Chemistr 31 Final Exam Ma 0, 013 Name Lab Section # Some Useful Equations and Constants: Addition/subtraction: multiplication/division: Exponents: a + b or a b S S + S a b Or Error (error 1 ) + (error ) +... Add & sub use abs., Mult & div use rel. Standard deviation: a b or a / b Confidence Intervals: t-based: µ x ± t s n S Sa a s!(x i " x) n "1 Sb + b a S Mean Value: n S a n a x n x i n number of samples, t student t value ΔG ΔG + RTlnQ, ΔG RTlnK (at equilibrium), G H T S J R 8.314 mol K 0.0806 L atm mol K, 0 C 73 K h 6.66 x 10 34 J sec c.998 x 10 8 m sec E h! h c! Abs!bc hc! Avagadro's number 6.0 x 103 K [s ] [s 1 ] k t r t m t m K V s V m µ 1 ( Mi (Zi) ) γ 10 0.510 Z! 1 + ( size 305! ) Be careful with these. ph pka + Log [ A ] [ HA ] or Log γ 0.510 Z! 1 + ( size 305! ) ph 1 ( pk1 + pk ) 1
As announced in class, this exam was taken directl from homework. Problems chosen for the exam are listed here. In the answer pages of the exam the homework problem number will be listed in green. Chapter 1: Problems: 1, 3, 37 Chapter 3: Problems:, 11, 16, and 3-A Chapter 4: Problems: 11 Chapter 6: Problems: 8, 1 Chapter 7: Problems: 3, 9, 1 Chapter 8: Problems: 3, 6 Chapter 10: Problems: 3, Chapter 17: Problems: 1, 4, 11, Chapter : Problems: 1,, -C
Multiple Choice / Fill in the Blank / Short Answer Section. Onl one correct answer for multiple choice questions unless otherwise stated. 1.(4 pts)1-1 The SI base units for length, mass, time and amount are: a) meter, kilogram, second, and mole b) centimeter, gram, second, and mole c) meter, pound, minute and dozen d) meter, gram, minute, and mole..(4 pts)1-37 The point in a titration when an obvious color change occurs is called the ( end point or equivalence point )? 3.3-11 Suppose that in a gravimetric analsis, ou forget to dr the filter crucibles before weighing them for the empt weight. After filtering the product, ou dr the product and crucibles thoroughl before weighing them. Empt weight alwas high. (4 pts) Is the apparent mass of product alwas high or alwas low? (4 pts) Is the error in mass sstemtic or random? 4.(4 pts)1-3 A bottle of concentrated aqueous sulfuric acid, labeled 98.0 wt% H SO 4, has a concentration of 18.0 M. How man milliliters of this should be used to make 1.000 L of 1.00 M H SO 4? Dilution M1V1 MV 1 L 1 M V1 0.05555 L 55.5 ml. 18 M 5.(4 pts)3- Round the number 0.1451 to two significant figures. 0.15 6.(4 pts) Rewrite the number 3.13456 ( ± 0.16789 % ) in the form: number ( ± absolute uncertaint ) with an appropriate number of digits. 3.13456 x 0.16789 % 3.13456 x 0.0016789 0.0054 3.13 ( ± 0.005 ) 7.(4 pts)6-1 Given the following constants for solubilit: AgCl Ksp 1.8 x 10 10 AgI Ksp 8.3 x 10 17 AgBr Ksp 5.0 x 10 13 Ag CrO 4 Ksp 1. x 10 1 If a solution containing 0.10 M Cl, Br, I, and CrO 4, is treated with Ag +, in what order will the anions precipitate? First I, Br, Cl, and CrO 4 Last Since each of these ions starts at 0.10 M, use that value in each K equation to find the Ag + necessar to start precipitation: for I Ag + 8.3 x 10 16, for Br Ag + 5.0 x 10 1, for Cl Ag + 1.8 x 10 9, and for CrO 4 Ag + 1. x 10 11 3.5 x 10 6 3
