CHAPTER 2: DAVIS BOOK

Question 1

In the following table, we have listed the isotopes of magnesium, the percentages at which they are found naturally, and the isotopic weights of each. Calculate the atomic weight of magnesium and compare the value you calculate to the value given in the list of elements in the front of your textbook.

Answer 1

24.3096 amu.

Question 2

A solution of calcium chloride is prepared in a 1.00-L volumetric flask. A 60.00-g sample of CaCl2 is added to a small amount of water in the flask, and then additional water is added to bring the total volume of solution to 1.00 L. What is the concentration of calcium chloride in units of molarity?

Answer 2

0.5406 M

Question 3

Calcium can be removed from natural waters by the addition of sodium hydroxide according to the unbalanced reaction
Ca(HCO3)2 + NaOH = Ca(OH)2 + NaHCO3
Balance this reaction.

Answer 3

Ca(HCO3)2 + 2NaOH = Ca(OH)2 + 2NaHCO3

Question 4

In the atmosphere, sulfur dioxide (released by coal-burning power plants) reacts relatively slowly with oxygen and water vapor to form sulfuric acid, a pollutant that has the potential to cause
severe respiratory distress and acid rain.
The chemical equation is
SO2 + O2 + H2O −→ H2SO4
Balance this reaction.

Answer 4

2SO2 + O2 + 2H2O −→ 2H2SO4

Question 5

The following reaction is important in lake and river sediments that are devoid of oxygen.

SO2−4 + H+ + CH2O HS− + CO2 + H2O
Balance this reaction.

Answer 5

SO2−
4 + H+ + 2CH2O HS− + 2CO2 + 2H2O

Question 6

If you add 30 g of calcite (CaCO3) to a 1.00-L volumetric flask and bring the final volume to 1.00 L, what would be the concentration of calcium (Ca2+) in solution? Assume that the calcium in solution is at equilibrium with CaCO3(s) and the temperature of the solution is 25◦C. The pKs for calcite is 8.48.

Answer 6

5.75 × 10−5 M (moles per liter)

Question 7

Assume you added 30 g of calcite to water to make up 1.00 L of a solution containing 0.01 M NaCl. What would be the concentration of calcium (Ca2+) in solution?

Assume that the calcium in solution is at equilibrium with calcite (s) and the temperature of the solution is 25◦C.

Answer 7

[Ca2+] = [CO2−

3 ] = 8.7 × 10−5 M

Question 8

Assume you added 30 g of calcite to 1 L of water having
the composition given below. What would be the concentration of calcium (Ca2+) in solution? Assume that the calcium in solution is at equilibrium with calcite (s) and the temperature of the
solution is 25◦C.

Answer 8

[Ca2+] = [CO2−

3 ] = 2.5 × 10−4 M

Question 9

What is the solubility of dolomite in water that contains 100 mg · L−1 CO2−3 ? The solubility product of dolomite is 10−17.09. Assume that the effects of ionic strength are negligible.

Answer 9

1.704 × 10−6 M.

Question 10

A solution is initially supersaturated with CO2−3 and Ca2+ such that the concentrations are both
50.0 mg · L−1. When equilibrium is ultimately reached what will be the final concentration of Ca2+?

Answer 10

16.7 mg · L−1

Question 11

If 100 mg of H2SO4 (MW = 98) is added to water, bringing the final volume to 1.0 L, what is the final pH?

Answer 11

2.69

Question 12

A solution of HOCl is prepared in water by adding 15 mg HOCl to a volumetric flask, and adding water to the 1.0 L mark. The final pH is measured to be 7.0. What are the concentrations of HOCl
and OCl? What percent of the HOCl is dissociated? Assume the temperature is 25◦C.

Answer 12

[OCl−] = 6.39 × 10−5 M.
[HOCl] = 2.22 × 10−4 M
percentage of OCl− = 22.4%

Question 13

The concentration of carbon dioxide in water at 20◦C is determined to be 1.00 · 10−5 M. The Henry’s constant for carbon dioxide dissolution in water is 3.91 · 10−2 M atm−1 at 20◦C. What is the partial pressure of CO2 in the air?

Answer 13

2.56 × 10−4 atm

Question 14

You are working at a site where a leaking underground storage tank has contaminated the soil and groundwater below a gas station. The groundwater was analyzed and the concentrations of
benzene and methyl tertiary butyl ether (MTBE) were found to be 45 and 500 g/L, respectively. At the soil temperature (10◦C) the dimensionless Henry’s constants, H, for benzene and
MTBE are 0.09 and 0.01, respectively. Calculate the soil vapor concentrations of both chemical compounds.

