Chemistry 60 - Exam 2
26 January 2016
Name _________________________
Show all work for credit. State any assumptions made to solve a problem.
Give all numerical answers with the correct number of significant figures and
the correct units. All answers in scientific notation must be in correct
scientific notation (i.e., 6.0221023 not 6.022E23 or 6.022e23). Each instance
of incorrect scientific notation will result in the loss of 3 points. All numbers
that require units should have the units written. All instances of numbers
without units will result in the loss of 3 points each.
1. (2 points each) Balance the following chemical equations:
a. 2 Na2O2 + 2 H2O  4 NaOH + _____O2
b. 2 NaOH + _____Cl2  ______NaCl + _____NaClO + _____H2O
c. 4 Si2H3 + 11 O2  8 SiO2 + 6 H2O
d. ______H2SO4 + 8 HI  ______H2S + 4 I2 + 4 H2O
2. (28 points) A binary compound of magnesium and nitrogen is analyzed, and 1.2791 g of the
compound is found to contain 0.9240 g of magnesium. When a second sample of this
compound is treated with water and heated, the nitrogen is driven off as ammonia, leaving
a compound that contains 60.31% magnesium and 39.69% oxygen by mass. Calculate the
empirical formulas of the two magnesium compounds. Write the chemical equation that
occurs between the first magnesium compound and water. Calculate the mass of ammonia
produced in the reaction.
1 mol Mg
= 0.03801687 mol Mg/0.02535215 mol
=
=
×2 3
N 1.49955
24.3050 g Mg
1 mol N
1.2791 g Mg xNy − 0.9240 g Mg ×
=0.02535215 mol N/0.02535215 mol N =1× 2 =2
14.0067 g N
0.9240 g Mg ×
(
)
The first compound is Mg3N2.
1 mol Mg
=
1.000267 ≈ 1
2.481382 mol Mg/2.480718 mol O =
24.3050 g Mg
1 mol O
39.69=
g O×
2.480718 mol =
O/2.480718 mol O 1
15.9994 g O
60.31 g Mg ×
The second compound is MgO.
Mg3N2 + 3 H2O → 2 NH3 + 3 MgO
1 mol Mg3N2
2 mol NH3 17.0305 g NH3
? g NH=
1.2791 g Mg3N2 ×
×
×
= 0.43167 NH3
3
100.9284 g Mg3N2 1 mol Mg3N2
1 mol NH3
3. (20 points) Calculate the following. SHOW ALL WORK FOR FULL CREDIT. (Conversion factors
are on the last page.)
a. The number of fluorine atoms in 35.00 g of carbon tetrafluoride.
1 mol CF4
4 mol F 6.022 × 10 23 at F
? at F =
35.00 g CF4 ×
×
×
=
9.580 × 10 23 at F
88.0043 g CF4 1 mol CF4
1 mol F
b. Calculate the number of grams of iron in a 125.0 mL of steel that has a density of
7.84 g mL-1 and is 97.0% Iron by mass.
? g Fe= 125.0 mL steel ×
7.84 g steel
97.0 g Fe
×
= 951 g Fe
1 mL steel 100.0 g steel
c. The temperature of the sun’s surface is about 1.0×104 F. What is this temperature
in K?
(1.0 × 10
5K
F − 32.0 F    + 273.15 K =
6 × 10 3 K
9 F
4 
)
d. Calculate the volume, in in3, of silver containing 1.56×1024 atoms of silver. The
density of silver is 10.3 g/cm3.
3
? in3 Ag =×
1.56 10 24 at Ag ×
1 mol Ag
107.8682 g Ag 1 cm3 Ag  1 in

×
×
×
=
1.66 in3 Ag
23
1 mol Ag
10.3 g Ag  2.540 cm 
6.022 × 10 at Ag
e. The fastest spacecraft ever launched is the New Horizons spacecraft which just
passed Pluto last year. It’s traveling at 23 km per second. What is this speed in
inches per day?
in
23 km 60 s 60 min 24 h 103 m 1 cm
1 in
?
=
×
×
×
×
× −2 ×
=7.8 × 1010 in/d
day
s
1 min
1h
1d
1 km 10 m 2.540 cm
4. (19 points) 163.883 g of manganese(VI) chloride reacts with ammonium oxalate in a double
replacement reaction. Calculate the number of grams of solid produced if there is a 96.10
% yield.
MnCl6 + 3 (NH4)2C2O4 → Mn(C2O4)3 + 6 NH4Cl
? g Mn ( C2O4 )3 =163.883 g MnCl6 ×
×
1 mol Mn ( C2O4 )3
1 mol MnCl6
×
267.656 g MnCl6
1 mol MnCl6
318.9950 g Mn ( C2O4 )3 t
1 mol Mn ( C2O4 )3
×
96.10 g Mn ( C2O4 )3 a
100.00 g Mn ( C2O4 )3 t
=
187.7 g Mn ( C2O4 )3 a
5. (25 points) Calculate the mass of water produced when 40.00 g of decanol (C10H21OH) reacts
with 40.00 g of oxygen gas in a combustion reaction.
C10H21OH + 15 O2 → 10 CO2 + 11 H2O
? g H2O= 40.00 g C10H21OH ×
? g H2O = 40.00 g O2 ×
1 mol C10H21OH
11 mol H2O
18.0153 g H2O
×
×
= 50.08 g H2O
158.2811 g C10H21OH 1 mol C10H21OH
1 mol H2O
1 mol O2
11 mol H2O 18.0153 g H2O
×
×
= 16.51 g H2O
31.9988 g O2 15 mol O2
1 mol H2O
16.51 g of water can be produced.
Conversion Factors
1 qt = 0.9463 L
1 lb. = 453.6 g
1 in = 2.540 cm
2 pt = 1 qt
1 gal = 4 qt
2 c = 1 pt
8 fl. oz. = 1 c
8 furlongs = 1 mi
12 in = 1 ft
1 mi = 5280 feet
8 drams = 1 fl. oz.
1 stone = 14 lb