Deck 16: Applications of Aqueous Equilibria

Question 1

Calculate the pH of a buffered solution.

Answer 1

A buffer solution contains both a weak acid and its conjugate base, or a weak base and its conjugate acid, as major species in solution.

Question 2

Explain how to prepare a buffered solution of known pH and capacity.

Answer 2

NOT ANSWER

Question 3

Calculate an acid or a base concentration from titration data.

Answer 3

At the stoichiometric point, just enough hydroxide ions have been added to react with every acidic proton present in the acid solution before the titration was started.

Question 4

Use the concepts of K_sp and the common-ion effect to calculate solution concentrations.

Question 5

Calculate the concentrations of species involved in complex-ion formation.

Question 6

When 0.1 moles of HC₂H₃O₂ and 0.1 moles of NaC₂H₃O₂ are added to make 1 L of aqueous solution, the major species present are

A) Na⁺, H₂O and HC₂H₃O_2.
B) HC₂H₃O₂ and C₂H₃O₂^-.
C) H₂O, Na⁺ and C₂H₃O₂^-.
D) NaC₂H₃O₂, HC₂H₃O₂ and H₂O.
E) Na⁺, H₂O, HC₂H₃O₂, C₂H₃O₂^-.

Question 7

When 0.1 moles of HCl are added to 1 L of solution containing 0.12 moles of aqueous Na₂CO₃, the major species present are

A) Na⁺, Cl^-, H₂O, HCO₃^-.
B) Na⁺, Cl^-, H₂O, CO₃^2-.
C) Na⁺, Cl^-, H₂O, HCO₃^-, CO₃^2-.
D) HCl, Na⁺, CO₃^2-.
E) Na⁺, Cl^-, H₂O, H₂CO₃, CO₃².

Question 8

What are the major species present when 10 ml of 0.1 M NaOH are added to 40 ml of 0.0.05 M CH₃COOH?

A) Na⁺, OH^-, H₂O and CH₃COOH
B) CH₃COOH and CH₃COO^-
C) H₂O, Na⁺ and CH₃COO^-
D) NaOH, CH₃COOH and H₂O
E) Na⁺, H₂O, CH₃COOH, CH₃COO^-

Question 9

Which of the following mixtures would make a buffer of pH = 4.74? (pK_a of acetic acid is 4.74)

A) 100 mL of 0.10 M HCl and 100 mL of 0.10 M NaC₂H₃O₂
B) 100 mL of 0.10 M HCl and 100 mL of 0.10 M HC₂H₃O₂
C) 50 mL of 0.10 M HC₂H₃O₂ and 100 mL of 0.10 M NaC₂H₃O₂
D) 100 mL of 0.10 M NaOH and 100 mL of 0.10 M NaC₂H₃O₂
E) 50 mL of 0.10 M NaOH and 100 mL of 0.10 M HC₂H₃O₂

Question 10

Which of the following solutions is a buffer?
I. 50 mL of 0.1 M acetic acid mixed with 25 mL of 0.05 M NaOH
II. 100 mL of 0.1 M acetic acid mixed with 25 mL of 0.5 M NaOH
III. 50 mL of 0.1 M acetic acid mixed with 25 mL of 0.05 M sodium acetate
IV. 50 mL of 0.1 M acetic acid mixed with 25 mL of 0.2 M HCl
V. 50 mL of 0.1 M sodium acetate mixed with 25 mL of 0.05 M NaOH

A) I and III
B) III only
C) I, II and V
D) III, IV and V
E) II, IV and V

Question 11

A solution is made by the addition of 0.34 mole of Na₂HPO₄ and 0.65 mole of NaH₂PO₄ and sufficient water to give a total volume of 1.2 L. How many grams of NaOH would need to be added to increase the pH by 0.2 pH units?If needed, use the following equation: pH = pK_a + log(A^-/HA)

A) 0.34
B) 3.4
C) 4.4
D) 5.4
E) 6.5

Question 12

Two buffer solutions of the same pH are prepared. After adding 15 millimoles of acid to 250 mL of one solution, the pH changes by 0.02 pH unit. Adding 15 millimoles of acid to 250 mL of the second solution results in a 0.05 pH unit change. Which statement most accurately describes the differences between the two buffer solutions?

