Chem. 1B Final Practice Test 2 Solutions

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1 First letter of last name Chem. 1B Final Practice Test 2 Solutions Name Print Neatly. You will lose 1 point if I cannot read your name or perm number. Student Number If you are sitting next to someone with the same version of the test, you will be moved and lose 5 points. All work must be shown on the exam for partial credit. Points will be taken off for incorrect or no units. Calculators are allowed. Cell phones may not be used for calculators. On fundamental and challenge problems you must show your work in order to receive credit for the problem. If your cell phone goes off during the exam you will have your exam removed from you. Fundamentals (of 72 possible) Problem 1 (of 13 possible) Problem 2 (of 17 possible) Problem 3 (of 18 possible) Problem 4 (of 20 possible) Multiple Choice (of 60 possible) Extra Credit (of 5 possible) Final Total (of 200 possible) 1

2 Fundamental Questions Each of these fundamental chemistry questions is worth 6 points. You must show work to get credit. Little to no partial credit will be rewarded. Make sure your answer includes the proper units. 1) 6 pts The equilibrium constant K for the following reaction at 900 C is What is KP at this temperature? CS2(g) + 4H2(g) CH4(g) + 2H2S(g) K = [CH 4][H 2 S] 2 [CS 2 ][H 2 ] 4 K P = P CH 4 P H2 S 4 P CS2 P H2 C = n V = P RT K = P CH4 RT P CS2 RT 2 P H2 S R 2 T 2 4 P H2 R 4 T 4 K P = 1 R 2 T 2 K K P = = R 2 T P 2 2 CH 4 P H2 S 4 = R 2 T 2 K P CS2 P P H2 1 (0.0028) ( L atm K )2 (1173K) 2 K P = ) 6 pts When 1 of a fuel burns at constant pressure it produces 3452 k of heat and does 11 k of work. What are the values of ΔE and ΔH for the combustion of the fuel? Report your answer in k q = 3452 k w = 11 k E = q + w = 3452 k + 11 k = 3463 k H = q at constant temperature H = 3452 k 2 3) 6 pts If ΔH is k and ΔS is 0.91 what is ΔG at 75 C and is the reaction K spontaneous. G = H T S = 12.4 k (348K) ( k K ) = 12.7 k Since ΔG is negative the reaction is spontaneous. 2

3 4) 6 pts What is the ar solubility of CaF2 in M LiF? CaF2(s) Ca 2+ (aq) + 2F - (aq) Ca 2+ F - Initial M Change +S +2S Equilibrium S S K sp = [Ca 2+ ] [F ] 2 = S( s) 2 = Since Ksp is small assume S = S(0.010) 2 = S = Check Assumption 2( ) 100% = % Good Molar Solubility S= ) 6 pts ΔG rxn for 2SO2(g) + O2(g) 2SO3(g) is k at C. What is ΔGrxn when the partial pressure of each gas is 100. atm? What is the spontaneous direction of the reaction under these conditions? G = G + RTln(Q) G = G + RTln ( P SO 3 2 ) P O2 2 P SO2 G = k + ( k ) (298.15K)ln ( K ) G = k Since ΔG is negative the reaction is spontaneous in the forward direction. 6) 6 pts What is K for the following reaction at 25 C? Sn 4+ (aq) + 2Ag(s) Sn 2+ (aq) + 2Ag + (aq) E =0.65 V Are there more reactants or products at equilibrium? Sn e - Sn 2+ 2(Ag Ag + + e - ) E = RT nf ln(k) 0.65 V = ( K )(298K) C (2)(96,485 ) ln(k) ln(k) = 51 K = More products 3

4 7) 6 pts The preparations of two aqueous solutions are described in the table below. For each solution, write the chemical formulas of the major species present at equilibrium. You can leave out water itself. Write the chemical formulas of the species that will act as acids in the 'acids' row, the formulas of the species that will act as bases in the 'bases' row, and the formulas of the species that will act as neither acids nor bases in the 'other' row. You will find it useful to keep in mind that NH3 is a weak base. 0.5 of HNO3 is added to 1.0 L of a 0.5 NH3 solution. acids: NH4 + bases: none other: NO3-0.1 of NaOH is added to 1.0 L of a solution that is 0.3 M in both NH3 and NH4Br. acids: NH4 + bases: NH3 other: Na +, Br - 8) 6 pts A solution has a ph of 4.90, what is the poh, [H + ], and [OH - ]? poh = ph = = 9.10 [H + ] = 10 ph = = M [OH ] = = M 9) 6 pts What is Ecell for the following reaction at 25 C? I2(s)+ Fe(s) Fe 2+ (aq, 0.20 M) + 2I - (aq, 0.30 M) E = E RT nf ln(q) Fe Fe e - I2 + 2e - 2I - E =0.44 V E = 0.54 V Fe + I2 Fe I - E =0.98 V Q = [Fe 2+ ][I ] 2 E = 0.98V ( K )(298K) C ln((0.20)(0.30) 2 ) = 1.03 V (2)(96,485 ) 4

