CHEM 112 BUFFERS AND TITRATIONS SB ASSIGNMENT

Consider a general buffer system made from a weak acid, HA, and its conjugate base A-. Which of the following options correctly describe the behavior of this system when strong acid is added to it? Select all that apply.

The ratio [HA]/[A-] will increase. The [A-] in solution will decrease. The overall pH will decrease only slightly.

Match each compound to the substance(s) that would increase the solubility of the compound when present in aqueous solution.

Look below.

Match each description concerning the solubility of a metal hydroxide (M(OH)nM(OH)n) in acidic solution to the correct aqueous equilibrium.

Look below.

Match each stage of a weak acid-strong base titration with the correct description of how to calculate [H3O+] and/or pH.

Look below.

Match each term to the correct definition, with reference to an acid-base titration.

Look below.

Overall equilibrium for the dissolution of a metal hydroxide in acidic solution

M(OH)n (s) + nH3O+ (aq) ⇌ Mn+ (aq) + 2nH2

Solubility equilibrium of a metal hydroxide

M(OH)n ⇌ Mn+ (aq) + nOH− (aq)

Solve for the molar solubility of AgCl in a 0.20-M solution of Na2S2O3. AgCl (s) + 2S2O32- (aq) ⇌ Ag(S2O3)23- (aq) + Cl- (aq) Koverall = 7.5 x 103

9.9 x 10-2 M Reason: Koverall = [Ag(S2O3)23-][Cl-]/[S2O32-]^2 7.5 x 10^3 = s2/(0.20 − 2s)^2, take the square root of both sides: 86.6 = s/(0.20 − s)ss = 9.9 x 10-2 M

Which of the following statements correctly describe the equivalence point of a strong acid (HX), strong base (MOH) titration? Select all that apply.cWhich of the following statements correctly describe the equivalence point of a strong acid (HX), strong base (MOH) titration? Select all that apply.

At the equivalence point of a strong acid (HX), strong base (MOH) titration equimolar amounts of acid and base have reacted. At the equivalence point of a strong acid (HX), strong base (MOH) titration the pH of the solution is 7.000.

0.050 moles HA and 0.075 moles MOH

Beyond the equivalence point

initial pH

Calculate [H3O+] using Ka and [HA]initial

Complete the following equilibrium concerning the formation of a complex ion. Cr3+ + 4OH- ⇌ ?

Cr(OH)4- Reason: The total charge on both sides of the equilibrium must be the same. (3+) + 4(1-) = 1-. The complex formed is Cr(OH)4-.

Which of the following equations correctly represents the neutralization of a weak acid (HA) with a strong base (MOH)? Select all that apply.

HA (aq) + MOH (aq) → MA (aq) + H2O (l) HA (aq) + OH- (aq) → A- (aq) + H2O (l) Reason: H+ (aq) + OH- (aq) → H2O (l) is the net ionic equation for the neutralization of a strong acid with a strong base.

MgCO3

HCl

AgCl

NH3

Al(OH)3

NaOH or HCl

Which of the statements correctly describe the formation constant (Kf) for a complex ion? Select all that apply.

The formation constant has a value much greater than 1. The formation constant is the equilibrium constant for the complex ion formation. The larger the formation constant, the more stable the complex ion is.

Which of the following is true regarding the curve resulting from the titration of a weak base with a strong acid?

The pH at the equivalence point is less than 7. Reason: The titrated weak base produces a conjugate acid that can donate protons for a slightly more acidic solution at the equivalence point. Reason: The pH of the solution before titration is dependent upon the concentration and Kb of the weak base.

What conditions must be met in order to have a buffer solution? Select all that apply.

The pH of the buffer cannot be more than one pH unit different from the pKa of the weak acid it contains. The concentrations of a weak acid and conjugate base differ by less than a factor of 10.

Which of the following statements correctly reflect the relationship between buffer composition and solution pH given that Ka = [H3O+][A−]/[HA]? Select all that apply.

The pH of the solution depends on the ratio [HA]/[A−]. If the relative amount of HA is increased, the solution pH will decrease.