A) The first solution has a lower capacity than the second solution.
B) The first solution has a higher concentration of conjugate acid than conjugate base.
C) The second solution has a lower concentration of conjugate base that the first solution.
D) The second solution has a higher acid to conjugate base ratio than the first solution.
E) The first solution has a higher capacity than the second solution.

Question 13

The pK_a's of the three acid-conjugate base pairs derived from phosphoric acid, H₃PO₄, are 2.12, 7.21 and 12.32. How many g of sodium hydroxide would have be added to 150 mL of 0.1 M H₃PO₄ to prepare a buffer of pH = 7.41?

A) 0. 21 g
B) 0.34 g
C) 0.56 g
D) 0.83 g
E) 0.91 g

Question 14

A biochemist wishes to study the activity of an enzyme that produces one mole hydrogen ion for every mole of product produced. However, the enzyme is sensitive to changes in pH and will become inactive at pH less than 7.1. If the initial pH of the solution is 7.21 set with a phosphate buffer (the pK_a's of the three acid-conjugate base pairs derived from phosphoric acid, H₃PO₄, are 2.12, 7.21 and 12.32) and the reaction is designed to produce 0.15 moles of product in a volume of 250 mL, what is the minimum concentration of the conjugate acid in the buffer solution?

A) 0.46 M
B) 0.53 M
C) 0.82 M
D) 1.12 M
E) 1.25 M

Question 15

Which of the following will have the smallest pH change upon the addition of 0.001 mole strong acid?

A) 50 mL of a total concentration of 0.10 M pH = 5.2 acetate buffer
B) 100 mL of a total concentration of 0.050 M pH = 5.2 acetate buffer
C) 75 mL of a total concentration of 0.067 M pH = 5.2 acetate buffer
D) 250 mL of a total concentration of 0.020 M pH = 5.2 acetate buffer
E) 25 ml of total concentration of 0.20 M pH = 5.2 acetate buffer

Question 16

In which of the following titrations will the pH be less than 7 at the equivalence point?

A) acetic acid titrated with sodium hydroxide
B) perchloric acid titrated with lithium hydroxide
C) sodium hydroxide titrated with hydrochloric acid
D) ammonium bromide titrated with perchloric acid
E) perchloric acid titrated with methyl amine

Question 17

In which of the following titrations will the pH be greater than 7 at the equivalence point?

A) perchloric acid titrated with lithium hydroxide
B) acetic acid titrated with sodium hydroxide
C) sodium hydroxide titrated with hydrochloric acid
D) ammonium bromide titrated with perchloric acid
E) perchloric acid titrated with methyl amine

Question 18

The equivalence point for titration of 50 ml of 0.1 M formic acid, HCO₂H (a weak acid) requires what volume of 0.2 M sodium hydroxide?

A) 50 ml
B) 25 ml
C) less than 25 ml as formic acid is a weak acid
D) 100 ml
E) between 25 and 50 ml as formic acid is a weak acid

Question 19

The equivalence point for titration of 50 ml of 0.1 M formic acid, HCO₂H (a weak acid) requires what volume of 0.2 M methylamine, CH₃NH₂ (a weak base)?

A) 50 ml
B) 25 ml
C) impossible to determine unless K_a for formic acid is known
D) impossible to determine unless K_b for methalamine is known
E) impossible to determine unless both K_a for formic acid and K_b for methylamine are known

Question 20

The equivalence point for titration of 50 ml of 0.1 M formic acid, HCO₂H (a weak acid) requires what volume of 0.2 M calcium hydroxidem, Ca(OH)₂?

A) 50 ml
B) 25 ml
C) less than 12.5 ml as formic acid is a weak acid
D) 12.5 ml
E) between 12.5 and 25 ml as formic acid is a weak acid

Question 21

Sodium carbonate, also called soda ash, is often analyzed by titration with strong acid. Initial concentrations of the carbonate ions are about 0.1 M. What indicator(s) would be suitable for detecting the stoichiometric point in this titration? (for H₂CO₃: pK_a1 = 3.75 pK_a2 = 10.33)

A) I only
B) I and II
C) III only
D) IV only
E) I and V

Question 22

Your friend at Podunk U. urgently emails asking if you remember any acid-base chemistry. His problem is that for his final organic lab problem he needs to report the pK_a of his acid. He did the pH titration, but only wrote down the following data for the titratant volumes: V = 14.27 mL; pH = 4.73. V_{equivalence point} = 22.43 mL. What is the pK_a of the unknown acid?

