CHEM 151 USF General Chemistry for Science and Engineering Questions

Homework 4CHEM151: General Chemistry for Science and Engineering
Homework set 4 – Total points: 100
Instructions: Please type your answers to the following questions and submit an electronic copy of your
completed assignment on Canvas under Homework Set 1. Points for each question are indicated in front
of the question. Please name your file as CHEM151_HW4_FirstNameLastInitial
This homework set is on the material covered in week 4: Acids and Bases
Question 1 (5 pts)
As the [H+] proton concentration increases, the pH __________ (increases/decreases).
As the [OH-] hydroxide ion concentration increases, the pH ___________(increases/decreases).
Question 2 (10 pts)
a. What is the pH and the pOH of a solution that has a [H+] concentration of
1 x 10-13 M? Show your work.
b. Is this solution acidic or basic?
Question 3 (35 pts)
In class, we looked at the ionization of ammonia (NH3) as a base. Ammonia, the unionized form,
is more toxic than ammonium, the ionized form. The conjugate acid, ammonium (NH4+), further
dissociates in water. The pKa is 9.25 for the dissociation ammonium.
a. Write the equation for the dissociation of ammonium, NH4+.
b. Balance the equation.
c. Label the acid and its conjugate base.
d. Write out the equilibrium constant, Ka. What is the value of Ka?
e. What is the equation for the total concentration (C)?
f. What is the formula for the alpha fraction for NH4+?
g. What is the formula for the alpha fraction for NH3?
h. Using excel, calculate and plot the alpha fraction for pH=1 to pH=14.
i. As pH increases, which species dominates?
j. As pH decreases, which species dominates?
k. When are the fractional concentrations equal? How is this point related to the pH
and pKa?
Question 4 (20 pts)
a. If a significant amount of atmospheric carbon dioxide is dissolved in ocean water, what
would happen to the pH of the surface ocean water?
b. If 1.0 ton of carbon dioxide was dissolved in 1.0*106 gallons of ocean water, what would
be the pH of this sample of ocean water? Assume this sample of ocean water does not
significantly mix with nearby ocean water, and assume that other chemical components
in the seawater that might counteract the effect of carbon dioxide, such as salts, can be
ignored. The equilibrium constant for the reaction of carbon dioxide with sea water is
1.2*10-3. The acid dissociation constant for carbonic acid, H2CO3, is 4.3*10-7. Note: 1 ton
= 2000 lb, 1 lb = 0.45 kg, 1 gallon = 3.785 L.
Homework 4
CHEM151: General Chemistry for Science and Engineering
Question 5 (20 pts)
Limestone (CaCO ) will dissolve in acidic rainwater in the Midwest to produce the weak acid
bicarbonate (HCO ). This is an important component of natural buffer systems. The pH of the
resulting water will be resistant to the strong acid because of the bicarbonate ions (a water
quality parameter called alkalinity). As more acid is deposited by acid rain, more limestone will
be dissolved to react with it, preventing the water from turning acidic.
a. If you have 1.0 L of pure water at 25 °C and water K = 1.0*10 , how would you calculate
the pH. Write the equation(s). Review section 9.2 of the textbook again if you need to.
b. If you add 200.00 mL of 1.0000 M nitric acid to the 1.0 L water in the previous question,
how would you calculate the new pH? Write the equations.
c. If you add 100.00 mL of 1.000 M CaCO3 to the 1.0 L of water, how would you calculate
the new pH?
Question 7 (10 pts)
Read about pH on the EPA website:
a. What are some of the sources and effects of low pH in environmental waters?
b. What are some of the sources and effects of high pH in environmental waters?
1 pt Extra credit:
a. How long did it take you to complete this assignment?
b. What percent of the acid-base material covered in class and assignments #3 and #4 was
new? What percent was review (you had already learned it)?

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