8.(4 pts) Assuming ou have correct molarities and volumes, a buffer can 8-6 be made b which of the following methods? Circle all that work to make a buffer. There ma be more than one. (a) add HCl to NaOH strong acid with strong base does NOT make a buffer. (b) add HCl to NaC H 3 O Using correct molarities and volumes (c) add HC H 3 O to NaC H 3 O these three all give a weak acid in (d) add HC H 3 O to NaOH solution with its conjugate base. 9.(4 pts) If ou are extracting a substance from water into hexane is it more effective -1 to do one extraction with 300 ml of hexane or three extractions with 100 ml? 10.(4 pts) The energies of which part of the electromagnetic spectrum corresponds 17-4 mostl to stretching and bending of molecules? (a) X-ras (b) UltraViolet (c) Visible (d) InfraRed (e) Microwave 11.(4 pts) Fill in the blank. If ou double the frequenc of electromagnetic 17-1 radiation ou ( double ) the energ. more wiggles more energ. Also E h! 4
B. Problem Section. Show all needed calculations to receive full credit. The number of points are shown in parentheses. Use the back side of the page if needed. Do NOT use activities or mass/charge balance unless told to!!! 1.3-11 The uncertainties indicated are all absolute uncertainies. Perform the calculation and write the answer with the correct number of figures based on uncertaint and indicate both the absolute and relative uncertaint. 6.843 ( ± 0.008 ) x 10 4.09 ( ± 0.04 ) 1.63 ( ± 0.01 ) 6.843 x 10 4 (a)(5 pts) Answer 1.48761 x 10 5 1.5 x 10 5.09 1.63 (b)(5 pts) relative uncertaint 0.09 or 9% denominator: 0.04 + 0.01 0.0413 abs 0.46 0.08963 rel numerator: 0.008 x 10 4 6.843 x 10 4 0.00117 rel Division: (0.00117) + (0.08963) 0.08964 rel or 0.09 or 9%. (c)(5 pts) absolute uncertant 0.1 x 10 5 1.48761 x 10 5 x 0.08964 1.33 x 10 4 or 0.1 x 10 5..(5 pts)17-11 The absorbance of a 0.0013 M compound is 0.8 at a wavelength of 66 nm in a 10.0 cm sample cell. Calculate the molar absorbtivit of the compound. Abs!bc so! Abs b c 0.8 (10 cm) (0.0013 M) 66.8 cm 1 M 1 3.-, -c Solvent passes through a column in.0 minutes, but solute requires 8.0 minutes. (a)(5 pts) Calculate the retention factor. k t r t m t m 8.0.0.0 6.0.0 3.0 (b)(5 pts) What fraction of time is the solute in the mobile phase in the column?.0 min in mobile phase 8.0 min total in column 0.5 5
4. The dibasic compound B ( pkb1 4.00 and pkb 8.00) was titrated with 1.00 M HCl. The initial solution of B was 100.0 ml of 0.10 M B. Find the ph at the following volumes of acid added. 10-3 (a)(5 pts) 0 No acid. B + H O HB + + OH (b)(5 pts) Kb1 x reacts 0.10 M x x x x 0.10 5 ml 1.0 x 10 4. x 1.0 x 10 5 x [OH ] 3. x 10 3 poh.50 ph 11.50 B + H + HB + Half wa to equivalence point 1. [B] [HB + ] I 0.010 0.005 0 ICE tables use moles. K uses molarit. C.005 0.005 +0.005 poh pkb1 4.0 E 0.05 0 0.05 ph 10.00 (c)(5 pts) 10 ml B + H + HB + Equivalence point 1. HB + is both acid and base. I 0.010 0.010 0 must either use sstematic method or approx. 1 C.010 0.010 +0.010 poh (pkb1 + pkb) 1 (4.00 + 8.00) E 0 0 0.010 poh 6.00 ph 8.00 (d)(5 pts) 15 ml can start from E.P. 1, or from original. both work HB + + H + H B + Half wa to eq. point. [H B + ] [HB + ] I 0.010 0.005 0 C.005 0.005 +0.005 poh pkb 8.0 E 0.005 0 0.005 ph 6.00 (e)(5 pts) 0 ml HB + + H + H B + Equivalence point. Onl H B + I 0.010 0.010 0 must switch to Ka1. C.010 0.010 +0.010 E 0 0 0.010 0.010 mol in 0.10 L 0.08333 M. New eqn. H B + HB + + H + Ka1 x dissoc. 0.08333 x x x Ka1 1.0 x 10 6 x 0.08333 Kw Kb 1.0 x 10 14 1.0 x 10 8 1.0 x 10 6 x [H + ].887 x 10 4 ph 3.54 6
