Ch. 15

1
Ch. 15 16 Review

• Everything except Polyprotics Lewis
  Acids/Bases!!

2
Practice 1

• Write the equilibrium-constant expressions for
  the following processes
• H2(g) I2(g) ? 2HI(g)
• Cd4(aq) 4Br-(aq) ? CdBr4(aq)
• P4 (s) 5 O2 (g) ? P4O10 (s)

3
Practice 2

• For the UNBALANCED reaction
• NH3 (aq) ? N2 (g) H2 (g)
• Write an equilibrium expression
• Calculate the value of K at 127C for
• NH3 3.1x10-2M
• N2 8.5x10-1 M
• H2 3.1x10-3 M
• Calculate the value of K with the above
  concentrations for the following reaction
• 1/2 N2 3/2 H2 ? NH3

4
Practice 3

• A mixture of hydrogen and nitrogen in a reaction
  vessel is allowed to attain equilibrium at 472C.
  The equilibrium mixture of gases was analyzed and
  found to contain 7.38 atm H2 , 2.46 atm N2 , and
  0.166 atm NH3. From these data, calculate the
  equilibrium constant Kp for the reaction

5
Practice 4

• Sulfur trioxide decomposes at high temperature in
  a sealed container
• Initially, the vessel is charged at 1000 K with
  SO3(g) at a partial pressure of 0.500 atm. At
  equilibrium the SO3 partial pressure is 0.200
  atm. Calculate the value of Kp at 1000 K.

6
Practice 5

• At 448C the equilibrium constant Kc for the
  reaction
• is 50.5. Predict in which direction the reaction
  will proceed to reach equilibrium at 448C if we
  start with 2.0 ? 102 mol of HI, 1.0 ? 102 mol
  of H2, and 3.0 ? 102 mol of I2 in a 2.00-L
  container.

7
Practice 6

• Consider the equilibrium
• In which direction will the equilibrium shift
  when
• N2O4 is added
• NO2 is removed
• the total pressure is increased by addition of
  N2(g)
• the volume is increased
• the temperature is decreased

8
7.) Determine if the following is acidic, basic,
or neutral.

• Cu(NO3)2

9
Answer

• Ions Cu2, NO3-
• Cu2 H2O ? Cu(OH)2 H
• weak base acidic (due to H)
• NO3- H2O ? HNO3 OH-
• strong acid neutral
• (will dissociate, so H and OH- will form
  water)
• Salt Acidic

10
8.) Determine if the following is acidic, basic,
or neutral.

• KClO4

11
Answer

• Ions K, ClO4-
• K H2O ? KOH H
• strong base neutral
• (will dissociate, so H and OH- will form
  water)
• ClO4- H2O ? HClO4 OH-
• strong acid neutral
• (will dissociate, so H and OH- will form
  water)
• Salt neutral

12
9.) Determine if the following is acidic, basic,
or neutral.

• NaH2PO4

13
Answer

• Ions Na, H2PO4-
• Na H2O ? NaOH H
• strong base neutral
• (will dissociate, so H and OH- will form
  water)
• H2PO4- H2O ? H3PO4 OH-
• weak acid basic (due to OH-)
• Salt basic

14
10.) Determine if the following is acidic, basic,
or neutral.

• LiF

15
Answer

• Ions Li, F-
• Li H2O ? LiOH H
• strong base neutral
• (will dissociate, so H and OH- will form
  water)
• F- H2O ? HF OH-
• weak acid basic (due to OH-)
• Salt basic

16
11) Determine if the following is acidic, basic,
or neutral.

• (NH4)2CO3
• Extra info
• Kb (of NH3) 1.8x10-5
• ka (of HCO3-) 5.6x10-11

17
Answer

• Ions NH4, CO3-2
• NH4 H2O ? NH3 H3O
• weak base acidic (due to H3O)
• CO3- H2O ? HCO3- OH-
• weak acid basic (due to OH-)
• must compare Kb vs. Ka to decide pH of salt
• Kb (of NH3) 1.8x10-5
• Ka of NH4 5.6x10-10
• ka (of HCO3-) 5.6x10-11
• kb of CO3-2 1.8x10-4
• Salt basic since Kb is greater than ka

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12.) Complete the following table
pH H pOH OH Acidic, basic, or neutral?
5.4 x 104
7.8 x 10-10
10.75
5.00
19
Answers

• ROW1
• pH 3.27
• pOH 10.73
• OH 1.9 x 1011
• acidic (since pH lt 7)
• ROW 2
• pH 4.89
• H 1.3 x 105
• pOH 9.11
• acidic (since pH lt 7)

• ROW 3
• H 1.8 x 10-11
• pOH 3.25
• OH 5.6 x 104
• basic (since pH gt 7)
• ROW 4
• pH 9.00
• H 1.0 x 109
• OH 1.0 x 105
• basic (since pH gt 7)

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Practice 13

• Calculate the pH of a 0.0430 M HNO3 solution.