55 mL of a 0.25-M solution of the weak acid, HA, is titrated with 0.30 M NaOH. Which of the following statements correctly describes this titration after 60. mL of the NaOH has been added?

The titration is beyond the equivalence point. The solution is basic (pH > 7). Reason: The [OH-] from the excess NaOH is the primary factor that affects the pH of the solution.

What can be determined from the weak acid-NaOH titration curve shown? Select all that apply.

The volume of strong base needed to reach the equivalence point The pKa value of the weak acid The pH value at the equivalence point

Compared to the titration curve for titration of a strong acid with a strong base, the curve for titration of a weak acid with a strong base has which of the following features? Select all that apply.

a higher initial pH for equal acid concentrations a shorter vertical region near the equivalence point Reason: A weak acid-strong base titration curve consists of a more basic equivalence point. Reason: A weak acid-strong base titration curve consists of a more gradual change in pH as base is added.

Which of the following will NOT become more soluble if the pH of the solution is decreased? Select all that apply.

AgBr PbCl2 Reason: S2- is a strong base and reacts with H3O+ to form HS-. ZnS will become more soluble if the pH decreases.

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

neutralization of a strong acid with a strong base

CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)

neutralization of a weak acid by strong base

Place the steps for preparing a buffer in the correct order, beginning with the first step at the top of the list.

1. Choose an appropriate cojugate acid-base pair 2. Calculate the ratio of buffer component concentrations needed 3. Convert the ratio of buffer component concentrations to appropriate molar quantities 4. Mix components together and correct pH if needed.

25.0 mL of a 0.20 M solution of the weak acid CH3COOH (Ka = 1.8 × 10-5) is titrated with 0.25 M NaOH. Place the steps required to calculate the pH at the equivalence point in the correct order, starting with the first step at the top of the list.

1. Moles CH3COOH 2. Volume NaOH 3. [CH3COO-] 4. [OH-] 5. pH

A metal hydroxide is combined with each of the following solutions. List the solutions in order of maximizing the solubility of the metal hydroxide. The solution that dissolves the most metal hydroxide belongs at the top of the list.

1. acidic solution 2. neutral solution 3. basic solution

The quantity 0.0300 moles is equal to how many millimoles?

3.00 × 10^1 mmol 1 mol = 1000 mmol

Which of the following options correctly describes an acid-base buffer? Select all that apply.

A buffer maintains a relatively narrow pH range of a solution. A buffer is a solution that contains a weak acid and its conjugate base (also weak). A buffer resists changes in pH upon addition of small amounts of either an acid or a base.

Which of the following options correctly describes the function of an acid-base buffer?

A buffer minimizes changes in pH when acid or base is added to the solution.

The Ksp for Fe(OH)2 is 1.6 x 10-14. Which of the following procedures would increase the solubility of Fe(OH)2? Select all that apply.

Addition of 1.0 M NaCN (aq) Addition of 1.0 M HCl (aq) Reason: Adding Fe(NO3)2 would introduce the common ion, Fe3+, which would decrease the solubility of Fe(OH)2. Reason: Adding NaOH would introduce the common ion, OH-, which would decrease the solubility of Fe(OH)2.

Which of the following statements correctly describe the equivalence point of a strong acid-weak base titration? Select all that apply.

At the equivalence point of a strong acid (HA)-weak base (NH3) titration equimolar amounts of acid and base have reacted. At the equivalence point of a strong acid-weak base titration [H3O+] > 1.0 x 10-7 M.

Which of the following statements correctly describe the equivalence point of a strong acid (HX), strong base (MOH) titration? Select all that apply.

At the equivalence point of a strong acid (HX), strong base (MOH) titration equimolar amounts of acid and base have reacted. At the equivalence point of a strong acid (HX), strong base (MOH) titration the pH of the solution is 7.000.

Which of the following options correctly describe how to calculate the pH at various points during the titration of a weak acid against a strong base? Select all that apply.

At the equivalence point, [A-] = initialmolesofHAtotalvolumeinitialmolesofHAtotalvolume. At the equivalence point the pH calculation is based on the reaction of the conjugate base A- with H2O. The initial [H3O+] is calculated from [HA]init and Ka.