On this page some, or all problems require sstematic method. Notice this!!! 8-3 5.(a)(5 pts) Calculate the ph of a 5.0 x 10 8 M NaOH solution. small amt base in water. [Na + ] 5.0 x 10 8 M [H + ] [OH ] 1.0 x 10 14. charge bal [OH ] [Na + ] + [H + ] [H + ] ( 5.0 x 10 8 + [H + ] ) 1.0 x 10 14. substitute [OH ] 5.0 x 10 8 + [H + ] [H + ] + 5.0 x 10 8 [H + ] 1.0 x 10 14 0 [H + ] 5.0 x 10 8 ± (5.0 x 10 8 ) + 4.0 x 10 14 5.0 x 10 8 +.0616 x 10 7 [H + ] 7.808 x 10 8 ph 7.107 7.11 basic, not acidic. (b)(5 pts) What fraction of the OH in the solution in (a) comes from dissociation of water? [OH ]total 5.0 x 10 8 + [H + ] 5.0 x 10 8 + 7.808 x 10 8 1.808 x 10 7 [OH ]from water [H + ] 7.808 x 10 8 Fraction 7.808 x 10 8 1.81 x 10 7 0.6096 61% from water. (Most of it.) 6.7-1 Write charge and mass balances for a solution made b dissolving MgBr to give Mg +, Br, MgBr +, and MgOH +. (hint write equations first) * MgBr Mg + + Br * so [Br ] [Mg + ] * Mg + + Br MgBr + Mg + + OH MgOH + H O H + + OH (a)(5 pts) Charge [positives] [negatives] [Mg + ] + [MgBr + ] + [MgOH + ] + [H + ] [Br ] + [OH ] (b)(5 pts) Mass from the * equation above, [all Br] [all Mg] or [Br ] + [MgBr + ] ( [Mg + ] + [MgBr + ] + [MgOH + ] ) [Br ] [Mg + ] + [MgBr + ] + [MgOH + ] another focuses on the hdrogen / hdroxide relationship: [H + ] [MgOH + ] + [OH ] 7
7.(6 pts)7-3 Calculate the ionic strength of a 0.0000 M La(IO 3 ) 3 solution (assuming complete dissociation at this low concentration and no other reactions in solution). La(IO 3 ) 3 La 3+ + 3 IO 3 0.0000 0.0000 0.00060 1 µ ( (0.0000) (3) + (0.00060) ( 1) 1 ) ( 0.00180 + 0.00060 ) 1 µ ( 0.0040 ) 0.0010 8.(6 pts)7-9 Calculate the molar solubilit of AgCl in water if Ksp 1.8 x 10 10, and γag + 0.964, and γcl 0.964. AgCl(s) Ag + (aq) + Cl (aq) Ksp 1.8 x 10 10 [Ag + ] (0.964) [Cl ] (0.964) x (0.964) x 1.8 x 10 10 (0.964) 1.9369 x 10 10 x 1.36 x 10 5 9.(6 pts)6-8 For the reaction HCO 3 H + + CO 3 G + 59.0 kj mol at 98 K. Find the value of the equilibrium constant for the reaction. G R T ln K ln K 3.81 59000 J mol (8.314 J mol K ) (98 K) ln K K e 3.81 4.55 x 10 11 1.4-11The percentage of an additive in gasoline was measured six times with the following results: 0.13%, 0.1%, 0.16%, 0.17%, 0.0%, and 0.11%. For the percentage of the additive find: (a)(5 pts) Average 0.13% + 0.1% + 0.16% + 0.17% + 0.0% + 0.11% 6 0.148% (b)(5 pts) Standard Deviation s (.148.13) + (.148.1) + (.148.16) + (.148.17) + (.148.0) + (.148.11) 5 s 0.0343% or s 0.03% (c)(5 pts) 90% Confidence interval expressed as x ± C.I. From table 4 the 90% Student's t value with ( 6 1 ) 5 degrees of freedom.015. (.015) (0.0343) C.I. 0.08 6 answer is: 0.15% ± 0.03% 8