Deck 17: Redox Titrations

Question 1

The percent purity of a malonic acid reagent must be calculated.A student weighs out 0.1511 grams of malonic acid,CH₂(CO₂H)₂,and quantitatively transfers the solid to a 250 mL Erlenmeyer flask.To the flask 50.00 mL of distilled water is added to dissolve the malonic acid.After dissolution,the student uses a 50 mL volumetric pipet to add 50 mL of 0.200 M Ce⁴⁺,which is allowed to react with the malonic acid.After complete reaction,the excess Ce⁴⁺ is back titrated with 14.58 mL of 0.100 M Fe²⁺ solution.Calculate the percent purity malonic acid.
CH₂(CO₂H)₂ + 2 H₂O + 6 Ce⁴⁺ → 2 CO₂ + HCO₂H + 6 Ce³⁺ + 6 H⁺
Ce⁴⁺ + Fe²⁺ ⇋ Ce³⁺ + Fe³⁺

Answer 1

98.0% pure malonic acid

Question 2

Potassium permanganate is a strong oxidant commonly used in redox titrations.Which is(are)TRUE for permanganates titration chemistry?
I In strongly acidic solution the permanganate half-reaction is + 8 H⁺ + 5 e^- ⇋ Mn²⁺ + 4 H₂O.
II In neutral or alkaline solution the permanganate half-reaction is + 4 H⁺ + 3 e^- ⇋ MnO₂ (s)+ 2 H₂O.
III In strongly alkaline solution the permanganate half-reaction is + e^- ⇋ .

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

Answer 2

I,II,and III

Question 3

Sodium thiosulfate is used almost exclusively to titrate .Which of the following is NOT true for sodium thiosulfate titrant?

A)In neutral or acidic solution,triiodide oxidizes thiosulfate to tetrathionate.
B)One mole of

Is equivalent to two moles of I₂.
C)In basic solution,

Disproportionates to I⁻ and HOI,which can oxidize

To

)
D)Sodium thiosulfate is standardized with freshly made

Solution prepared from KIO₃ and KI.
E)Dissolved CO₂ and metal ion catalyzed atmospheric oxidation of thiosulfate decreases the concentration of prepared thiosulfate solution.

Answer 3

One mole of

Is equivalent to two moles of I₂.

Question 4

Potassium permanganate is NOT a primary standard.Why?
I Solid KMnO₄ contains traces of MnO₂.
[ _actual] < [ _calculated]
II Permanganate reacts with organic material in distilled water.
[ _actual] > [ _calculated]
III Permanganate is unstable in aqueous solution,producing MnO₂,O₂,and OH⁻.
[ _actual] < [ _calculated]
IV The presence of MnO₂,Mn²⁺,heat,light,acids,and bases increase the rate of reaction between permanganate and water.
[ _actual] < [ _calculated]

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

Question 5

For redox titrations,the oxidation state of the analyte may need adjusting.The analyte can be preoxidized with an oxidizing agent to quantitatively oxidize the analyte,with the excess oxidizing agent eliminated to prevent interference with titration.Which of the oxidizing agents is paired with INCORRECT elimination chemistry?

A)Excess peroxydisulfate is eliminated by boiling,converting the peroxydisulfate to sulfate.
B)Excess silver(I,III)oxide in mineral acid is eliminated by boiling,converting silver(III)cation to silver(I)cation.
C)Excess solid sodium bismuthate is removed by filtration.
D)Excess hydrogen peroxide in basic solution is eliminated by boiling,converting hydrogen peroxide to water and oxygen gas.
E)Excess stannous chloride in hot HCl is eliminated by boiling,converting stannous cation to stannic cation.

Question 6

____________________ is the titration of iodine produced by a chemical reaction.

A)Iodigravimetric titration
B)Iodimetry
C)Iodogravimetric titration
D)Iodometry
E)Iodotration

Question 7

Calculate the potential for a redox titration when 50.00 mL of 0.100 M Co³⁺ is titrated with 35.00 mL of 0.130 M .The potential is measured against the Ag-AgCl reference electrode,E = 0.197 V. Co³⁺ + e^- ⇋ Co²⁺____________________E^o = 1.92 V + e^- ⇋ E^o = 0.56 V

A)0)42 V
B)0)62 V
C)1)66 V
D)1)86 V
E)1)24 V

Question 8

Starch is an indicator for iodine.For iodimetry,starch solution is added:

A)at the beginning of the titration.
B)at the half-way point of the titration.
C)right before the equivalence point of the titration.
D)after the equivalence point of the titration has been reached.
E)to the titrant before starting the titration.

Question 9

A student is given the assignment to determine the mass iron in a brand of women's vitamins.He begins by taking four vitamin tablets,grinding the tablets into a fine powder to undergo the necessary sample preparation to free all iron from the sample matrix and to reduce all iron to the +2 state.The resulting Fe²⁺ solution is transferred to a 250 mL volumetric flask and diluted to volume with distilled water.A 50 mL aliquot of the sample requires 42.98 mL of 1.00 x 10⁻³ M .Calculate the mg Fe per tablet. ⇋

Question 10

Calculate the potential for a redox titration when 50.00 mL of 0.100 M Co³⁺ is titrated with 50.00 mL of 0.110 M .The potential is measured against the Ag-AgCl reference electrode,E = 0.197 V.
Co³⁺ + e^- ⇋ Co²⁺____________________E^o = 1.92 V + e^- ⇋ E^o = 0.56 V

Question 11

Which of the statements below are true for the titration of Fe²⁺ with standard Ce⁴⁺?
I The indicator electrode potential before the addition of Ce⁴⁺ solution is calculated using the Fe³⁺ half-reaction after calculating the equilibrium concentrations of Fe³⁺ and Fe²⁺ in solution.
II Before the equivalence point,the indicator electrode potential is calculated using the Fe³⁺ half-reaction because only [Fe³⁺] and [Fe²⁺] are known.
III At the equivalence point,the indicator potential is the average of E^o for Fe³⁺ and Ce⁴⁺.
IV After the equivalence point,the indicator electrode potential is calculated using the Ce⁴⁺ half-reaction because only [Ce⁴⁺] and [Ce²⁺] are known.