In an experiment, an aqueous solution of base is slowly added from a burette to a vessel containing an acid with a known concentration and volume. Which substance in this experiment is the titrant?

Base Reason: The titrant is the solution that is added from the burette.

0.050 moles HA and 0.025 moles MOH

Before the equivalence point

Which of the following combinations could be used in an acid-base buffer system? Select all that apply.

CH3COOH/NaCH3COO NH3 /NH4Cl HNO2 /KNO2

Calcium sulfate is added to a solution of 0.10 M CaCl2. Which of the following steps are required to calculate the molar solubility of CaSO4? (Select all that apply.)

CaSO4(s) ⇄ Ca2+(aq) + SO42-(aq) Ksp = 4.93 × 10-5 = [Ca2+][SO42-]

Which of the following quantities must be known in order to calculate the pH of an acid-base buffer solution using the Henderson-Hasselbalch equation? Select all that apply.

Concentration of conjugate base (or conjugate acid) Concentration of weak acid (or weak base) Ka of weak acid or Kb of weak base

The formation of the complex ion Cu(CN)42- is correctly represented by which of the following equilibrium expressions?

Cu2+ + 4CN- ⇌ Cu(CN)42-

Which of the following statements correctly describe how pH and/or hydroxide and hydronium ion concentration influence the solubility of a metal hydroxide (M(OH)nM(OH)n)? M(OH)n (s) ⇌ Mn+ (aq) + nOH− (aq)

Decreasing the pH of solution increases the solubility of M(OH)n. The addition of 0.10 M HCl to the solution increases the solubility of M(OH)n.

The neutralization of a strong acid (HX) with a strong base (MOH) is correctly represented by which of the following equations? Select all that apply.

H3O+ (aq) + OH- (aq) → 2H2O (l) H+ (aq) + OH- (aq) → H2O (l) HX (aq) + MOH (aq) → H2O (l) + MX (aq)

A complex ion consists of a central metal _____ covalently bonded to one or more ions or molecules.

Ion

25.0 mL of 0.250 M HCl is titrated with a 0.125 M solution of the weak base aniline (C6H5NH2; Kb = 3.80 x 10-10). The equivalence point is reached when 50.0 mL of base has been added. Which of the following options correctly reflect the steps required to calculate the pH at the equivalence point? Select all that apply.

Ka of the conjugate acid C6H5NH3+ = 2.63 x 10-5 pH = 2.82 [C6H5NH3+] at the equivalence point = 6.25x10^-3/0.025+0.050L = 8.33 x 10-2 M

Which of the following will NOT become more soluble if the pH of the solution is decreased? Select all that apply.

PbCl2 AgBr Reason: Since Cl- is the conjugate base of a strong acid, it is not affected by decreasing pH. Reason: Br- is the anion of a strong acid and is not affected by decreasing pH.

Which of the following statements correctly describe how to name an ester? Select all that apply.

The alkyl portion is named first, followed by the name for the acyl portion. The alkyl and acyl portions of the ester structure are named separately. The portion of the ester directly bonded to the oxygen atom is named as an alkyl (or aryl) group, with the -yl ending. The -ic ending of the parent carboxylic acid is replaced by the suffix -ate.

______ have/has a particular tendency to form complex ions.

Transition metals

Consider a buffer solution consisting of 0.35 M HNO2 and 0.50 M KNO2, which has an initial pH of 3.30 (Ka for HNO2 = 7.1 x 10-4). If 0.030 mol of HCl are added to 1.0 L of this solution, select all the options that correctly reflect the steps required to calculate the change in pH.

[H3O+] = 7.1 x 10^-4 x 0.38/0.47 pH = -log(7.1×10^−4 × 0.38/0.47) = 3.24

before equivalence point

[H3O+] = Kax[HA]/[A-]; [HA] and [A-] determined using reaction table

Initial pH

[H3O+] = [HA] and pH = -log[H3O+] [H3O+] = [HA] and pH = -log[H3O+]

A chemist titrates a 25.00-mL portion of 0.15 M HCl with a 0.20 M solution of KOH. Which of the following options correctly describe how to calculate the pH at the beginning of this titration AND after 15.00 mL of base has been added? Select all that apply.

