acetic acid is 1.8 x 10–5. What is the molarity of the acetic acid solution?Question 6. A solution of the compound, X3Z, is found to have a Z3– concentration of 0.00010M. Calculate Ksp for thiscompound. Calculate your answer to 3 significant figures.Question 7. 4.67 g of Ag2SO4 are placed in 1000 mL of distilled water in a 2 L beaker. After sufficient time is allowed,how many g of solid, undissolved silver sulfate remain in the beaker? Ksp for silver sulfate is 1.2x10–5.Assume that the addition of the silver sulfate does not change the volume of the solution.Name: __________________________Page 4 of 10Question 8. Write a balanced net ionic reaction for each of the following dissolution reactions, and use the appropriateKsp and Kf values in the appendix of your text to calculate the equilibrium constant for each.a) Zinc hydroxide in aqueous NaOH to form Zn(OH)42–b) Copper(II) hydroxide in aqueous ammonia to form Cu(NH3)42+Name: __________________________Page 5 of 10c) Silver bromide in aqueous NH3 to form Ag(NH3)2+Question 9. What is the molar solubility of barium hydroxide in water? Would you characterize barium hydroxide asbeing “soluble”? Why?Question 10. What is the molar solubility of barium hydroxide in a pH 9.0 buffer?Name: __________________________Page 6 of 10Question 11. For the vaporization of benzene, ∆Hvap = 30.7 kJ/mol, and ∆Svap = 87.0 J/molK. Calculate ∆Stotal at 70 °C.Question 12. Will benzene boil at 70 °C? Why?Question 13. Calculate, to three significant figures, the normal boiling point for benzene in °C.Name: __________________________Page 7 of 10Question 14. For melting of NaCl, ∆Hfusion = 30.2 kJ/mol, and ∆Sfusion = 28.1 J/molK. Calculate ∆Stotal at 1050 K. Watchyour significant figures.Question 15. Does NaCl melt at 1050 K? Why?Question 16. Calculate the melting point of NaCl. Report your answer in °C. Report your answer to three signficantfigures.Name: __________________________Page 8 of 10Question 17. What ratio of acid to conjugate base ([HA]/[A– ]) is required to prepare a buffer with a pH of 3.6 using lacticacid. Ka for lactic acid, HC3H5O3 is 1.4 × 10–4. Give two methods by which this buffer can be prepared.Answer. Ka = [H3O+][A–]/[HA] = 1.4×10–4 = 10–3.6 [A–]/[HA]; [A–]/[HA] = 0.557; [HA] / [A–] = 1.79One method that this buffer can be made is with a solution of lactic acid and a solution ofsodium lactate. Mix the two solutions in the ratio calculated above for the correct pH.Another method involves the mixing of lactic acid and sodium hydroxide. A solution oflactic acid of sufficient titration is titrated with a solution of NaOH until the lactic acid /lactate ion concentrations are that calculated in the first part of this question.Part IIQuestion 1. Discuss chemical spontaneity in terms of movement towards or away from equilibrium.Answer. A spontaneous process in one where the process is moving towards equilibrium. Aprocess moving away from equilibrium is a non-spontaneous process.Question 2. Discuss thermodynamic stability and kinetic stability.Answer. A stable substance is one that does not readily undergo reaction under a certain setof conditions. This stability can be the result of a slow kinetic process where a reaction istoo slow to occur in a timely manner. A substance can also be thermodynamically stablewhere a reaction is non-spontaneous.Name: __________________________Page 9 of 10Question 3. How can a reaction be endothermic, yet spontaneous? Offer an example with an explanation.Answer. To be spontaneous ∆Stotal needs to be positive. Since ∆Stotal = ∆Ssys + ∆Ssurr, thereare two changes in entropy that must be considered. ∆Ssurr is dependent on the enthalpychange change of the reaction. For an endothermic reaction ∆Ssurr is negative. Thus,∆Ssys must be suffciently positive to counter-act the negative ∆Ssurr. An example of this isthe dissolution of NH4NO3. This is an endothermic process, yet it is spontaneous due tothe entropy-favorable provcess of dissociation.Question 4. Explain the statement, “A disordered state is more probable than an ordered state.” Be sure to offerexamples.Answer. The greater the number of ways that a particular state can be arranged, the higherthe probability of any single state occuring. Consider two flasksconnected by a stopcock.One flask is evacuated and the other is filled with N2. If the stopcock is opened the N2 willspontaneously flow into the evacuated container until the pressure is the same in bothcontainers. While possible, it is highly improbable that the N2 would collect preferentially ina single flask. Therefore, the state with the N2 equally distributed is more probable andmore disordered.Question 5. Explain the statement, “In any spontaneous process at constant temperature and pressure, the free energy ofthe system always decreases.”Answer. ∆G for a spontaneous process is negative and ∆G for all processes is 0 atequilibrium. Since a spontaneous process is heading toward equilibrium, the ∆G for thisprocess is approaching zero, thus decreasing.Question 6. Discuss the notion that free energy is the maximum useful work that can be produced by a system.Answer. In all spontaneous processes there is energy that can be used to do useful work.Some energy, however, goes to heating the surroundings and increasing the entropy ofthe