Houston Community College System
Chemistry 1305
EXAM # 3
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HOUSTON COMMUNITY COLLEGE
CHEM 1305
Name: _____________________
Exam # 3 (Chap. 8-12)
Score:
PART I – Multiple choice: (3 points each)
1) Which of the following is a physical change?
A) baking a cake
B) digesting a meal
D) a spoonful of salt is dissolved in a bowl of soup
C) fermentation of grapes
E) tanning the skin
2) In a ________ reaction, two or more elements or compounds form one product.
A) decomposition
D) double replacement
B) single replacement
E) combination
C) dehydration
3) What are the coefficients for oxygen and carbon dioxide in the balanced equation?
C5 H12  ?O2  ?CO2  ? H 2O
A) 8, 6
B) 1, 5
C) 5, 8
D) 6, 8
E) 8, 5
4) A certain laboratory procedure requires the use of 0.250 moles of magnesium. How many grams of
magnesium would you mass out on the balance? Molar mass: Mg – 24.31 g/mol
A) 6.1
B) 6.08
C) 0.010
D) 0.0103
E) 4.51
5) If you find the mass of a sample of glucose (C6H12O6) to be 90.0 g, how many moles of glucose do you
have? Molar masses: C- 12.01 g/mol; H – 1.01 g/mol; O – 16.00 g/mol
A) 0.4995 moles
B) 0.499 moles
C) 0.500 moles
D) 2.00 moles
E) 2.002 moles
6) How many moles of oxygen gas react with 4 mol of hydrogen gas according to the balanced chemical
equation:
___H2(g) + ___O2(g) → ___H2O(g)
A) 1 mol
B) 2 mol
C) 3 mol
D) 4 mol
7) When 2 moles of Mg 3 N 2 are allowed to react, how many moles of H 2 O also react?
Mg3 N2  6 H2O  3 Mg(OH)2  2 NH3
A) 1 mole
B) 4 moles
C) 6 moles
D) 8 moles
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E) 12 moles
8) An unknown molecule is found to consist of 24.2% carbon by mass, 4.0% hydrogen by mass and the
remaining mass is due to chlorine. What is the empirical formula of the molecule?
A) CH2Cl
B) C2H4Cl2
C) CH2Cl2
D) C2H3Cl
E) CHCl
9) Determine % by weight of oxygen in hypochloros acid.
A) 39.5%
B) 9.9%
C) 36.2%
D) 79.0%
E) none of the above
10) A 10.0 cm3 container of helium is sealed at 22.0 °C and 1.0 atm pressure. What pressure would be
exerted by the helium if the container were heated to 220 °C? (1 cm3 = 1 mL)
A) 10.0 atm
B) 1.67 atm
C) 2.01 atm
D) 1.08 atm
E) 2.08 atm
11) A barometer indicates that the atmospheric pressure is actually 768.2 mmHg. What is the
pressure in kPa?
A) 14.85
B) 102.4
C) 7.582
D) 1.011
E) 7.784
12) A gas sample occupies a volume of 16.4 L at 27 °C and 0.300 atm. How many moles of gas are
present?
A) 0.200
B) 0.450
C) 3.50
D) 10.0
E) 31.0
13) A sample of carbon monoxide gas occupies 3.20L at 125°C. At what temperature will the gas occupy a
volume of 1.54L if the pressure remains constant?
A) 218.20K
B) 208.21K
C) 191.54K
D) 200.07K
E) 193.20K
14) Which of the following molecules or ions has the most valence electrons.
A) C2H6
B) SO3
C) BF4¯
D) NH4+
E) SCl2
15) What is the electron domain geometry of NF3 ?
A) trigonal pyramidal
B) linear
C) trigonal planar
D) tetrahedral
16) The octet rule indicates that:
A) all of the noble gases have eight total electrons
B) all of the energy levels in an atom hold a maximum of 8 electrons
C) all of the main group elements have 8 valence electrons
D) atoms lose, gain or share electrons to have 8 valence electrons
E) the noble gases react with other compounds to get 8 valence electrons
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E) bent
17) Nitrite ion has ------ resonance structure(s):
A) none
B) one
C) two
D) three
E) four
18) Which noble gas is isoelectronic with a chloride ion?
A) helium
B) neon
C) argon
D) krypton
19) In an electron-dot structure of an element, the dots are used to represent:
A) all the electrons in an atom
B) the complete electron arrangement
C) only the electrons that participate in bond formation
D) the valence electrons
E) the electrons that the element will gain when it forms a compound
20) Which of the following molecules is polar?
A) BF3
B) N2
C) NaCl
D) O3
E) BeCl2
PART II – Show All Your Work: (8 points each)
1) a) What is the density of chlorine gas, Cl2, at STP?
b) Assuming similar conditions, how many liters of water vapor, H2O, react to produce 2L of hydrogen gas?
C(s) + H2O(g)  CO(g) + H2(g)
2) a) What is the mass of calcium nitrate (in kilograms) is needed to obtain 2.5 Kg of calcium phosphate? The
unbalanced equation is below:
K3PO4  Ca(NO3 )2  Ca 3 (PO4 )2  KNO3
b) What is the volume of nitrogen gas at STP from the decomposition of 25.0 g of ammonia?
___NH3(g)  ___H2(g) + ___N2(g)
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3) Phosphorus and chlorine react as follows: P4(s) + Cl2(g)  PCl3(l). Balance it! Assume
125 g P4(s) react with 323 g of Cl2(g).
a) What is the limiting reactant?
b) What mass of phosphorus trichloride can be formed from the reaction?
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4) If 90.0 cm of oxygen gas is at 0.500 atm and -123°C, what is the volume at STP?
5) a) Chromium (III) oxide reacts with hydrogen sulfide (H2S) gas to form chromium (III) sulfide and water:
Cr2O3 (s) + 3H2S (g) 
Cr2S3 (s) + 3H2O (l)
to produce 101 g Cr2S3, how many moles of Cr2 O3 are required?
b) How many moles of magnesium are needed to react with 16 g of oxygen gas?
___Mg + ___O2  ____MgO
BONUS QUESTION – Show All Your Work: (10 points)
Draw the Lewis structure for the following molecules and indicate the number of bonding electrons, nonbonding electrons and the shape of each one. NH3, H2O, CCl4, O3, SO2
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Answer Key
Part I- Multiple Choice
1) D
8) A
15) A
2) E
9) E
16) D
3) E
10) B
17) C
4) A
11) B
18) C
5) C
12) A
19) D
Part II- Show Your Work
1) a) 3.16 g/L b) 2 L
2) a) 4.0 Kg
b) 16.5 L
3) Cl2, 699 g product
4) 81.9 cm
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5) a) 0.664 mol Cr2O3
b) 1.00 mol Mg
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6) B
13) C
20) D
7) E
14) C