5 10) 6 pts A piece of iron of mass 20.0 g at 100. C is placed in a vessel of negligible heat capacity but containing 50.7 g of water at 22.0 C. Calculate the final temperature of the water. Assume that there is no energy lost to the surroundings. q metal = q water q = mc T (20.0 g Fe) (0.45 ) (T g f 100. ) T f = 25 = (50.7 g H 2 O) (4.18 g ) (T f 22.0 ) 11) 6 pts Consider the following exothermic reaction at 25 C and 1 atm. 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) Circle the correct answer for each of the following quantities: w, ΔH, ΔE, ΔS, and ΔG. w<0 w>0 w=0 ΔH<0 ΔH>0 ΔH=0 ΔE<0 ΔE>0 ΔE=0 ΔS<0 ΔS>0 ΔS=0 ΔG<0 ΔG>0 ΔG=0 12) 6 pts Identify if the following reactions are redox reactions or not. If they are redox reactions, specify what is oxidized and what is reduced. Cu(OH)2(s) + 2HNO3(aq) Cu(NO3)2(aq) + 2H2O(l) Not a redox reaction Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) Redox Reaction Fe +3 0 gained electrons Fe is reduced C lost electrons C is oxidized 5

6 Challenge Questions Each of the following short answer questions are worth the noted points. Partial credit will be given. Make sure to show work, report answers to the correct number of significant figures and use the proper units. 1) 13 pts A metal forms the salt MCl3. Electrolysis of the ten salt with a current of A for 6.63 h produced 3.00 g of the metal. What is the ar mass of the metal? Determine the es of electrons n = It F 60 min 60 s t = 6.63h ( ) ( ) = 23,900 s 1 h 1 min n = It (0.700 A)(23,900 s) = F 96,485 C = Because M forms the compound MCl3 the ion that M forms is M 3+ M e - M Determine the es of M e ( 1 M 3 e Determine ar mass M 3.00 g g = ) = M 6

7 2) 17 pts Consider the process A(l, 75 C) A(g, 155 C) which is carried out at constant pressure. The total ΔS for this process is known to be For A(l) and A(g), K CP values are 75.0, and K 29.0, respectively, and are not dependent on temperature. Calculate K ΔHvap for A(l) at 125 C (its boiling point). Break this process into 3 steps A(l) (75 C to 125 C) A(l) to Al(g) 125 C A(g) (125 C to 155 C) Step 1: S = C P ln ( T 2 T 1 ) = (75.0 )ln (398K ) = 10.1 K 348K K Step 2: Phase change, therefore, T is constant. The problem indicates that P is also constant. Step 3: S = q T = H vap T = H vap 398K S = C P ln ( T 2 T 1 ) = (29.0 ) ln (428K K 398K ) = 2.11 S tot = H vap K 398 K = 75.0 K H vap = k = 25.0 K K 7

8 3) Calculate the number of es of HCl(g) that must be added to 1.0 L of 1.0 M NaC2H3O2 to produce a solution buffered at each ph. 3a) 6 pts ph = pka Na +, C2H3O2 -, H +, and Cl - Yes a reaction will go to completion H + + C2H3O2 - HC2H3O2 H + C2H3O2 - HC3H5O2 Initial () x Final () x x Major Species: C2H3O2 -, HC2H3O2, Na +, and Cl - Buffer When the ph = pka [HC2H3O2] = [C2H3O2 - ] [A 1.0 x ] V x [HA] V x = es of HCl is needed 3b) 6 pts ph = 4.20 Still have a buffer 3c) 6 pts ph = 5.00 ph = pk a + log ( [A ] [HA] ) pk a = log( ) = x 4.20 = log ( V x ) V 1.0 x 0.29 = x x = 0.78 M 0.78 es of HCl is needed. Still have a buffer ph = pk a + log ( [A ] [HA] ) 5.00 = log ( 1.0 x 1.8 = x x = 0.36 M 0.36 es of HCl is needed. 1.0 x V x ) V 8

9 4a) 10 pts A technician carries out the reaction 2SO2(g) + O2(g) 2SO3(g) at 25 C and 1.00 atm in a constant-pressure cylinder fitted with a piston. Initially, SO2 and O2 are present in the cylinder. The technician then adds a catalyst to initiate the reaction. How much work takes place, and is it done by the system or on the system. Assume that the reaction goes to completion and the temperature of the system is constant. w = P V Determine the change in volume Determine initial volume PV = nrt L atm ( )( K = )(298K) V = nrt = 1.47 L P 1.00 atm Determine the limiting reagent and final es of substances SO2 (L.R) O2 SO3 I F Total es of gas in the container after the reaction comes to completion = Determine final volume V = nrt L atm ( )( = K )(298K) = 1.10 L P 1.00 atm Calculate work V = 1.10L 1.47L = 0.37L Pa P = 1.00atm ( ) = Pa 1 atm w = P V = (1.00 atm)( 0.37 L) = 0.37 L atm L atm ( 1 L atm Work is done on the system 4b) 6 pts What is the change in enthalpy for the reaction in? H rxn = H f (prod) H f (react) H rxn = 2 H f (SO 3 ) 2 H f (SO 2 ) H f (O 2 ) H rxn = 2 ( 396 k k k ) 2 ( 297 ) (0 ) = 198 k This is the amount of heat released when 1 e of O 2 reacts with 2 of SO 2 Calculate ΔH for O k 0.15 O 2 ( ) = 3.0 k = O 2 4c) 4 pts What is the change in internal energy for the reaction in? Since the reaction takes place at constant pressure ΔH = q E = q + w = =

