CHEMISTRY 321/323 EXAM 2 October 21, 2008

1. There are twenty (20) multiple-choice questions. Code answers for the multiple-choice questions on the scan sheet.

2. Write your name and student 10 number on the answer sheet. 3. Write your Graduate Instructors name on the line for Ulnstructor" on the

answer sheet. 4. Use a #2 HB pencil to code all information onto the answer sheet. 5. Code your name and student 10 number on the answer sheet. 6. Code 0321 or 0323 as the ·'Section Number" on the answer sheet. 7. Code the best answer to each question on the answer sheet. 8. Relevant equations, figures, and tables are provided. Use the information

provided as needed to answer any of the questions. 9. Each multiple-choice question is worth ten (10) points. The total score for this

exam is two hundred (200) points.

CONFIRM THAT YOUR EXAM INCLUDES ALL TWENTY (20) QUESTIONS

1.

I. II. 111. IV. V.

2.

3.

In a typical UV/vis absorption experiment involving an aqueous solution (wavelengths longer than 180 nm and shorter than 780 nm), the absorbed radiation excites the following motion(s):

electron motion in strong single bonds high frequency vibration electron motion in multiple bonds electron motion in non-bonding orbitals on heteroatoms high frequency rotation

a. land III b. II and V c. I, III, and IV @ III and IV e. all of the motions listed in I-V

When a calibration curve is generated under conditions of low stray light but results for the unknown are collected under conditions of intense stray light, the concentration of the unknown derived from the calibration curve:

a. varies randomly above and below the true value. b. is lower than the true value and the difference is independent of

concentration. c. is higher than the true value and the difference is independent of

concentration. d. is higher than the true concentration and the deviation increases

with increasing concentration. is lower than the true concentration and the deviation increases with increasing concentration.

Which of the following is least likely to give rise to a non-linear dependence of absorbance (A) on concentration (C)?

a. stray light b. high concentration c. polychromatic light

C9J low molar absorptivity e. all of the above can give rise to non-linear A vs. C behavior

4. Which of the curves in Figure 1 best represents the shape of the concentration error, se, (dependent variable) versus concentration (independent variable) curve in an absorbance measurement?

a. 1 b. 2 c. 3 ~ 4 e. none of the above.

5. For a method with a linear response vs. concentration, a plot of the sensitivity of the method versus concentration is best represented in Figure 1 by curvenine:

a. 1 b. 2 c. 3 d. 4

@ 5

6. Which of the plots in Figure 1 best represents the shape of the relative concentration error, sclC, at low concentration?

@ 1 b. 2 c. 3 d. 4 e. 6

7. The use of a blank solution to establish zero absorbance minimizes errors resulting from:

a. stray light b. wavelength error

@ light losses that are independent of the analyte d. concentration error e. fingerprints

A solution of molecule X was subjected to an absorbance measurement at 214 nm (£ = 30.0 cm-1'M-1

) using a cuvette with a pathlength of 2.0 em. Use this information to answer questions 8 and 9.

8. What units are associated with the sensitivity of the response? (i.e., absorbance) to changes in concentration for this measurement? (au = absorbance units)

tbc ::"V\:~\EtrJ a. cm-1'M-1

@ M-1 • L' M- 1CiA fI\.) •• £v ,~c. aU'cm-2'M-1

d. M·cm2·au·1

e. em

9. If there is a systematic error in absorbance of +0.15 at the concentration of the solution of X subjected to measurement, what will be the resulting systematic error in the determination of the concentration of X?

a. 7.5 x 10-2 M ~ :: ~~ .. ::. <t. \? b. 4.5 M ~c IJ (/ () I s- 3

@ 2.5x10-3M /Jc= ----: • : 2.~'<ro- f1 d. 5.0x10-4M £b (30.0)(1.) e. Insufficient information provided

10. In an absorbance measurement using a cuvette with an inner diameter of 1.0 em, the fraction of the transmitted light was PIPo =217. If the measurement were to be repeated using a cuvette of 2.0 em, what value of PIPo would be expected? f A

,... -;:. /~ :: I,) a. 0.29 ' .-.- 1. b. 0.04 ~)., C ':: A ': - \t>~ I = ~ \°3 ~

@ 0.08 1. r b , ()d. 0.14 _ l~ ~ :: A :. O."""'Lf -:. to b c. .\-0 r ~. G-.

e. 0.53

k{J r b ~ 2.0 (.,tNt-J

f}-:-. f .tJ»1S _,4 ~,.o1rB I =:a I~ ::: I 0 0;::: O. 0 ~

For all acidlbase problems, assume pKw =14.0, all solutions are aqueous, and all activity coefficients are unity. Any Ka or I<t> values that may be needed can be found or derived from the attached tables at the end of the exam.

