CHEM 31
Introductory Chemistry
EXAM #1
September 18, 2002
Name:
Serkey, Anne
SSN:
Lab T.A.:
Who, Idunno
INSTRUCTIONS: Read through the entire exam before you begin. Answer all of the
questions. For questions involving calculations, show all of your work -- HOW you
arrived at a particular answer is MORE important than the answer itself! Circle your
final answer to numerical questions.
The entire exam is worth a total of 150 points. Attached are a periodic table and a
formula sheet jam-packed with useful stuff!
Good Luck!
2
Possible
Points
21
Points
Earned
21
3
4
33
34
5
6
7
26
20
16
33
34
26
20
16
Page
TOTAL:
150
150
1. Carry out the following operations, and express the answer with the appropriate
number of significant figures:
a. (3 pts) 21.2405 – 5.80 =
21.2405 – 5.80 = 15.4405 → 15.44
b. (4 pts) (0.0045 x 20,000.0) + (6104.5/35.359) =
90.0 + 172.643457 = 262.643457
= 263 → 2.63 x 102
2. (2 pts each) Round each of the following numbers to three significant figures and
express the result in scientific notation:
a. 456,500 rounds to:
4.56 x 105
b. 33, 987.22 rounds to:
3.40 x 104
3. (5 pts each) Liquid nitrogen is cool, but liquid Neon is cooler! The boiling point of
Neon is a cool –246.1 oC.
a. Express this temperature in oF.
o
F = (9/5)oC + 32 = (9/5)(-246.1 oC) + 32
= -442.98000 + 32 = -410.9800 → -411.0
o
F
b. Express this temperature in Kelvins (K).
K = oC + 273.15 = -246.1 + 273.15 = 27.050 → 27.0 K
2
4. (15 pts) A 32.55-g sample of a solid is placed in a flask. Toluene, in which the
solid is insoluble, is added to the flask so that the volume of the solid and liquid
together is 50.00 mL. The solid and liquid toluene together weigh 52.65 g. The
density of toluene at the temperature of the experiment is 0.866 g/mL. What is
the density of the solid?
solid + toluene = 52.65 g
solid =
32.55 g
Mass of toluene = 20.10 g toluene
20.10 g toluene x
= 23.21016 mL toluene
1 ml toluene
0.866 g toluene
solid + toluene =
toluene =
solid =
50.00 mL
23.21016 mL
26.7898 mL
density of the solid = 32.55 g
26.7898 mL
= 1.215013 g/mL
1.22 g/mL
4. (18 pts) Fill in the gaps in the following table:
Symbol
121
182
Sb
Protons
51
Neutrons
70
108
Electrons
51
74
0
0
Net Charge
W
74
192
Os2+
76
116
74
2+
3
5. (2 pts each) From this list of elements: Xe, Cu, N, Al, Sr, I, Sb, K, O; pick
the one that best fits each of the following descriptions. You may use each
element only ONCE.
a.
b.
c.
d.
e.
f.
g.
h.
i.
an alkali metal:
K
an alkaline earth metal:
Sr
a noble gas:
Xe
I
a halogen:
a metalloid:
Sb
a transition metal:
Cu
a metal that forms a 3+ ion:
Al
a nonmetal that forms a 2- ion: O
an element that resembles phosphorous:
N
6. (1 pt each) Give the atomic symbol for the following elements:
a. sodium:
Na
b. gold:
Au
c. bromine:
Br
d. mercury:
Hg
7. (1 pt each) Give the name of the element for the following atomic symbols:
a. C:
Carbon
b. Zn:
Zinc
c. He:
Helium
d. Sn:
Tin
8. (2 pts each) Give the formula for the following compounds:
a. perchlorate ion:
ClO4-
b. copper (II) hydroxide:
Cu(OH)2
c. dinitrogen pentoxide:
N2O5
d. nitrous acid:
HNO2
4
9. (2 pts each) Give the name of the following compounds:
a. H3PO4:
Phosphoric Acid
b. P4F6: Tetraphosphorous Hexafluoride
c. NH4Cl:
Ammonium Chloride
d. Fe(NO3)3:
Iron (III) Nitrate
10. (6 pts each) Write a complete, balanced chemical equation for each of the
following reactions:
a. Solid potassium chlorate is heated and decomposes to form solid
potassium chloride and oxygen gas.
2KClO3 (s) → 2KCl (s) + 3O2 (g)
b. N2(g) + H2(g) → NH3(g)
N2(g) + 3H2(g) → 2NH3(g)
c. C5H12O(l) + O2(g) → CO2(g) + H2O(l)
2C5H12O(l) + 15O2(g) → 10CO2(g) + 12H2O(l)
5
11. (10 pts) Copper has only two naturally-occurring isotopes (63Cu and 65Cu),
with atomic masses of 62.9296 amu and 64.9278 amu. If the natural
abundances of these two isotopes are 69.17% and 30.87%, respectively,
calculate the average atomic mass of copper.
avg atomic mass = (0.6917)(62.9296) + (0.3087)(64.9278)
=
43.52840
+
20.043212
= 63.571612
= 63.57 amu
12. (10 Pts) How big is Avogadro’s number? Suppose you bought a brand new
computer and overclocked the Pentium™ 4 processor so that it ran at
3.0 GHz. You then programmed the computer to count to 6.02214199 x 1023
so that it counted to 3.0 x 109 every second (that’s one count for every clock
cycle of the microprocessor). How long would it take for the computer to
count to Avogadro’s number? Express your answer in reasonable time units
(days, weeks, months, years, etc.).
We need to reach 6.02214199 x 1023 counts and we can count at a rate
of 3.0 x 109 counts/sec:
6.02214199 x 1023 counts x
sec
= 2.00738066 x 1014 seconds
TOO BIG!
3.0 x 109 counts
2.00738066 x 1014 s x
min
60 s
x
hr
60 min
x
day
24 hr
x
year
=
365.25 day
= 6.3610055 x 106 years
= 6.4 million years!
(Windows would crash WELL before
it came close to this ☺)
6
13. (2 pts each) Indicate whether each of the following compounds is molecular
or ionic:
a. B2H6
ionic
molecular
(circle one)
b. CsBr
ionic
molecular
(circle one)
c. SCl2
ionic
molecular
(circle one)
14. (2 pts each) Using the periodic table as a guide, predict the charge of the
most stable ion of each of the following elements:
a. Sr
+2
b. Al
+3
c. F
-1
d. Sb
-3
e. Cs
+1
7