A) 3.5
B) 4.0
C) 4.5
D) 4.8
E) 4.9

Question 23

The following graph shows the titration of a 0.150 M benzoic acid solution. At what pH range would benzoic acid be appropriate to make a buffer?

A) 2.2
B) 3.5
C) 4.0
D) 7.5
E) 10.0

Question 24

What is the solubility product expression for barium phosphate?

A) K_sp = [Ba²⁺][PO₄^3-]
B) K_sp = [Ba²⁺]²[PO₄^3-]³
C) K_sp = [Ba²⁺]³[PO₄^3-]²
D) K_sp = [Ba²⁺]³[PO₄^3-]³
E) K_sp = [Ba²⁺][PO₄^3-]³

Question 25

What is the solubility product expression for iron (III) hydroxide?

A) K_sp = [Fe³⁺][OH^-]
B) K_sp = [Fe³⁺][OH^-]³
C) K_sp = [Fe³⁺]²[OH^-]³
D) K_sp = [Fe³⁺]³[OH^-]
E) K_sp = [Fe³⁺]³[OH^-]³

Question 26

At what pH will an 0.0010 M of iron (III) nitrate (K_sp (Fe(OH)₃)= 4.0 x 10^-38) begin to precipitate?

A) 2.55
B) 4.22
C) 5.55
D) 7.41
E) 9.03

Question 27

Which of the following is NOT one of the equilibria in the complexation of Ru(II) by ammonia (coordination number = 6)?

A) Ru²⁺(aq) + NH₃(aq) $\rightleftharpoons$ Ru(NH₃)²⁺ (aq)
B) Ru²⁺(NH₃)(aq) + NH₃(aq) $\rightleftharpoons$ Ru²⁺(NH₃)₂ (aq)
C) Ru²⁺(NH₃)₄ (aq) + NH₃(aq) $\rightleftharpoons$ Ru²⁺(NH₃)₅ (aq)
D) Ru²⁺(NH₃)₃ (aq) + NH₃(aq) $\rightleftharpoons$ Ru²⁺(NH₃)₄ (aq)
E) Ru²⁺(NH₃)₆ (aq) + NH₃(aq) $\rightleftharpoons$ Ru²⁺(NH₃)₇ (aq)

Question 28

In which solution will copper nitrate (Cu(NO₃)₂) be the least soluble?

A) 0.1 M NaNO₃
B) 0.1 M NH₃
C) pure water
D) 0.1 M CuCl₂
E) 0.1 M NaOH

Question 29

Write the equation showing how Na₂HPO₄ and NaH₂PO₄ act as a buffer. Also write an equation showing how this buffer reacts when HCl is added.

Question 30

When is a solution a buffer?

Question 31

What is the pH of a solution made by the addition of 0.34 mole of Na₂HPO₄ and 0.65 mole of NaH₂PO₄ and sufficient water to give a total volume of 1.2 L?If needed, use the following equation: pH = pK_a + log(A^-/HA)

Question 32

What is the pH upon adding 10 mL of 1.0 M NaOH toa) 1.2 L of water and b) a solution made by the addition of 0.34 mole of Na₂HPO₄ and 0.65 mole of NaH₂PO₄ and sufficient water to give a total volume of 1.2 L?If needed, use the following equation: pH = pK_a + log(A^-/HA)

Question 33

What is the pH of the resulting solution when 0.1 moles of HCl are added to 1 L of solution containing 0.12 moles of aqueous Na₂CO₃?If needed, use the following equation: pH = pK_a + log(A^-/HA)

Question 34

A solution is prepared by adding 0.14 mole Na₂HPO₄ and 8.2 grams of NaH₂PO₄ to sufficient water to prepare 1.00 L of solution. What is the pH of the solution? What is the pH of the solution after 0.041 moles HCl are added?If needed, use the following equation: pH = pK_a + log(A^-/HA)

Question 35

Determine the ratio of conjugate base to acid for a buffered solution at pH = 4.3 made from propionic acid, (K_a = 1.34 x10^-5).

