105Fa11- Hour Exam 4 : KEY
For full credit, you must show all conversion factors and the final answer with complete units, use the correct
number of significant figures, and round off correctly. WRITE CLEARLY: if the grader cannot read your
writing, no credit will be assigned. If you need the value of a constant, look on the last page.
Question 21 (10 pts)
Exams G and I. An Alka-Seltzer pill containing an unknown amount of sodium carbonate was treated in a
closed flask with excess hydrochloric acid. The resulting wet carbon dioxide gas (vol = 689.5 mL) was collected
over water using a water buret. Calculate the mass (g) of sodium carbonate contained in the pill. Atmospheric
pressure that day = 755.00 mm Hg, and room temperature = 25° C. (Hint: first write the balanced chemical
equation.)
Na2CO3 + 2HCl  H2CO3 + 2 NaCl
H2CO3  CO2 + H2O
PCO  Ptot  PH O  755.00  24.0 mm Hg  731.0 mm Hg
2
2


1 atm
 731.0 mm Hg 
  0.6895 L
760 mm Hg 
PV 
n

 0.027106 mol CO 2
L  atm
RT
0.08206
 273.15  25 K
K  mol
 1 mol Na 2 CO3  105.99 g Na 2 CO3 
  2.87 g Na 2 CO3

0.027106 mol CO 2  
 mol CO 2  mol Na 2 CO3 
Exams H and J: An Alka-Seltzer pill contains 2.873 g of sodium carbonate. It was treated in a closed flask with
excess hydrochloric acid and the resulting wet carbon dioxide gas was collected over water using a water buret.
Calculate the total volume of gas collected in the buret. Atmospheric pressure that day was 755.00 mm Hg, and
the temperature was 25° C. (Hint: first write the balanced chemical equation.)
Na2CO3 + 2HCl  H2CO3 + 2 NaCl
H2CO3  CO2 + H2O
PCO  Ptot  PH O  755.00  24.0 mm Hg  731.0 mm Hg
2
2
2.873 g Na 2 CO3 
nRT
V 

P
1 mol Na 2 CO3
1 mol CO 2

 0.0271063 mol CO 2
105.99 g Na 2 CO3 1 mol Na 2 CO3
L  atm
 (273.15  25) K
0.66319 L  atm
K  mol

1 atm
0.96184 atm
731.0 mm Hg 
760.00 mm Hg
0.0271063 mol  0.08206
V  0.68949 L  0.689 L
1
Many are possible isomers in each of these; only a few are shown here. Systematic names were not required, but are shown
for your information. Recall that truly different isomers have different systematic names.
Question 22 (10 pts)
Exam G. Write clear Lewis formulas of THREE structural isomers of C8H14 that contain at least three methyl
groups (-CH3) and one C=C. Show all hydrogens. Geometric isomers do not count.
CH3
H
H
H3C
H
CH3
H
H
H3C
H
H
etc
H
CH3
CH3
etc
H
H
H
1,2,3-trimethyl cycl opentene
H
CH2
CH3
H
H
1,4,5-trimethyl cycl opentene
H
H
H3C
H
H3C
H
H
CH3
6-methyl-2,4-hepta diene 2,3,4-trimethyl -1,3-pentadiene
H
H3C
H3C
H
CH3
CH3
H3C
H
CH3
CH3
H
CH3
H
H
H3C
H
H
2-methyl-2,4-hepta diene
H3C
CH3
H
H
etc
CH3
H
3-ethyl-1,2-di methyl cycl obutene 1-(1-methyl -1-propenyl)-1-methylcyclopropa ne
Exam H. Write clear Lewis formulas of THREE structural isomers of C8H14 that contain at least four methyl
groups (-CH3) and one CC. Show all hydrogens.
H
H
CH3
CH3
H
CH3
H
CH3
2,5-di methyl-3-hexyne
H3C
CH3
CH3
CH3
H3C
3,4,4-trimethyl-1-pentyne
H3C
H
H3C
H
CH3
CH3
H3C
5,5-di methyl-2-hexyne
H
CH3
2,2-di methyl-3-hexyne
2
CH3
H H
4,4-di methyl-2-hexyne
H
H3C
H3C
H3C
CH3
etc
Exam I. Write clear Lewis formulas of THREE structural isomers of C8H14 that contain at least three methyl
groups (-CH3) and one ring. Show all hydrogens.
H3C
H3C
H
CH3
H
H
H
H
H
H3C
H
H
CH3
H
H3C
H3C
H
H
1,2,3-tri methyl cycl opentene 1,2,4-tri methyl cycl opentene
H3C
CH3
H
H
H
H
1-i s opropyl i dene-3-methyl cycl obuta ne
H
H
etc
H
1,3,4-tri methyl cycl opentene
etc
H
H3C
H
CH3
CH3
H
H3C
CH3
H
H
CH3
H
H
3-i s opropyl -1-methyl cycl obutene
Write clear Lewis formulas of THREE structural isomers of C7H10 that contain
at least two methyl groups (-CH3)
and one CC
and one C=C.
Show all hydrogens.
H3C
H
H
H3C
CH3
H
H
H
H
H
H3C
CH3
H
H
hept-2-en-5-yne
CH3
H3C
CH3
H
CH3
2-methyl hex-2-ene-4-yne
CH3
H
H2C
H
3,4-di methyl pent-3-ene-1-yne
H3C
H2C
H
H3C
H3C
CH3
H
H H
H
hept-2-ene-4-yne
hept-4-ene-2-yne
CH3
etc
H
2-methyl hex-1-ene-3-yne
3
CH3
3,4-di methyl pent-4-ene-1-yne