Answer 14

Cbenzene = 0.36 g/L
CMTBE = 5 g/L

Question 15

As a result of the Aykhal Crystal detonation in 1974 (in the former Soviet Union), cesium-137 was measured (in 1993) in the soil at a concentration of 2 × 104 Bq · kg−1 soil.* If the background concentration of 137Cs is 0.5 Bq · kg−1 soil, how many years will it take before the concentration of 137Cs reaches background levels? The decay of radionuclides occurs by a first-order reaction.

Answer 15

t = 459 years

Question 16

An environmental engineering student was very interested in the reaction of the chemical
2,4,6-chickenwire. She went into the lab and found that 2,4,6-chickenwire degrades in water.
During her experiments she collected the data in the following table. Plot the data to
determine if the reaction is zero-, first-, or second-order with respect to the concentration of
2,4,6-chickenwire.

Answer 16

first-order.

Question 17

A falling raindrop initially has no dissolved oxygen. The saturation concentration for oxygen in the drop of water is 9.20 mg · L−1. If, after falling for 2 s, the droplet has an oxygen concentration of 3.20 mg · L−1, how long must the droplet fall (from the start of the fall) to achieve a concentration of 8.20 mg · L−1? Assume that the rate of oxygen exchange is first-order.

Answer 17

t = 10.4 s

Question 18

Commercial sulfuric acid (H2SO4) is often purchased as a 93 wt% (weight percent) solution. Find the concentration of this solution of H2SO4 in units of milligram per liter, molarity, and normal-
ity. Sulfuric acid (100%) has a specific gravity of 1.839. Assume that the temperature of the solution is 15◦C.

Answer 18

36.30 Eq · L−1 or 36.3 N

Question 19

Find the mass of sodium bicarbonate (NaHCO3) that must be added a 1.00 L volumetric flask containing distilled water to make a 1.0 M solution. Find the normality of the solution.

Answer 19

84 g · L−1

Question 20

Determine the equivalent weight of each of the following: Ca2+, CO2−3 , CaCO3.

Answer 20

Ca2+
20.04 g · g-Eq−1 or 20.04 mg · mEq−1

CO2-3
30.01 g · g-Eq−1 or 30.01 mg · mEq−1

CaCO3
50.05 g · g-Eq−1 or 50.05 mg · mEq−1

Question 21

The pH of a water is measured to be 7.5. The concentration of bicarbonate was measured to be 1.3 × 10−3 M. What are the concentrations of carbonate, carbonic acid, and CT? Assume this system is closed to the atmosphere.

Answer 21

[H2CO3] = 8.2 × 10−5 M

[CO2−3 ] = 1.9 × 10−6 M

CT= 1.384 × 10−3 M ≈ 1.4 × 10−3 M

Question 22

The pH of a water is measured to be 7.5. What are the concentrations of carbonate, bicarbonate, carbonic acid, and CT? Assume this system is open to the atmosphere. The temperature is 25◦
C.
The Henry’s constant for carbon dioxide is 10−1.47 M · atm−1 at this temperature. The partial
pressure of carbon dioxide is 10−3.53 atm.

Question 23

A water contains 100.0 mg · L−1 CO2−3 and 75.0 mg · L−1 HCO− 3 at a pH of 10 (T = 25◦
C). Calculate the exact alkalinity. Approximate the alkalinity by ignoring the appropriate chemical species.

Answer 23

Alkalinity = 233 mg · L−1 as CaCO3

Question 24

A soil sample is collected and the soil water is analyzed for the chemical compound
1,2-dichloroethane (DCA). The concentration in the water is found to be 12.5 g · L−1. The
organic matter content of the soil is 1.0%. Determine the concentration of DCA that would be sorbed to the soil and that associated with the organic matter. DCA has a Kd of
0.724 (g · kg−1 )(g · L−1)−1 .

Answer 24

905 g · kg−1

Question 25

A 1-m3 sample of air was found to contain 80 g · m−3 of SO2. The temperature and pressure
were 25.0◦C and 103.193 kPa when the air sample was taken. What was the SO2 concentration in
parts per million?

Answer 25

0.030 ppm of SO2