PRACTICE EXAM 3 (Chapter 4)
1. Problems 32, 46, 58, 79, 83
2. Iron ore is impure Fe2O3. When the ore is heated with excess carbon, this reaction occurs:
Fe2O3 (s) + 3 C (s) → 2 Fe (l) + 3 CO (g)
When a sample of ore weighing 938 g is heated, 523 g of pure Fe are obtained. Calculate the mass of
Fe2O3 in the ore sample and the percent Fe2O3 by mass in the ore.
3. Molarity problems:
a. Determine [Cl1–] in a solution that contains 13.68 g BaCl2 in 500.0 mL solution.
b. How many grams of K2SO4 are needed to prepare 250.0 mL of a solution that is [K1+] = 0.100 M?
c. Calculate [NaOH] in a solution prepared by diluting 10.0 mL of 3.392 M NaOH to 250.0 mL.
4. Identify each of these water-soluble compounds as a strong electrolyte, weak electrolyte, or
nonelectrolyte, then draw simple diagrams to illustrate how their aqueous solutions are different: HF,
KI, CH3OH.
Answers to Practice exam 3
1. Text problems
32. 82.9 g H2SO4; 1.71 g H2
46. 10.1 g Mg can yield 16.8 g MgO; 10.5 g O2 can yield 26.4 g MgO. The limiting reactant is Mg
and the theoretical yield is 16.8g MgO. The percent yield is 71.0%.
58. 1700 mL (1667mL rounded to 2 sigfigs)
79. 0.337 M
83. a. Ag = 0 b. Ag = +1 c. Ca = +2, F = –1
d. H = +1, S = –2 e. C = +4, O = –2 f. Cr = +6, O = –2
2. 748 g Fe2O3; the ore is 79.7% Fe2O3.
3. a. [Cl1–] = 0.2628 M
4.
KI is a strong electrolyte
(all dissociated)
b. 2.18 g K2SO4
c. 0.136 M NaOH
HF is a weak electrolyte
(only one shown dissociated)
CH3OH is a non-electrolyte
(none dissociated)
AP Chem Practice Test 3: Chapter 5 (Gases)
1. Gas law problems
a. A small 12 L tank filled with gas at 6.8 atm pressure is connected to a large 185 L tank that has
been evacuated. When the stopcock connecting the tanks is opened, what will be the final
pressure in this system (assuming the temperature returns to its original value)?
b. A sample of Ne gas has volume 886 mL at 752 mm Hg and 26 °C. What will be the volume of this
sample if the temperature is lowered to –20 °C with the pressure and amount of gas held constant?
c. What is the volume of 89.2 g CO2 gas at 37 °C and 737 mm Hg?
d. A 0.418 g sample of gas has a volume of 115 mL at 66.3 °C and 743 mm Hg. What is the molar
mass of the gas?
e. A weather balloon has volume 25 L at the surface of the earth (23 °C and 750 mm Hg). What is its
volume at high altitude, where the temperature is –20 °C and the pressure is 188 mm Hg?
f. What is the density of CO2 gas at 32.7 °C and 758 mm Hg? Would it more dense or less dense than
SO2 gas?
2. Gas stoichiometry problems
a. What volume of hydrogen gas is required to react with 5.85 L of nitrogen gas at 625 °C and 7.5
atm? The unbalanced equation for the reaction is N2 (g) + H2 (g)  NH3 (g)
b. How many L of hydrogen gas at STP are produced when 4.38 g Zn react with excess HCl?
The unbalanced equation for the reaction is Zn (s) + HCl (aq)  ZnCl2 (aq) + H2 (g)
c. How many grams of CO2 gas are produced when 35 L of CH4 burn? Assume 25 °C and 1 atm.
The unbalanced equation for the reaction is CH4 (g) + O2 (g)  CO2 (g) + H2O (l)
3. A student collects 28.7 mL of nitrogen gas over water. The barometric pressure is 768 mm Hg, the
temperature is 23 °C, and the vapor pressure of water at 23 °C is 21 mm Hg.
a. After the water levels inside and outside the collection tube are equalized, what is the pressure of
the gas mixture in the collection tube?
b. What gases are in the tube?
c. Determine the partial pressure and mole fraction of each gas in the tube.
4. Consider 3 balloons of gas, each containing 5.0 L, all at the same temperature and pressure. One
balloon contains 3.3 g CH4, one contains 14 g Cl2, and the third contains 8.8 g CO2.
a. Compare the number of molecules, molecular kinetic energies, & molecular speeds of these gases.
b. Which gas will be retained in its balloon the longest? Explain.
c. Which balloon(s) would float if released? The average molar mass of air is 29 g/mol.
5. Sketch a Maxwell-Boltzmann distribution. Label the axes and mark the root mean square speed on
your sketch.
a. How will the sketch change if the temperature is increased?
b. How will the sketch change for a heavier substance? a lighter substance?
6. a. A sample of Cl2 effuses in 28.6 s. How long would it take for the same amount of NO to effuse
under the same conditions?
b. In a diffusion experiment you obtain these data: Gas A travels 52 cm in the same time gas B
travels 115 cm. You know that gas A is Kr. Calculate the molar mass of gas B.
Answers to Chapter 5 Practice Test
1. a. 0.41 atm
b. 750. mL
c. 53.2 L
d. 103 g/mol
e. 85 L
f. 1.75 g/L; less
2. a. 17.6 L H2
b. 1.50 L H2
c. 63 g CO2
3. a. The pressure is equal to barometric pressure, 768 mm Hg
b. There are two gases in the tube: N2 and H2O vapor
c. PH2O = 21 mm Hg, PN2 = 768 – 21 = 747 mm Hg
21mmHg
747mmHg
[mole fraction = pressure fraction]
XH2O 
 0.027
X N2 
 0.973
768mmHg
768mmHg
4. a. All contain the same number of molecules (Avogadro's hypothesis). They all have the same average
molecular kinetic energy, because they are at the same temperature. Speed is inversely
proportional to molecular mass, so CH4 is fastest, CO2 is intermediate, and Cl2 is slowest.
b. The heaviest gas effuses most slowly, so Cl2 is retained longest.
c. Gas density is proportional to molar mass; CH4 is less dense than air (16 g/mol vs. 29 g/mol), so
the CH4 balloon would float; the other gases are more dense than air (44 g/mol and 71 g/mol, vs.
29 g/mol), so those balloons would sink.
5. See pg 218
6. a. 18.6 s
b. 17 g/mol