[H3O+] after base has been added = 7.5×10^−4/0.0400L initial pH = -log(0.15) = 0.82 initial moles of H3O+ = 3.75 × 10^-3

The pH of a saturated metal hydroxide solution is 11.12. If the pH were 11.50, the solubility of the metal hydroxide in solution would ______.

decrease Reason: Increasing the pH means more OH−OH- ions in solution, which would push the equilibrium of the metal hydroxide towards the reactants side or increase the amount of solid metal hydroxide.

The _____ point of a titration is the point at which the indicator changes color. The indicator is chosen so that the color change occurs at a pH as close as possible to the pH of the ______ point.

end, equivalence

A measure of the tendency of a metal ion to form a particular complex ion is given by the ______ constant.

formation Reason: The equilibrium constant for the formation of a complex ion is specifically called the "formation" constant.

Match the initial moles of weak acid (HA) and strong base (MOH) to the correct stage of an acid-base titration.

look below.

at equivalence point

matches Choice, Calculate [OH-] usingKband [A-]; [H3O+] =Kw[OH-] Calculate [OH-] using Kb and [A-]; [H3O+] = Kw/[OH-]

Which of the following equalities are correct at the equivalence point of a weak acid (HA), strong base (MOH) titration? Select all that apply.

moles HA = moles OH- pH > 7.000

Which of the following equalities are correct at the equivalence point of a strong acid-strong base titration? Select all that apply.

moles OH- = moles H3O+ [OH-] = [H3O+] pH = 7.000

Which of the following equalities are correct at the equivalence point of a strong acid-weak base titration? Select all that apply.

pH < 7.000 moles weak base = moles H3O+ Reason: [OH-] < [H3O+] due to the hydrolysis of the conjugate acid of the weak base that is formed. Reason: [weak base] < [conjugate acid] Reason: [H3O+] > 1.00 x 10-7 M due to the hydrolysis of the conjugate acid of the weak base that is formed.

When calculating the pH of a saturated metal hydroxide solution X(OH)2, which of the following calculations and equations are required? (Select all that apply.)

pH = 14 − pOH OR [H+] = Kw/[OH−] X(OH)2 (s) ⇆ X2+ (aq) + 2OH− (aq)

When calculating the pH of a saturated metal hydroxide solution X(OH)2X(OH)2, which of the following calculations and equations are required? (Select all that apply.)

pH = 14 − pOH OR [H+] = Kw/[OH−] X(OH)2 (s) ⇆ X2+ (aq) + 2OH− (aq)

A buffer solution consists of 0.23 M HF and 0.31 M NaF (pKa for HF = 3.18). Which option shows the correct calculation for the pH of the buffer using the Henderson-Hasselbalch equation after 0.010 mol of HCl is added to 1.0 L of the solution?

pH = 3.18 + log (0.30/0.24) Reason: The 0.010 mol of HCl reacts with F- so that [F-]new = 0.30 M. [HF]new = 0.24 M.

A buffer solution consists of 0.45 M HCOOH and 0.63 M HCOONa (pKa for HCOOH = 3.74). Which option shows the correct calculation for the pH of the buffer after 0.020 mol of solid NaOH is added to 1.0 L of the solution?

pH = 3.74 + log (0.65/0.43)

Consider a buffer made by combining equal volumes of 0.15 M CH3COOH and 0.32 M NaC2H3O2. What is the pH of this buffer if the Ka for CH3COOH is 1.8 x 10-5?

pH = 5.07

Which of the following is the correct form of the Henderson-Hasselbalch equation?

pH = pKa + log [conjugate base]/[weak acid]

0.050 moles HA only

prior to the addition of any base

In acid base titration, the _____ is the solution that is added from the burette into the titration vessel.

titrant

Which of the following can NOT be used as an acid-base buffer solution? Select all that apply.