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

Question 12

In addition to redox indicators and starch-iodine complex,the equivalence point for a redox titration can be determined using a Gran plot.Which is NOT true for Gran plots?

A)Gran plots give best results using redox data well before the equivalence point.
B)Gran plots are constructed by plotting the volume titrant added on the x-axis and V10^{−nE/0.05916} on the y-axis.
C)The plot of V versus V10^{−nE/0.05916} is a straight line with the x-intercept = equivalence point volume.
D)Constant ionic strength over the course of the titration minimizes error in the equivalence point.
E)Gran plots are more error prone than first derivative and second derivative plots.

Question 13

Which statements are true for redox indicators?
I.The color change occurs when the indicator goes from its oxidized form to its reduced form.
II. The color for the oxidized state of the indicator occurs when the ratio of the reduced state to the oxidized state is 1:10.
III. The range over which the indicator functions is E^o .
IV. The color for the reduced state of the indicator occurs when the ratio of the reduced state to the oxidized state is is 1:10.

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

Question 14

Which are TRUE for the use of indicators with dichromate?
I Dichromate is orange and changes color to green when reduced.The color change is abrupt enough to serve as an indicator.
II The equivalence point for dichromate can be determined using Pt and calomel electrodes.
III Dichromate end points are determined using the distinct color changes of diphenylamine sulfonic acid or diphenylbenzidine sulfonic acid indicators.
IV In basic solution,the orange dichromate solution changes color to yellow when reduced to chromate anion.The color change is abrupt enough to serve as an indicator.

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

Question 15

A 50.00 mL aliquot of a water sample is reacted with excess potassium iodide in acidic solution to generate .Carbon dioxide is bubbled through the solution to remove nitrogen monoxide generated.The water sample is transferred to a 500 mL volumetric flask and diluted to volume.A 50.00 mL aliquot is then titrated against 1.092 x 10⁻⁴ M thiosulfate,requiring 15.48 mL to reach the starch end point.What is the concentration in ppm? Assume solution density of 1.000 g/mL. ⇋ ⇋

Question 16

Cerium(IV)is a strong oxidizing agent commonly used in redox titrations.Which of the following is NOT true for cerium(IV)?

A)Ce⁴⁺ is yellow and Ce³⁺ is colorless.The color change is abrupt enough that Ce⁴⁺ can serve as its own indicator.
B)Reduction of Ce⁴⁺ to Ce³⁺ proceeds cleanly in acidic solutions.
C)Ce⁴⁺ does not form aqueous complexes when dissolved in water.
D)Ce⁴⁺ is indefinitely stable in sulfuric acid solution.
E)Ce⁴⁺ reacts rapidly with chloride in hot hydrochloric acid solutions to form chlorine gas.

Question 17

For redox titrations,the oxidation state of the analyte may need adjusting.The analyte can be prereduced with a reducing agent to quantitatively reduce the analyte,with the excess reducing agent eliminated to prevent interference with titration.Which of the reducing agents is paired with INCORRECT elimination chemistry?

A)Stannous chloride in hot HCl is eliminated by the addition of excess HgCl₂,converting stannous cation to stannic cation.
B)Chromous chloride is eliminated by boiling,converting chromium(II)cation to chromium(III).
C)Sulfur dioxide is expelled by boiling in acid.
D)Hydrogen sulfide is expelled by boiling in base.
E)Excess hydrogen peroxide in acidic solution is eliminated by boiling,converting hydrogen peroxide to water and oxygen gas.

Question 18

A Ce⁴⁺ standard solution is prepared with primary-standard-grade ammonium hexanitratocerate(IV),(NH₄)₂Ce(NO₃)₆.Which of the follow will NOT decrease the concentration of Ce⁴⁺ in solution over time?

A)Ce⁴⁺ solution prepared in distilled water.
B)Ce⁴⁺ prepared in hot HCl solution.
C)Ce⁴⁺ prepared in H₂SO₄ solution.
D)Ce⁴⁺ prepared in HClO₄ solution.
E)Ce⁴⁺ prepared in HNO₃ solution.

Question 19

A permanganate solution is prepared by dissolving 20.0123 g KMnO₄ in 500 mL of distilled water and boiled for 1 hour to remove any organic material.Following sintered-glass filtration,the solution is quantitatively transferred to a 1.0 L volumetric flask and diluted to volume with distilled water.The permanganate solution was titrated against 0.1023 M oxalic acid prepared in sulfuric acid solution.A 50.00 mL aliquot of oxalic acid solution required 16.68 mL of permanganate solution.A titration blank required 0.04 mL of permanganate.What is the permanganate molarity? ⇋

Question 20

Which is NOT true for oxidation with potassium dichromate?

A)In acid solution,dichromate ion is orange.
B)In acid solution,dichromate ion is reduced to chromous ion.
C)Dichromate is a less powerful oxidizing agent than permanganate and cerium(IV).
D)In basic solution,dichromate is converted into yellow chromate ion.
E)Potassium dichromate is a primary standard.