Chem 107 - Hughbanks
Exam 1 - Spring 2013
Name (Print)
UIN #
Section 502
Exam 1, Version # B
On the last page of this exam, you’ve been given a periodic table and some physical constants. You’ll
probably want to tear that page off to use during the exam – you don’t need to turn it in with the rest of
the exam.
The exam contains 10 problems, with 5 numbered pages. You have the full 75 minutes to complete the
exam. Please show ALL your work as clearly as possible – this will help us award you partial credit
if appropriate. Even correct answers without supporting work may not receive credit. You may use an
approved calculator for the exam, one without extensive programmable capabilities or the ability to store
alphanumeric information. Print your name above, provide your UIN number, and sign the honor code
statement below.
On my honor as an Aggie, I will neither give nor receive unauthorized assistance on this exam.
SIGNATURE:
Name (Print)
Version B
(For grading)
Scores
(1)
(7 points) Consider the complete neutralization of 1 L of 0.1 M HCl with 1 L of
0.1 M NaOH by combining the two solutions. Assuming the reactant solutions
and product solution have approximately the same density as water, which of the
following statements is true? There may be more than one correct choice; enter
ALL the correct letters, in alphabetical order.
(a) The approximate concentration of chloride ions in the original HCl solution
and in the product solution is the same.
(b) In the total of both the reactant solutions and in the product solution, 0.1
moles of sodium ions are present.
(c) In the total of both the reactant solutions and in the combined product
solution, the total number of electrons is the same.
(d) The combined mass of the product solution is the same as the combined mass
of the reactant solutions.
(e) In all the products, the total number of atoms exceeds the total number of
atoms in all the reactants.
1
2
3
4
5
6
7
8
9
10
Tot.
/7
/7
/10
/8
/10
/10
/10
/12
/16
/10
/100
Ans. 1__b, c, d____
(2) (7 points) In the blanks provided, place these gases in order of decreasing average molecular speed at
25 ˚C:
(A) N2
.
(B) Ar
D
>
A
(C) CH3Cl
>
B
>
(D) CH4
C
>
(E) CF4
E
Insert the letters for each choice in the spaces. Be legible. This is an all-or-nothing question, the
order must be entirely correct to get any credit.
(3) (10 points) In the first row of blanks beneath each of the following compounds, classify them as
follows: weak acid (wa), strong acid (sa), weak base (wb), strong base (sb), or as a salt (s) that is
neither acid or basic. In the second row of blanks, classify the same compounds as a weak
electrolyte (we), strong electrolyte (se), or nonelectrolyte (ne).
NaCl
HClO4
CH3COOH
CH3NH2
s
se
KOH
sa
wa
wb
sb
se
we
we
se
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Name (Print)
(4)
Version B
(8 points) The following full sets of potential compounds are made up of common cations and
anions. Which full sets are most likely to actually exist? (All 3 compounds in a set must be sensible
to qualify.) There may be more than one correct choice; enter ALL the correct letters, in alphabetical
order.
(a) Ca3(PO4)2, Ag2SO4, NH4NO3 (b) (NH4)2PO4, Li2CO3, Ca3(PO4)2 (c) Rb(SO4)2, CaCO3, ZnClO4
(d) NH4PO4, NaCO3, KNO3
(e) K2SO4, KClO4, Ca(OH)2
Ans. 4______ a, e____
(5) (10 points) Calcium hydroxide can be dissolved in phosphoric acid to form calcium phosphate and
water.
(i) (5 pts) Balance the equation below by filling in the blanks with coefficients:
__3__ Ca(OH)2 + __2__ H3PO4 → __1__ Ca3(PO4)2 + __6__ H2O
(ii) (5 pts) What volume (in mL) of 1.23 M H3PO4 is needed to react with 12.0 g of Ca(OH)2? (Write
the letter of the correct answer below.)
(a) 0.088 mL
(b) 0.197 mL
(c) 131.7 mL
(d) 197.5 mL
(e) 87.8 mL
Ans. 5 (ii) ___e___
(6) (10 points) Write the letters for the endings of the following sentence that form a true statement in
the blank provided. (Enter ALL the correct letters, in alphabetical order.)
