CHEM 1A: Exam 3 Practice Problems:
These practice problems are not a comprehensive list of all questions to be asked on the exam. Refer to the suggested
textbook HW, other practice problems on the review page, iclicker questions, old quizzes & challenge problem sets for a
comprehensive review.
Redox Chemistry:
1. Classify each reaction as an acid–base reaction, a precipitation reaction, or a redox reaction, or state if there is
no reaction; then complete and balance the chemical equation:
a. Pt2+(aq) + Ag(s) →
b. HCN(aq) + NaOH(aq) →
c. Fe(NO3)3(aq) + NaOH(aq) →
d. CH4(g) + O2(g) →
2. Using the activity series (shown to the right), predict what happens in
each situation. If a reaction occurs, write the complete ionic equation
for the reaction. If a reaction will not naturally occur, write NR.
a. Platinum wire is dipped in hydrochloric acid.
b. Manganese metal is added to a solution of iron(II) chloride.
c. Tin metal is dipped in an aqueous solution of silver (I) nitrate.
d. Hydrogen gas is bubbled through a solution of lead(II) nitrate.
3. Arsenic acid (H3AsO4) is a highly poisonous substance that was once used as a pesticide. The reaction of
elemental zinc with arsenic acid in acidic solution yields arsine (AsH3, a highly toxic and unstable gas) and
Zn2+(aq). Balance the equation for this reaction in ACIDIC CONDITIONS using oxidation states and half
reactions:
H3AsO4(aq) + Zn(s) → AsH3(g) + Zn2+(aq)
4. Dentists occasionally use metallic mixtures called amalgams for fillings. If an amalgam contains zinc, however,
water can contaminate the amalgam as it is being manipulated, producing hydrogen gas under basic conditions.
As the filling hardens, the gas can be released, causing pain and cracking the tooth. Write a balanced chemical
equation for this reaction.
Gas Laws:
1. Radon (Rn) is a radioactive gas formed by the decay of naturally occurring uranium in rocks such as granite. It
tends to collect in the basements of houses and poses a significant health risk if present in indoor air. Many
states now require that houses be tested for radon before they are sold. Calculate the density of radon at 1.00
atm pressure and 20°C and compare it with the density of nitrogen gas, which constitutes 80% of the
atmosphere, under the same conditions to see why radon is found in basements rather than in attics.
2. You are in charge of interpreting the data from an unmanned space probe that has just landed on Venus and
sent back a report on its atmosphere. The data are as follows: pressure, 90 atm; temperature, 557°C; density, 58
g/L. The major constituent of the atmosphere (>95%) is carbon. Calculate the molar mass of the major gas
present and identify it.
3. Sulfuric acid, the industrial chemical produced in greatest quantity worldwide (almost 45 million tons per year in
the United States alone), is prepared by the combustion of sulfur in air to give SO2, followed by the reaction of
SO2 with O2 in the presence of a catalyst to give SO3, which reacts with water to give H2SO4. The overall chemical
equation is as follows:
2 S + 3 O2 + 2 H2O→ 2 H2SO4
What volume of O2 (in liters) at 22°C and 745 mmHg pressure is required to produce 1.00 ton of H2SO4?
4. Sodium azide (NaN3) decomposes to form sodium metal and nitrogen gas according to the following balanced
chemical equation:
2 NaN3(s) → 2 Na(s) + 3 N2(g)
This reaction is used to inflate the air bags that cushion passengers during automobile collisions. The reaction is
initiated in air bags by an electrical impulse and results in the rapid evolution of gas. What volume of N2 gas
could be produced from the decomposition of a 5.00 g sample of NaN3? Assume the gas would be produced at
22°C and 762 mmHg.
5. One method for preparing hydrogen gas is to pass HCl gas over hot aluminum; the other product of the reaction
is AlCl3. If you wanted to use this reaction to fill a balloon with a volume of 28,500 L at sea level (1.00 atm) and a
temperature of 78°F, what mass of aluminum would you need? What volume of HCl at STP would you need?
6. For an ideal gas at constant temperature, the product of pressure and volume (PxV) should be constant,
regardless of the pressure. Experimental data for methane at 0 oC, however, show that the value
of PxV decreases significantly as the pressure ranges from 0 atm to 120 atm. The decrease in PxV over the same
pressure range is much smaller at 100°C. Explain why PxV decreases with increasing pressure. Why is the
decrease less significant at higher temperatures?
At a fixed temperature, a higher pressures results in a higher density of gas
particles. With the gas particles closer together, the presence of non-zero
intermolecular forces becomes significant enough to reduce the pressure from
that expected for an ideal gas (non-interacting particles). The intermolecular
forces act to draw the gas particles together, which reduces the force of collisions.
At higher temperatures, the average kinetic energy of the moving gas particles is
sufficiently large to overcome the intermolecular forces and a smaller decrease in
the pressure is observed.
7. A 10.0 L cylinder contains 500 g of methane. Calculate its pressure to two significant figures at 27°C using A)
the ideal gas law & B) the van der Waals equation of state. Then explain why the two pressures are NOT the
same.
Reference information for methane:
a = 2.273 b = 0.0430
Titrations:
1. The structure of vitamin C (ascorbic acid, a monoprotic acid) is
shown to the right reacting with sodium hydroxide in an acid base
neutralization.
An absence of vitamin C in the diet leads to the disease known as
scurvy, a breakdown of connective tissue throughout the body and of
dentin in the teeth. Because fresh fruits and vegetables rich in vitamin C
are readily available in developed countries today, scurvy is not a major
problem. In the days of slow voyages in wooden ships, however, scurvy
was common. Ferdinand Magellan, the first person to sail around the
world, lost more than 90% of his crew, many to scurvy. Although a diet
rich in fruits and vegetables contains more than enough vitamin C to
prevent scurvy, many people take supplemental doses of vitamin C,
hoping that the extra amounts will help prevent colds and other illness.
Suppose a tablet advertised as containing 500 mg of vitamin C is dissolved in 100.0 mL of distilled water that
contains a small amount of the acid–base indicator bromothymol blue, an indicator that is yellow in acid solution
and blue in basic solution, to give a yellow solution. The addition of 53.50 mL of a 0.0520 M solution of NaOH
results in a change to green at the endpoint, due to a mixture of the blue and yellow forms of the indicator.
What is the actual mass of vitamin C in the tablet?
HINT: The molar mass of ascorbic acid is 176.13 g/mol.
2. Glutathione is a low-molecular-weight compound found in living cells that is produced
naturally by the liver. Health-care providers give glutathione intravenously to prevent
side effects of chemotherapy and to prevent kidney problems after heart bypass
surgery. Its structure is shown to the right.
Glutathione is found in two forms: one abbreviated as GSH (indicating the presence of an –
SH group) and the other as GSSG (the disulfide form, in which an S–S bond links two
glutathione units). The GSH form is easily oxidized to GSSG by elemental iodine:
2 GSH(aq) + I2(aq) → GSSG(aq) + 2 HI(aq)
A small amount of soluble starch is added as an indicator. Because starch reacts with excess
I2 to give an intense blue color, the appearance of a blue color indicates that the
equivalence point of the reaction has been reached.
Adding small volumes of a 0.0031 M aqueous solution of I2 to 194 mL of a solution that
contains glutathione and a trace of soluble starch initially causes no change. After 16.3 mL
of iodine solution have been added, however, a permanent pale blue color appears because
of the formation of the starch-iodine complex. What is the concentration of glutathione (in
mol/L) in the original solution?