Question 36

In the selective precipitation of metal ions as sulphide salts, usable concentrations of the S^2- ions can be maintained by bubbling H₂S through a solution buffered at a pH of about 10.0. How many mL of 6 M HCl would need to be added to 3.5 mL of a solution 5.6 M in aqueous ammonia to form a solution buffered at pH 10.0?

Question 37

A buffer is made by adding 0.82 moles of monohydrogen phosphate and 0.82 moles of dihydrogen phosphate to sufficient water to give a total volume of 1.0 L. Hydrogen chloride, 0.60 moles, is absorbed into the solution. What is the pH?

Question 38

A buffer solution made from ammonia, NH₃, and ammonium chloride, NH₄Cl, is prepared with pH 9.0. If K_b of NH₃ is 1.78x10^-5 and the total concentration of all nitrogen species is 0.25 M, what are the nitrogen concentrations?

Question 39

What mass of sodium acetate must be added to 250 mL of 0.10 M acetic acid (pK_a = 4.75) to give a buffer of pH = 4.90?

Question 40

What mass of ammonium chloride (pK_a (NH₄⁺) = 9.25) must be added to 2.0 L of 0.65 M aqueous ammonia to give a buffer with pH = 8.75?

Question 41

How would you make a buffer of pH = 3.75 from the following solutions with a total volume of 300 ml? (K_a = 1.77 x10^-4 for Formic Acid, HCO₂H); 0.100M HCOOH; 0.100 M HClO₄; 0.100 M NaOH; 0.100 M NH₃

Question 42

How would you make a buffer of pH = 3.75 from the following solutions with a total volume of 450 ml? (K_a = 1.77 x10^-4 for Formic Acid, HCO₂H); 0.100M NaHCOO; 0.100 M HClO₄; 0.100 M NaOH, 0.100 M NH₃

Question 43

Calculate the pH in the titration of a 0.325 g sample of acetylsalicylic acid (see line drawing below) initially in 25.0 mL water with 0.102 M NaOH when (a) 6.23 mL and (b) 20.00 mL of titrant have been added.

Question 44

Calculate the pH in the titration of a 0.325 g sample of acetylsalicylic acid (see line drawing below) initially in 25.0 mL water with 0.102 M NaOH when (a) 8.84 mL and (b) 17.68 mL of titrant have been added.

Question 45

A qualitative sketch of the titration curve for acetylsalicylic acid is shown at right.Given the indicators listed, which one would be suitable for titrations of acetylsalicylic acid?

Question 46

Consider the titration of 25.0 mL of an 0.11 M lactic acid solution CH₃CH(OH)COOH (K_a= 8.4 x10^-4) with 0.150 M NaOH.a) Calculate the pH after 10 mL of the NaOH solution has been added.b) Calculate the pH of the solution at 10 mL of NaOH past the stoichiometric point.c) Would phenol red (pK_in = 7.9) or phenolphthalein (pK_in = 9.4) be a better indicator for this titration?

Question 47

A 5.00 mL sample containing sulphurous acid (K_a1 = 1.5 x 10^-2; K_a2 = 1.0 x10^-7) is titrated with 0.1324M NaOH. The stoichiometric point is at 25.32 mL. How many g of H₂SO₃ are present in the sample and what is the pH at the stoichiometric point?

Question 48

At what pH would a solution with a total concentration of 0.20 M phosphate NOT be a buffer? Explain your answer.

Question 49

What is the concentration of phosphate in a solution made from a 0.22 M phosphoric acid whose pH has been adjusted to 7.1 by the addition of solid potassium hydroxide (assume no volume change)?

Question 50

What is the concentration of phosphate in a solution made from a 0.15 M phosphoric acid whose pH has been adjusted to 7.6 by the addition of solid sodium hydroxide (assume no volume change)?