21
Answer

• Since HNO3 is a strong acid, the nitric acid
  solution will be 100 ionized.
• Thus H NO3 0.0430 M.
• The pH - log 0.0430 1.37

22
Practice 14

• Calculate the pH of a 0.020 M Ba(OH)2 (aq)
  solution.

23
Answer

• Since Ba(OH)2 is a strong base it is 100
  ionized. Note that ionization gives 2 OH ions
  for each mole of Ba(OH)2.
• Thus OH 2 x 0.020 M 0.040 M
• pOH -log0.040 1.40
• pH 14 1.40 12.60

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Practice 15

• Calculate the pH of a 0.250 M HC2H3O2 solution.
  Ka(HC2H3O2) 1.8 x 10-5.

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Answer
Balanced Equation HC2H3O2 H C2H3O2
Initial Concentration (M) 0.250 0 0
Change (M) - x x x
Equilibrium Concentration (M) 0.250 - x x x
Answer
(a)
Thus x2 4.5 x 10-6 x 2.12 x 10-3 H. pH
2.67.
26
Practice 16

• Calculate the pH of a 0.600 M solution of
  methylamine CH3NH2. Kb 4.4 x 104.

27
Answer
Answer Since CH3NH2 is a weak base, the balanced
equation for the reaction is CH3NH2 H2O
CH3NH3 OH.
Balanced Equation CH3NH2 H2O CH3NH3 OH  
Initial Concentration (M) 0.600 ___ 0 0 0
Change (M) - x ___ x x x
Equilibrium Concentration (M) 0.600 - x ___ x x x
Thus x 1.62 x 10-2 OH, and pOH 1.79. The
pH 12.21.
28
Practice 17

• The pH of a 0.10 M solution of a weak base is
  9.67. What is the Kb of the base?

29
Answer
Answer The balanced equation for a weak base B
is given in Eq(10). The equilibrium table
required is given below.
Balanced Equation B H2O BH OH  
Initial Concentration (M) 0.10 ___ 0 0 0
Change (M) - x ___ x x x
Equilibrium Concentration (M) 0.10 - x ___ x x x
At equilibrium, OH BH x. Use the pH to
calculate the OH at equilibrium (which is the
value of x). Here pOH 14.00 pH 14.00
9.67 4.33. Thus
30
Practice 18

• Use the following acidity constants to help
  answer the questions below
• Ka(HC2H3O2) 1.8 x 10 5
• Ka(HCN) 4.9 x 1010
• Ka(HCOOH) 1.7 x 10-4
• Which of the three acids is the weakest?
• Which of the following bases is the strongest
  C2H3O2-, CN- , or HCOO- ?
• What is the pKa of HCN?
• What is the Kb for CN- ?

31
Answer

• (1) smallest k value HCN
• (2) strong base weakest acid lowest k value
  HCN (acid) CN- (base)
• (3) pKa -log Ka -log (4.9x10-10) 9.31
• (4) Ka x Kb 1x10-14
• kb 1x10-14 2.0 x 10-5
• 4.9x10-10

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19.) Predict whether an aqueous solution would be
acidic, basic or neutral?

• 1.sodium nitrate
• 2.ammonium iodide
• 3.sodium bicarbonate
• 4.ammonium cyanide
• 5.sodium hypochlorite
• 6.potassium acetate

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Predict whether an aqueous solution would be
acidic, basic or neutral? ANSWERS

• 1.sodium nitrate NaNO3 neutral
• 2.ammonium iodide NH4I acidic
• 3.sodium bicarbonate NaHCO3 basic
• 4.ammonium cyanide NH4CN basic (Kb ?)
• kb (for NH3) vs. Ka (for HCN)
• 1.8x10-5 vs. 4.9x10-10
• 5.sodium hypochlorite NaClO basic
• 6.potassium acetate KC2H3O2 basic

34
Practice 20 (from ch. 15 study questions)

• Suppose that 0.50 moles of hydrogen gas, 0.50
  moles of iodine gas, and 0.75 moles of hydrogen
  iodide gas are introduced into a 2.0 Liter vessel
  and the system is allowed to reach equilibrium.
• H2(g) I2(g) ? 2 HI(g)
• Calculate the concentrations of all three
  substances at equilibrium. At the temperature of
  the experiment, Kc equals 2.0 x 10-2.

35
Practice 21 (from ch. 15 study questions)

• Nitrosyl chloride NOCl decomposes to nitric oxide
  and chlorine when heated
• 2 NOCl(g) ? 2 NO(g) Cl2(g)
• At 600K, the equilibrium constant Kp is 0.060.
  In a vessel at 600K, there is a mixture of all
  three gases. The partial pressure of NOCl is 675
  torr, the partial pressure of NO is 43 torr and
  the partial pressure of chlorine is 23 torr.
• a. What is the value of the reaction quotient?
• b. Is the mixture at equilibrium?
• c. In which direction will the system move to
  reach equilibrium?
• d. When the system reaches equilibrium, what will
  be the partial pressures of the components in
  the system? (just set up the problem, do not
  solve all the way!!)