10 Multiple Choice Questions Each of the following multiple choice questions are worth 5 points. Your answers need to be filled in on the Scantron provided. Note: Your Scantrons will not be returned to you, therefore, for your records, you may want to mark your answers on this sheet. On the Scantron you need to fill in your perm number, test version, and name. Failure to do any of these things will result in the loss of 1 point. Your perm number is placed and bubbled in under the ID number. Do not skip boxes or put in a hyphen. In addition, leave bubbles blank under any unused boxes. The version number (A) is bubbled in under the test form. 1.The ph of a 0.10 M solution of a weak base is What is the Kb for this base? A) B) C) D) E) None of the above 2. For the vaporization of a liquid at a given pressure, A) G is negative at all temperatures. B) G is negative at low temperatures but positive at high temperatures (and zero at some temperature). C) G is positive at all temperatures. D) G is positive at low temperatures but negative at high temperatures (and zero at some temperature). 3. Predict which of the following reactions has a negative entropy change. I. CH4(g) + 2O2(g) CO2(g) + 2H2O(l) II. NH3(g) + HCl(g) NH4Cl(s) III. 2KClO4(s) 2KClO3(s) + O2(g) A) II B) I and II C) I D) III E) II and III 10

11 4.Arrange the following 0.10 M solutions from lowest to highest ph: NaF, NaC2H3O2, C5H5NHCl, KOH, HCN. (Ka for HCN is ; Ka for HF is ; Ka for HC2H3O2 is ; and Kb for C5H5N is ) A) HCN, C5H5NHCl, NaF, NaC2H3O2, KOH B) C5H5NHCl, HCN, NaF, NaC2H3O2, KOH C) NaF, NaC2H3O2, HCN, C5H5NHCl, KOH D) KOH, NaC2H3O2, NaF, HCN, C5H5NHCl E) None of the above 5. For a reaction in a voltaic cell, both H and S are positive. Which of the following statements is true? A) G > 0 for all temperatures. B) E cell will increase with an increase in temperature. C) E cell will decrease with an increase in temperature. D) E cell will not change when the temperature increases. E) None of the above statements is true. Use the following to answer questions 4-5: Consider the equation 2A(g) 2B(g) + C(g). At a particular temperature, K = Raising the pressure by lowering the volume of the container will A) cannot be determined B) none of the these C) cause [B] to increase. D) have no effect. E) cause [A] to increase. 7. If you mixed 5.0 B, 0.10 C, and A in a 1-L container, in which direction would the reaction initially proceed? A) The above mixture is the equilibrium mixture. B) To the left. C) We cannot tell from the information given. D) To the right. 8. A concentration cell is constructed using two Ni electrodes with Ni 2+ concentrations of 1.26 M (cathode) and M (anode) in the two half-cells. The reduction potential of Ni 2+ is 0.23 V. Calculate the potential of the cell at 25 C. A) V B) V C) V D) V E) V 11

12 9. At 25 C, the following heats of reaction are known: 2C2H2 + 5O2 4CO2 + 2H2O H = k C + O2 CO2 H = 394 k 2H2 + O2 2H2O H = 572 k At the same temperature, calculate ΔH for the following reaction: 2C + H2 C2H2 H =? A) 2422 k B) 226 k C) -2422k D) -226 k E) none of these 10. Of the following five ions or ecules, which is the strongest reducing agent? A) Cl2 B) Fe 2+ C) Cr 2+ D) F - E) H2 11. Ammonium metavanadate reacts with sulfur dioxide in acidic solution as follows (hydrogen ions and H2O omitted): xvo3 + yso2 xvo 2+ + yso4 2 The ratio x : y is A) 2 : 1 B) 1 : 1 C) 1 : 3 D) 1 : 2 E) None of the Above 12. The following reaction has a G value of 42.6 k/ at 25 C. HB(aq) + H2O(l) H3O + (aq) + B (aq) Calculate Ka for the acid HB. A) B) 14.0 C) 42,600 D) 17.2 E) 1.63 B, D, B, B, B, E, B, A, B, C, A, A 12

Chem. 1B Final Practice

Chem. 1B Final Practice Name Student Number All work must be shown on the exam for partial credit. Points will be taken off for incorrect or no units and for the incorrect number of significant figures.