11. A buffer solution was prepared from sodium hypochlorite (NaCIO) and hypochlorous acid to give a pH of 7.30. What is the [CIO-] I [HCIO] ratio in this solution (assume NaCIO to be highly water soluble)?

~::::3.0 )'1 0 -8' a. 1.00

[cfOJb. 1.04 c. 0.96 f l-\-: p h" 0.. + l~J E. If c. 'rD d. 1.67

@ 0.60 selo-l ::: o~~ 0

C(~Cto7

-r12. What is the pH of a 0.010 M ammonia solution? (.<'0 -:. I. g X 10 rt

a. 4.7 lOH/ -:: JKlo C(J -= ~.b\(,O-.r). (O.Df)b. 12.0

@ 10.6 -Lt -::: '{.2,L( ~ 10 ~

d. 2.0 e. 3.4 pO» :: J. l( :. p t1 ~ IO.J,

13. Which pH indicator would be the most appropriate for the titration of a 50.00 mL solution of 0.300 M formic acid, HCOOH(aq), with 0.150 M sodium hydroxide solution? -l. ~ C- :: V C.

f'H @. e,. r'" ,." f1 J? 8. Bromothymol Blue pKa =6.8 .J _ (O,os)(c>.3) b. Methyl Orange pKa =3.8 vB -- -: 0, \ L

(o.lr)c. Methyl Red pKa = 5.1 @ Thymol Blue pKa =8.8 e. V-r= 0.\ .. o.oS-~ O,IS- L..Trinitrobenzene pKa = 13.0

c --:: o .0 t S- \*A 0 \~ s A

cJ · l ) 1

14. Which of the following acids mixed with its sodium salt would be most appropriate for forming a buffer at pH = 8.7 with a high capacity?

,Name I, Formula

~Acetic

,Benzoic

IChloroacetic

~Cyanic

IFormic

,Hydrocyanic

,Hydrofluoric

IHydrogen chromate Ion

IHypobromous

~Hypochlorous

IHypoiodous

IIodic

ILactic

INitrous

IPhenol

IPropionic

II HC2H3~

II HC7Hs0 2

II HC2H2~CI

II HCNO

I, HCH02

II HCN

II HF

I, HCr04 ­

II lIBrO

II HCIO

II RIO

II RI03

II HC~S03

II HN02

II HCJIsO

II HCJIs~

II II II II II ,I II

II" II II II II ,I II

II"

Kat 1.8* 10-s

6.5*10-s

1.4*10-3

3.5*10-4

I I I I I

1.8*10-4 I 4.9*10-10 I 6.8*10-4 I 3.0*10-7 I 2*10-9 I

3.0*10-8 I 2*10-11

r

1.7*10-1 I 1.4*10-4 I 4.5*10-4 I 1.3*10-10 ! 1.3*10-s I

8. hydrocyanic acid b. lactic acid

@ hypobromous d. hypochlorous acid 8. any acid in the table with its sodium salt could be used to make

such a buffer and all would have the same buffer capacity

15. What is the pH of a 1.00 mM aqueous hypoiodous acid solution?

~ 6.8 ~tt -=- l. )C I{) -II

" -,..,b. 7.0 C(),"~f)(~KtO- )~ ~;c.ICl

c. 15.0 ()} .Of' l. ~ 10'0

d. 3.0 2.~\"-" e. none of the above

[""i""]-== JKCAC.~A .. 1C~

16. What is the pH of a 0.1 M aqueous solution of sodium phenoxide? (Assume high solubility for this compound.) kw - r

-10 K - --- -= ~ .. 7-~ ,0Ko.. :: I.J )ColO ".. '0-- r:::o... 8. 2.6

(6) 11.4 ((J ,rj = J-1.~ It 10- ':" (t) • \)

c. 1.0 d. 13.0 -::. 2.1-=7-¥ ,t) -JJ1 e. none of the above ,o0M ~ 1..6

f C~ ~ \1-"1

Use the following information regarding an acid, abbreviated as H2P, at 25°C in water to answer question 17:

