Practice test 3 - CEM 152
a.(20 points) Hydrazine, N2H4, is a weak base with Kb = 1.3 x 10-6. If the pH of an
aqueous solution of hydrazine is 10.5, what is the concentration of the base?
[N2H4] =
b. The skeletal structure of hydrazine is shown below. Complete the Lewis electron dot
structure of this molecule. (The dashes, or lines, in the structure provided below
represent electron pairs.)
H N N H
H H
1
Practice test 3 - CEM 152
1c. Draw the Lewis electron dot structure of the conjugate acid of hydrazine.
2. (20 points) The Ksp of Mn(OH)2 is 1.6 x 10-13 at 298 K. What is the maximum
concentration of Mn(II) that can be supported in neutral H2O(l)?
[Mn(II)] =
2
Practice test 3 - CEM 152
2b. If one drop, 0.05 mL, of 0.1M NaOH is added to a 10 mL sample of H2O, what is the
smallest concentration of Mn(II) that could be detected by precipitation?
[Mn(II)] =
3
Practice test 3 - CEM 152
3. (20 points) a. Kp = 160 for the formation of HI(g) from H2(g) and I2(g) at 500 K. If 3.0
moles of H2(g) and 3.0 moles of I2(g) are placed in a 5.0L tank at 500 K, how many
moles of HI are present at equilibrium? You may assume that the gases are ideal.
nHI =
b. The above reaction is exothermic. If the temperature is lowered to 400 K, will the
yield, or amount, of HI(g) that results from the process described in part a increase,
decrease or remain the same? Why?
4
Practice test 3 - CEM 152
4. (20 points) a. Ascorbic acid, H2C6H6O6 is a diprotic acid with first and second
ionization equilibrium constants, Ka1 = 8.0 x 10-5 and Ka2 = 1.6 x 10-12. A solution is
prepared by dissolving 0.2 M moles of potassium hydrogen ascorbate, KHC6H6O6, in
H2O(l). Write the chemical equations that are important for determining the pH of the
above solution.
b. What is the pH of the 0.2M solution of KHC6H6O6 described above?
pH =
5
Practice test 3 - CEM 152
5. (20 points)
a. A student decides to prepare a pH = 4.00 buffer solution from a 1.0 L solution of
0.10M acetic acid (CH3CO2H), Ka = 1.8 x 10-5, by adding a small amount of 6.0 M
NaOH to it. Write down the net chemical equation that describes this neutralization
process.
b. What concentrations of CH3CO2H and CH3CO2- are needed to realize the desired pH
= 4.00 buffer?
[CH3CO2H] =
[CH3CO2-] =
6
Practice test 3 - CEM 152
c. What volume of concentrated NaOH must be added?
volume NaOH =
7