If equal masses of He gas and Ar gas are mixed together in a flask,
(a) the number of collisions of the helium atoms against the walls of the flask is 10 times the number
of collisions of the argon atoms against the walls of the flask.
(b) raising the temperature of the gas mixture from –100 °C to 73 °C will double the total kinetic
energy of all the atoms in the flask.
(c) the average velocity of the Ar atoms is 10 times smaller than the average velocity of the He
atoms.
(d) the helium partial pressure is 10 times the argon partial pressure.
(e) the total kinetic energy of all the atoms in the flask will be 11 times the total kinetic energy of
just the argon atoms alone.
Ans. 6
b, d, e
2
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Version B
Questions 7-11 – show all work (48 pts)
(7)
(10 points) The average (root-mean-square) molecular speed for a common diatomic (two atoms
per molecule) gas at 273 K is 1775.3 km/hour. What is the molar mass (in g/mol) of this common
gas? Show all work.
½ Mv2 = 3/2 RT
v = (1775.3 km/h)(1000 m/km)/(3600 s/h) = 493.14 m/s
2
M = 3RT/v = 3(8.314 J/mol•K)(273 K)/(493.142 m2/s2) = 0.028 kg/mol = 28 g/mol
(8)
(12 points) Diborane, B2H6, and related compounds were briefly considered as potential rocket
fuels in the 1950s. A representative reaction for this class of molecules is that between B2H6 and
O2 to form B2O3 and H2O.
(a) (5 points) Write a balanced equation for the reaction between B2H6 and O2.
B2H6 + 3 O2 → B2O3 + 3 H2O
(b) (7 points) Compute the masses of B2O3 and H2O formed if 83.01 g of B2H6 reacts as completely
as possible with 315.6 g O2.
(315.6 g O2)/(32 g/mol) = 9.8625 mol O2
(83.01 g B2H6)/(27.6 g/mol) = 3.008 mol B2H6
B2H6 is the limiting reagent, so 3.008 mol B2O3 and 9.023 mol H2O will form
(3.008 mol B2O3)(69.6 g/mol) = 209.3 g B2O3
(9.023 mol H2O)(18.0 g/mol) = 162.4 g H2O
3
Name (Print)
Version B
(9) (16 points) Under appropriate conditions potassium chlorate, KClO3, might be used to generate pure
oxygen for use in breathing apparatus. When heated with a catalyst, it generates innocuous
potassium chloride, KCl, and oxygen:
.
2
KClO3 →
2
KCl
+
3
O2
Note: Catalysts are used to speed the rate of a reaction, but are not consumed in reactions –
therefore all the catalyst remains after the reaction has ended.
(a) (2 points) Using the blanks provided, balance this equation with the smallest possible integer
coefficients.
A 0.300 g mixture of KClO3 and a catalyst were heated together and the above reaction occurred in
quantitative (100 %) yield. All the oxygen gas was collected in a balloon at 27 ˚C and 1.0 atm.
pressure.
(b) (10 points) The KCl formed in the reaction is completely soluble in water and the catalyst is
completely insoluble in water. The catalyst/KCl mixture present after this reaction was
thoroughly washed with water to give 120 mL of a solution that was analyzed for its chloride
content with the result: [Cl–] = 0.0169 M. What was the volume of O2 collected in the balloon?
mol Cl– = (0.120 L)(0.0169 mol/L) = 2.028 × 10–3 mol
mol O2 = 3/2(1.352 × 10–3 mol) = 3.042 × 10–3 mol
Vol O2 = so (3.042 × 10–3 mol)(0.08206 L-atm/mol•K)(300 K)/(1.0 atm)
Vol O2 = 0.0749 L = 75 mL
(c) (4 points) What was the mass percent of KClO3 in the original KClO3/catalyst mixture?
Started with 2.028 × 10–3 mol KClO3
mass KClO3 = (2.028 × 10–3 mol)(122.55 g/mol) = 0.2485 g
% KClO3 = (0.2485 g)/(0.300 g) × 100% = 82.8%
4
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Version B
(10) (10 points) Shown below is the molecular structure of adrenaline, which is a stimulant produced by
the body in response to various stimuli.
(a) (6 pts) Give the molecular formula for adrenaline.
C9H13NO3
(b) (4 pts) Assuming your answer is correct; give the molar mass of adrenaline.
183.2 g/mol
5