PKa1 =3.2, PKa2 = 5.8

17. What is the pH expected for a 0.01 M aqueous solution of NaHP? (Assume that NaHP is highly soluble in water.) _ ~

IA" -= I J I 'f. \0-&.\ • f'<ct ::. I." )' to a. 9.0 r'~\ '0. J~

b. 4.3 (r-{~J ':. 1~.· t<.." -;:. 3. Jb 'lC \0- r c. 2.6

<a:> 4.5 11 H~ y. S­e. 6.1 l'

Use the following information to answer questions 18-20. 46.5 mL of a 0.10 M NaOH solution was required to reach the equivalence point of a monoprotic acid (Ka =6.5 x 10-5 M) initially present in a 30 mL solution.

18. What is the pH at the equivalence point? .5.1+ trf ~ ~k a.c.:o.t (L CJ L((, .r l.) (0·' f\) :. '1. ,,5'" )( 10-

3"'""0 I•.r 8.5® C 4 ·'~;(/O-J l...'

b~ 5~5 p,,~ -= J = b~ II{ 10- M @ eq.-pT;;

c. 1.2 (()-O;/l...T o .. O"l,~tJ."..--------------­d. 9.8 (0 H-J ~ J~ C,Nae1 ~ ~.S"JC/o-'OJ(~./,t IO-~-e. 7.0

= 3.' Xl0- h pOW: ~.~ pH =- "j.f'

19. What is the pH after addition of 10 mL of the NaOH solution? \ ~ -J A~

(O .. O(L) (O,lo~J -= t)C\O ~o\e" 0"'- .·.10 ~o\"" 11

a. 4.2 b. 8.7 c~ 4~3

@ 3~6

e. 4.8

20. What is the pH after addition of 50 mL of NaOH solution?

~o"""" L - Lfb- S' iN" L -;a J. S '"'" L ~ ~ C 4"S'S ~s e Sol'",.a. 2.4 v ~

@ 11.6 c. 11.8 d. 12.1 e. 13.0

Aqueous Equilibrium Constants Acid Dissociation Constants (25 C)

Name Formula Ka1 Ka2 Ka3 Acetic HC2H3O2 1.8*10-5 Arsenic H3AsO4 5.6*10-3 1.0*10-7 3.0*10-12 Arsenous H3AsO3 6*10-10 Ascorbic HC6H7O6 8.0*10-5 1.6*10-12 Benzoic HC7H5O2 6.5*10-5 Boric H3BO3 5.8*10-10 Carbonic H2CO3 4.3*10-7 5.6*10-11 Chloroacetic HC2H2O2Cl 1.4*10-3 Citric H3C6H5O7 7.4*10-4 1.7*10-5 4.0*10-7 Cyanic HCNO 3.5*10-4 Formic HCHO2 1.8*10-4 Hydroazoic HN3 1.9*10-5 Hydrocyanic HCN 4.9*10-10 Hydrofluoric HF 6.8*10-4 Hydrogen chromate Ion HCrO4

- 3.0*10-7 Hydrogen peroxide H2O2 2.4*10-12 Hydrogen selenate ion HSeO4

- 2.2*10-2 Hydrogen sulfide H2S 5.7*10-8 1.3*10-13 Hypobromous HBrO 2*10-9 Hypochlorous HClO 3.0*10-8 Hypoiodous HIO 2*10-11 Iodic HIO3 1.7*10-1 Lactic HC3H5O3 1.4*10-4 Malonic H2C3H2O4 1.5*10-3 2.0*10-6 Nitrous HNO2 4.5*10-4 Oxalic H2C2O4 5.9*10-2 6.4*10-5 Paraperiodic H5IO6 2.8*10-2 5.3*10-9 Phenol HC6H5O 1.3*10-10 Phosphoric H3PO4 7.5*10-3 6.2*10-8 4.2*10-13 Propionic HC3H5O2 1.3*10-5 Pyrophosphoric H4P2O7 3.0*10-2 4.4*10-3 Selenous H2SeO3 2.3*10-3 5.3*10-9 Sulfuric H2SO4 Strong Acid 1.2*10-2 Sulfurous H2SO3 1.7*10-2 6.4*10-8 Tartaric H2C4H4O6 1.0*10-3 4.6*10-5

Base Dissociation Constants (25 C)

