Answer Key
Chem 10113, Exam 1
September 24, 2014
1.
Write the complete symbol of the specific atom or ion that has 27 electrons, 36 neutrons,
and a mass number of 65.
65
2+
Cu
29
2.
(5 points)
3.
(8 points)
(3 points)
Balance the following chemical equations.
(a) 2 Al2O3 + 9 C → Al4C3 + 6 CO
(b) 4 NH3 + 5 O2 → 4 NO + 6 H2O
SHOW ALL WORK. An unknown compound of molecular mass 171 contains only
carbon, hydrogen, and bromine. Analysis shows that the compound contains 12 times as much
carbon as hydrogen by mass. Determine the molecular formula of the compound.
for one Br in the molecule: 171 - 80 = 91 for remaining C and H
convert C:H mass ratio to mole ratio:
(12 g C / 1 g H) (1 mole C / 12 g C) (1 g H / 1 mole H) = 1 mole C / 1 mole H
molar mass of CH = 13
91/13 = 7
∴ formula must be C7H7Br
4.
SHOW ALL WORK. Tartaric acid is the white powdery substance that coats sourtasting candies such as Sour Patch Kids. Combustion analysis of a 1.20 g sample of tartaric acid
(which contains only C, H, and O) produced 1.41 g of CO2 and 0.432 g of H2O. Determine the
empirical formula of tartaric acid. (molar masses: CO2 = 44.0, H2O = 18.0)
(12 points)
(1.41 g CO2) (1 mole / 44.0 g) (1 mole C / mole CO2) = 0.0320 mole C
(0.432 g H2O) (1 mole / 18.0 g) (2 mole H / mole H2O) = 0.0480 mole H
must determine moles of O in the sample by difference:
(0.0320 mole C) (12.0 g / mole) = 0.384 g C
(0.0480 mole H) (1.0 g / mole) = 0.0480 g H
mass of O = 1.20 - (0.384 + 0.048) = 0.768 g O
(0.768 g O) (1 mole / 16.0 g) = 0.0480 mole O
∴ empirical formula is C0.032 H0.048 O0.048 = C H1.5 O1.5 = C2H3O3
5
6.
(19 points)
Write the chemical formula for each of the following compounds.
Name
Formula
lead(IV) acetate
Pb(C2H3O2)4
aluminum oxalate
Al2(C2O4)3
chromic acid
H2CrO4
cobalt(II) phosphate octahydrate
Co3(PO4)2•8H2O
ammonium sulfite
(NH4)2SO3
mercury(I) cyanide
Hg2(CN)2
hydrosulfuric acid
H2S(aq)
hexane
C6H14
rubidium peroxide
Rb2O2
tetraphosphorus heptasulfide
P4S7
SHOW ALL WORK. When 100.0 mL of 0.360 M Ca(NO3)2 solution is mixed with
200.0 mL of 0.160 M Na3PO4, the following precipitation reaction occurs. The solid Ca3(PO4)2 is
collected, dried, and found to have a mass of 3.15 g. Determine the percent yield of this reaction.
{molar mass: Ca3(PO4)2 = 310.2}
3 Ca(NO3)2(aq) + 2 Na3PO4(aq) → Ca3(PO4)2(s) + 6 NaNO3(aq)
(12 points)
moles of reactants:
(0.100 L) {0.360 mole Ca(NO3)2 / L} = 0.0360 mole Ca(NO3)2
(0.200 L) {0.160 mole Na3PO4 / L} = 0.0320 mole Na3PO4
limiting reactant?
{0.0360 mole Ca(NO3)2} {1 mole Ca3(PO4)2 / 3 Ca(NO3)2}
= 0.0120 mole Ca3(PO4)2
{0.0320 mole Na3PO4} {1 mole Ca3(PO4)2 / 2 mole Na3PO4}
= 0.0160 mole Ca3(PO4)2
∴ Ca(NO3)2 is the limiting reactant!
theoretical yield = {0.0120 mole Ca3(PO4)2} (310.2 g / mole) = 3.72 g
percent yield = (3.15 g / 3.72 g) x 100 % = 84.7 %
7.
Write a complete, balanced chemical equation for each of the following processes.
(a) (4 points) The addition of nitric acid to water.
HNO3 + H2O
H3O+ + NO3
→
-
(b) (4 points) The preparation of barium perchlorate by a neutralization reaction.
2 HClO4 + Ba(OH)2
→
Ba(ClO4)2 + 2 H2O
(c) (4 points) The combustion of acetone, (CH3)2CO. = C3H6O
C3H6O + 4 O2
8.
3 CO2 + 3 H2O
In each of the following organic molecules, circle the main functional group and write the
appropriate family name (i.e., alkane, alcohol, etc.) below each one.
(8 points)
H3C
CH3
N
O
O
CH3O C CH2CH3
amine
9.
→
ester
C CH
alkyne
C CH CH3
H
CH2CH3
aldehyde
Complete the following statements for the substance Na2B12H12. (Work need not be
shown!) This substance has a molar mass of 187.8 g/mole and is 69.1 % boron by mass. The
empirical formula of this substance is NaB6H6. The number of boron atoms in 0.25 mole of
Na2B12H12 is 1.81 x 1024.
(8 points)
10. (5 points) Circle any of the following that are molecular compounds.
HSF3
AlPO4
HN3
KrF2
TeF4
NH4Br
Li2O2
SnS2
11. (8 points) SHOW ALL WORK. A crisp, new $100 bill weighs almost exactly 1.00 g. A typical 18wheeler has a hauling capacity of 24 tons. The next time that you pass a truck like that on the
highway, imagine it packed to full capacity with new $100 bills. (That's a lot of money, right?!?!)
The US national debt is currently 17.8 trillion dollars (i.e., 17.8 teradollars!). Determine the
number of trucks, every one packed full of $100 bills, that would be required to carry the national
debt. (Note: You must use the factor-label method to receive credit for your work.)
(17.8 x 1012 $) (1.00 g / 100 $) (1 lb / 454 g) (1 ton / 2000 lb) (1 truck / 24 ton)
= 8.17 x 103 trucks