Chem 2A – Exam 1
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Chem 2A – Exam 1
SCORE: ________ 100 pts possible
[1] (5 pts) Assign the oxidation state of nitrogen in each of the following gaseous
molecules:
N2O3
NO2
N2O4
NH3
NOCl
[2] (5 pts) What mass of magnesium bromide is required to prepare 250 mL of a solution
that is 5.00 × 10-3 M in bromide ions?
[3] (8 pts) Balance the following redox equation in acidic solution
Mn 2+ (aq) + NaBiO3 ( s ) ⎯⎯
→ MnO-4 (aq) + Bi3+ (aq)
[4] (4 pts) Write the chemical formulas for the following compounds
a) calcium chlorate
_______________________________________
b) aluminum oxide
_______________________________________
c) ferrous phosphate _______________________________________
d) cobalt(III) sulfite
_______________________________________
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Chem 2A – Exam 1
[5] (8 pts) An engineering student is asked to calculate the pressure of a 2000 L reaction
vessel containing 6.413 × 103 moles of SO2 gas at a temperature of 50oC. The student
uses the ideal gas law for the calculation. Calculate the difference in pressure of the
student’s result with the more accurate pressure determined using Van der Waal’s
equation? Is the ideal gas law appropriate to use in this instance?
[6] (12 pts) A 1.35 gram sample of a popular analgesic medicine known to contain C, H,
N, and O is subjected to combustion analysis. The nitrogen is converted to NO2 during
the combustion process. The products of the reaction are found to contain 3.1443 grams
of carbon dioxide, 0.7242 grams of water and 0.4109 grams of nitrogen dioxide.
Calculate the empirical formula of the compound.
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Chem 2A – Exam 1
[7] (15 pts) Potassium Chromate (K2CrO4), a bright yellow solid, is produced by the
reaction of chromite ore (FeCr2O4) with solid potassium carbonate and gaseous oxygen at
high temperatures according to the unbalanced reaction below. In a particular experiment
169 kg of chromite ore, 298 kg of potassium carbonate and 75.0 kg of oxygen were
sealed in a reaction vessel and reacted at high temperature. 194 kg of solid potassium
chromate (K2CrO4) was obtained. Calculate the percent yield.
FeCr2 O4 ( s ) + K 2CO3 ( s) + O2 ( g ) ⎯⎯
→ K 2CrO4 ( s ) + Fe2 O3 ( s ) + CO2 ( g )
[8] (8 pts) A 3.91 gram sample of zinc metal is placed in a 2.0 L sealed vessel containing
1.5 atm of HCl (g) at 25oC. The excess HCl gas reacts with the metal to produce ZnCl2
(s) and H2 (g). What is the pressure inside the vessel after the reaction is complete?
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Chem 2A – Exam 1
[9] (15 pts) A chemist weighs out 5.14 grams of a mixture containing unknown amounts
of BaO (s) and CaO (s) and placed the sample in a 1.50 L flask containing CO2 (g) at
30oC and 750 torr. After the reaction to form BaCO3 (s) and CaCO3 (s) was completed,
the pressure of CO2 (g)remaining was 230 torr. Calculate the percent mass of BaO (s)
and CaO (s) in the original sample.
[10] (6 pts) Several assumptions were made in the development of the kinetic molecular
explanation for the ideal gas law. Van der Waal’s equation is an attempt to correct for
two of these assumptions. Explain what the Van der Waals a and b constants correct for.
The Van der Waal’s a constant corrects for ….
The Van der Waal’s b constant corrects for….
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Chem 2A – Exam 1
[11] (8 pts) The manufacturer of a nutritional supplements claims the each tablet of
“Cold Buster 2000” contains 500 mg of vitamin C (C6H8O6). The vitamin C content can
be determined by titration with standardized NaOH solution. A 0.767 gram sample of
KHP (KHP = C6H5O4K, MW = 204.22 g mol-1) required 30.78 mL of NaOH solution to
reach the endpoint.
C6 H5O4 K + OH - ⎯⎯
→ C6 H 4O2-4 + K + + H 2O
A tablet is crushed and dissolved in 25 mL of water and placed in an Erlenmeyer flask.
The sample is titrated with the NaOH solution standardized above requiring 45.67 mL to
reach the endpoint.
C6 H8O6 + 2 OH - ⎯⎯
→ C6 H6O62- + 2H 2O
a) What is the concentration of the NaOH solution?
b) How much vitamin C does the tablet actually contain?
[12] (6 pts) Concentrated phosphoric acid (H3PO4) is sold as an 85% by mass aqueous
solution with a density of 1.685 g mL-1. Calculate the molarity, molality and mole
fraction of phosphoric acid in this solution. Treat water as the solvent.
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Chem 2A – Exam 1
Equations
x=
∑x
i
i
s=
n
PV = nRT
∑ (x
i
− x)2
i
n −1
μ=x±
ts
n
R = 0.08206 L ⋅ atm ⋅ mol-1 ⋅ K -1
N 0 = 6.02214 ×1023
R = 8.314 J ⋅ mol-1 ⋅ K -1
amu = 1.66054 × 10−27 kg
PV
PV
1 1
= 2 2
n1T1 n2T2
⎛
n2 ⎞
P
+
a
⎜
⎟ (V − nb) = nRT
V2 ⎠
⎝
nRT
n2
−a 2
P=
V − nb
V
Selected values for Van der Waals Constants
Molecule
NH3
HCl
SO2
a
(atm L2 mol-2)
4.170
3.667
6.714
b
(L mol-1)
0.03707
0.04081
0.05636
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