Name Formula Kb Ammonia NH3 1.8*10-5 Aniline C6H5NH2 4.3*10-10 Dimethlyamine (CH3)2NH 5.4*10-4 Ethylamine C2H5NH2 6.4*10-4 Hydrazine H2NNH2 1.3*10-6 Hydroxylamine HONH2 1.1*10-8 Methylamine CH3NH2 4.4*10-4 Pyridine C5H5N 1.7*10-9 Trimethylamine (CH3)CN 6.4*10-5

P/P0 = 10-εbc T = P/P0 A = -log T T = 10-A

A = εbC c = νλ λ = hc/E

sC = sA/ΦA,C sA=(-)0.434sT/T concentration error = measurement error/sensitivity Error propagation in arithmetic calculations: Example: Standard deviation of y (sy or sy/y)

cbay −+= 222cbay ssss ++=

cbay ⋅

= 222

⎟⎠⎞

⎜⎝⎛+⎟

⎠⎞

⎜⎝⎛+⎟

⎠⎞

⎜⎝⎛=

cs

bs

as

ys cbay

xay = ⎟⎠⎞

⎜⎝⎛=

as

xys ay

ay 10log= as

s ay 434.0=

aaantiy 10log10 == a

y sys

303.2=

For aA + bB cC + dD

[ ] [ ][ ] [ ]ba

dc

BADCK =

[ ][ ] [ ][ ] CatOHHOHOHKw

0143 2510−−+−+ ===

at 25 ºC, pH + pOH = 14

HA + H2O A- + H3O+ [ ][ ]

[ ]HAOHAKa

+−

= 3

A- + H2O HA + OH-

[ ][ ][ ]−

−

=AOHHAKb

wba KKK =

For HnA, Ka for full deprotonation = Ka1Ka2...Kan

Strong acid/base: [H3O+] ≈ CHA > 5 x 10-7 M

24

][2

3wHAHA KCC

OH++

=+ < 5 x 10-7 M

[OH-] ≈ CXOH > 5 x 10-7 M

24

][2

wXOHXOH KCCOH

++=−

< 5 x 10-7 M

Weak acid CHBKa ≥ 10-13, CHB/Ka ≥ 100 HBaCKH ≅+ ][

CHBKa < 10-13, CHB/Ka ≥ 100 wHBa KCKH +≅+ ][

CHBKa ≥ 10-13, CHB/Ka < 100 24

][2

HBaaa CKKKH

++−=+

CHBKa < 10-13, CHB/Ka < 100 exact solution Exact: [H+]3 + (CB-+Ka)[H+]2-(KaCHB+Kw)[H+]-KaKw=0

Weak Base

CBKb ≥ 10-13, CB/Kb ≥ 100 BbCKOH ≅− ][

CBKb < 10-13, CB/Kb ≥ 100 wBb KCKOH +≅− ][

CBKb ≥ 10-13, CB/Kb < 100 24

][2

Bbbb CKKKOH

++−=−

CBKb < 10-13, CB/Kb < 100 exact solution [OH-]3 + Kb[OH-]2-(KbCB+Kw)[OH-]-KbKw=0 exact

Buffer:

CHBKa ≥ 10-13, CHB/Ka ≥ 100

[ ]NaA

HAa C

CKOH ≅+3 ,

HA

NaAa C

CpKpH log+=

refined approximation: if CNaB, CHB are both ≥ 2 x 10-6 M

[ ] ( ) ( ) ( )2

42wHBaaNaBaNaB KCKKCKC

H+++++−

≈+

exact solution:

[H+]3 + (CNaB+Ka)[H+]2-(KaCHB+Kw)[H+]-KaKw=0

Salt of weak acid:

CNaBKb ≥ 10-13, CNaB/Kb ≥ 100 [ ] NaBbCKOH =−

refined approximation:

[ ]2

)(42wNaBbbb KCKKK

OH+++−

=−

exact solution:

0 = [OH-]3 + Kb[OH-]2 – (KbCNaB+Kw)[OH-] - KbKw

Amphiprotic salt:

[ ] [ ][ ] 1

2

1 a

wa

KHAKHAK

H −

−+

++

=

If [HA-] ≈ CNaHA, [ ]1

2

1 aNaHA

wNaHAa

KCKCKH

++

≈+

If CNaHA/Ka1 >> 1 and Ka2CNaHA >> Kw

[ ] 21 aa KKH ≈+

Figure 1. Selected curve shapes

1

23 4

5

Independent variable