Question 51

An aqueous solution is 0.2 M in both Mg²⁺ and Pb²⁺ ions. You wish to separate the two metal ions by adding oxalate, C₂O₄^2-. What is the highest possible oxalate-ion concentration such that only one of the two metal ions will form a solid?K_sp (MgC₂O₄) = 8.5 x 10^-5 K_sp (PbC₂O₄) = 2.7 x 10^-11

Question 52

An aqueous solution is 0.2 M in both Mg²⁺ and Pb²⁺ ions. You wish to separate the two metal ions by adding oxalate, C₂O₄^2-. What is the concentration of lead when Mg²⁺ begins to precipitate?K_sp (MgC₂O₄) = 8.5 x 10^-5 K_sp (PbC₂O₄) = 2.7 x 10^-11

Question 53

100 ml of aqueous solution is 0.2 M in both Mg²⁺ and Pb²⁺ ions. You wish to separate the two metal ions by adding oxalate, C₂O₄^2-. What is the mass of PbC₂O₄ precipitated before Mg²⁺ begins to precipitate?K_sp (MgC₂O₄) = 8.5 x 10^-5 K_sp (PbC₂O₄) = 2.7 x 10^-11

Question 54

At 100°C, 2.1 x 10^-2 g of AgCl dissolves in 1.00 L of water. What is the K_sp of AgCl at 100°C?

Question 55

What is the concentration of Ca²⁺ in a saturated solution of calcium phosphate Ca₃(PO₄)₂ (K_sp = 2.07 x 10^-33)?

Question 56

A real world application of solubility products is found in softening "hard water" by removing calcium and magnesium ions present as the sulphate and carbonate salts. We can decrease the amount of Ca²⁺ in solution by addition of sodium carbonate, a strong electrolyte:Na₂CO₃ (s) $\rightarrow$ 2 Na⁺ (aq) + CO₃^2- (aq)By increasing the concentration of carbonate, we tend to drive the solubility equilibrium toward the reactant:CaCO_{3 (s)} Ca²⁺ ₍aq₎ + CO₃^2- (aq) K_sp = 4.7 x 10^-9If the initial [Ca²⁺] = 5.0 x 10^-3 M, what percent of the [Ca²⁺] will be removed if the carbonate concentration is maintained at 1.0 x 10^-3 M?

Question 57

How many grams of AgNO₃ will dissolve in 1.32 L of a pH = 12.23 solution (K_sp AgOH = 1.52 x 10^-8)?

Question 58

What is the molar concentration of barium in a 0.01 M ammonium sulphate solution saturated with Ba(NO₃)₂?

Question 59

A 1.32 g sample of solid CuCl₂•2H₂O is added to 1.21 L of 2.0 M aqueous ammonia. The formation constant for tetra-amminecopper (II) is 1.0 x 10¹². What is the concentration of aqueous copper (II) in this solution?

Question 60

Trisbipyridylruthenium (II), Ru²⁺(BIPY)₃, has been used in many studies, most recently to understand electrical conductivity in DNA molecules. Write all the equilibria in the formation of this complex.

Question 61

1.1 mg of HgS (K_sp1.6 x 10^-54) is added to 1.0 L of 0.01 M NaCN ((Hg(CN)₄⁺²), K_f = 4 x 10⁴¹)? What is the value of the reaction quotient for Q_sp and will all of the HgS dissolve?

Question 62

Write the formation constant expression for the formation of [Zn(OH)₄]^-2 from Zn(OH)_2(s).Zn(OH)_2(s) + OH^-(aq) [Zn(OH)₃]^-1(aq)[Zn(OH)₃]^-1(aq) + OH^-(aq) [Zn(OH)₄]^-2(aq)

Question 63

Write the equilibrium constant expression and determine the solubility constant for CuS dissolving in an ammonia solution. (CuS, K_sp = 8 x10^-37) (Cu(NH₃)₄⁺², K_f = 1.1 x 10¹³)

Question 64

What is the concentration of [PbCl₃]^-prepared from a solution of 0.5 M CaCl₂ and solid PbCl₂?Pb²⁺ + 3Cl^- ? [PbCl₃]^- K_f = 2.4 x 10¹pK_sp(PbCl₂) = 4.77

Question 65

What is the concentration of [PbCl₃]^- when PbCl₂ solid first begins to precipitate on addition of NaCl salt to a 0.5 M PbNO₃ solution?
Pb²⁺ + 3Cl^- $\rightleftharpoons$ [PbCl₃]^-
K_f = 2.4 x 10¹pK_sp(PbCl₂) = 4.77

Question 66

Write the equilibrium constant expression and determine the solubility constant for HgS dissolving in a cyanide solution. (HgS, K_sp = 1.6 x 10^-54) ((Hg(CN)₂), K_f = 4 x 10⁴¹)