0.5 M HNO3 and 0.5 M NaNO3 0.3 M HCl and 0.3 M NaOH Reason: HNO3 is a strong acid. An acid-base buffer must consist of a conjugate acid-base pair of a weak acid or base. Reason: HCl and NaOH are not a conjugate acid-base pair. Reason: 0.4 M HClO and 0.3 M NaClO is an acid-base buffer solution consisting of a weak acid and a salt of its conjugate base.

1 mmol is equal to which of the following? Select all that apply.

1 M x 1 mL 1 M x 10-3 L

30.0 mL of a 0.15 M solution of the weak acid HClO is titrated with 0.20 M NaOH. Which of the following options correctly reflect how to calculate the pH after the addition of 16.5 mL of NaOH? Select all that apply. (The equivalence point has not yet been reached.) Ka for HClO is equal to 3.0 × 10-6.

1.2 × 10-3 moles of HClO remain in solution. pH = -log(3.0 × 10-6) + log (3.3×10^-3/1.2×10^-3)(1.2×10^-3) 3.3 × 10-3 moles of OH- have been added.

Which of the following conditions must be met in order to have a buffer solution? Multiple choice question. 10 ≥ [conjugate base][weak acid][conjugate base][weak acid] ≥ 0.1

10 ≥ [conjugate base]/[weak acid] ≥ 0.1

Calculate the pH of a saturated Zn(OH)2 solution. ksp (Zn(OH)2) = 1.8 × 10−14

9.53 Reason: ksp = (s)(2s)^2, 1.8 × 10^−14 = 4s3, s = 1.7 × 10−5 M, [OH−] = 2(1.7 × 10−5 M) = 3.4 × 10−5 M, pOH = 4.47, pH = 9.53

hich of the following statements correctly describe the equivalence point of a weak acid (HA), strong base (MOH) titration? Select all that apply.

At the equivalence point of a weak acid-strong base titration [OH-] > 1.0 x 10-7 M. At the equivalence point of a weak acid-strong base titration equimolar amounts of acid and base have reacted.

Which of the following statements correctly describe the equivalence point of a weak acid (HA), strong base (MOH) titration? Select all that apply.

At the equivalence point of a weak acid-strong base titration equimolar amounts of acid and base have reacted. At the equivalence point of a weak acid-strong base titration [OH-] > 1.0 x 10-7 M.

Which of the following options correctly describe how to calculate the pH at various stages during the titration of a strong acid against a strong base? Select all that apply.

At the equivalence point pH = 7.00. Initial pH = -log[HA].

Which of the following salts will dissolve more readily in aqueous nitric acid than in pure H2O? Select all that apply.

Fe(OH)3 CaCO3 Zn3(PO4)2 Reason: Cl- is the anion of a strong acid, HCl, and will therefore not readily react with a strong acid to increase the solubility of AgCl. Reason: ClO4- is the anion of a strong acid, HClO4, and will therefore not readily react with a strong acid to increase the solubility of Ba(ClO4)2.

hich of the following salts will dissolve more readily in aqueous nitric acid than in pure H2O? Select all that apply.

Fe(OH)3 Zn3(PO4)2 CaCO3 Reason: Cl- is the anion of a strong acid, HCl, and will therefore not readily react with a strong acid to increase the solubility of AgCl. Reason: ClO4- is the anion of a strong acid, HClO4, and will therefore not readily react with a strong acid to increase the solubility of Ba(ClO4)2.

Addition of ______ ions from the ionization of carbonic acid shifts the equilibrium in favor of ______ oxyhemoglobin molecules and causes the release of oxygen.

H+; unionized Reason: The H+ from the ionization of carbonic acid is involved with the conversion of ionized oxyhemoglobin to unionized oxyhemoglobin. The HHbO2 form of oxyhemoglobin is favored and releases oxygen more readily than the ionized version, HbO2−.

Hydroxide ions are neutralized in the presence of acidic solution

H3O+(aq)+OH-(aq)⇌2H2O(l)

Which of the following conjugate acid-base pairs is the best choice to prepare a buffer of pH 3.50?

HCOOH/HCOONa (pKa of HCOOH = 3.74) Reason: The pKa of the weak acid component should be as close as possible to the desired pH.

The neutralization of a strong acid with a strong base is correctly represented by which of the following equations? Select all that apply.

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l) HBr (aq) + KOH (aq) → KBr (aq) + H2O (l)

What are the principal buffer systems found in blood plasma and/or red blood cells? Select all that apply.

HHb (hemoglobin)/Hb− H2CO3/HCO3−

Which of the following equations correctly represent the neutralization of the weak base, ammonia (NH3), with the strong acid, nitric acid (HNO3)? Select all that apply.

HNO3 (aq) + NH3 (aq) → NH4NO3 (aq) H3O+ (aq) + NH3 (aq) → NH4+ (aq) + H2O (l)

Which of the following statements correctly describes an acid-base indicator? Select all that apply.

Many acid-base indicators are plant pigments. An acid-base indicator is selected whose color change coincides with the steepest part of the titration curve. An acid-base indicator is usually a weak organic acid or base for which the ionized and un-ionized forms are different colors.

Before equivalence point

Moles H3O+ remaining = (initial moles H3O+) - (moles H3O+reacted); use total volume to calculate new [H3O+] and pH

After equivalence point

Moles excess OH- present = (moles OH-added) - (moles OH-reacted); use total volume to calculate new [OH-]; calculate pH from pOH

A chemist titrates a 25.00-mL portion of 0.25 M HNO3 with a 0.25 M solution of NaOH. Which of the following options correctly reflect the steps required to calculate the pH once 35.00 mL of NaOH has been added? Select all that apply.

Moles of OH- added = 8.75 × 10^-3 Moles of OH- in excess = 2.5 × 10^-3 pH = 12.62

Which of the following statements correctly describe the titration curve for the titration of a strong acid with a strong base? Select all that apply.

The pH rise is very steep close to the equivalence point of the titration. The equivalence point is at a pH of 7.00. Reason: A strong acid-strong base titration will give a neutral solution at the equivalence point.

Which of the following should be considered when selecting/preparing a buffer solution? Select all that apply.

The pKa of the acid component of the buffer should be close to the desired pH. The acid and conjugate base used must both be weak. The pH of the buffer depends on the ratio of acid and base concentrations.

endpoint

The point at which the indicator changes color

equivalence point

The point at which the moles of OH- added are equal to the moles of H3O+ originally present

A buffer is made up using 2.5 L of 0.25 M sodium phenolate (C6H5ONa) and solid phenol (C6H5OH; pKa = 10.0). The desired buffer pH is 9.82. Which of the following options correctly show the calculations required to calculate the mass of phenol needed? Select all that apply.

[phenol] = 0.2510-0.180.2510-0.18 9.82 = 10.00 + log (0.25/[phenol]) Mass of phenol required = 2.5 L x 0.38 mol/1 L x 94.11 g/ 1 mol

Compared to the titration curve for titration of a strong base with a strong acid, the curve for titration of a weak base with a strong acid has which of the following features? Select all that apply.

a more acidic equivalence point a shorter vertical region near the equivalence point

In solution, an acid-base indicator can be represented by the following equilibrium where HIn and its conjugate base, In-, have distinctly different colors. HIn (aq) + H2O (l) ⇌ H3O+ (aq) + In- (aq) Match each type of medium to the expected color of solution based on the ionization of the indicator.

acidic medium - the color of the solution will be that of HIn basic medium - the color of the solution will be that of In-

0.050 moles HA and 0.050 moles MOH

at the equivalence point

Which of the following equalities are correct at the equivalence point of a weak acid (HA), strong base (MOH) titration? Select all that apply.

pH > 7.000 moles HA = moles OH- Reason: [OH-] > [H3O+] due to the hydrolysis of the conjugate base (A-) that is formed. Reason: [A-] > [HA]

after equivalence point

pH depends on excess base added

Which of the following correctly describes the reaction below? H3O+(aq) + B(aq) → BH+(aq) + H2O(l)

strong acid-weak base neutralization