Redox titrations

Question 1

an example of a powerful oxidising agent is

Answer 1

potassium manganate(VII)

Question 2

potassium manganate(VII) is used for the quantitative estimation of many reducing agents such as compounds of…………and for………….. and its salts

Answer 2

compounds of iron(II)

ethanedioic acid and its salts

Question 3

in acidic solution, the half equation for the reduction of the manganate(VII) ions is:

Answer 3

MgO4-(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4H2O(l)

Question 4

in alkaline solution, potassium manganate(VII) yields……………………..as a ……. precipitate, which interferes with the end point colour

Answer 4

manganese(IV) oxide as a brown precipitate

Question 5

because potassium manganate(VII) produces a brown precipitate in alkaline solution, potassium manganate(VII) is almost always used in sufficiently……….solution to prevent the formation of manganese(IV) oxide

Answer 5

acidic

Question 6

as the titration proceeds, potassium manganate(II) ions, Mg2+ ions, accumulate but give a colourless solution, but when an excess of potassium manganate(VII) ions is present, the solution becomes…….in colour

Answer 6

pink

Question 7

in the titration of potassium manganate(VII) with iron(II) ions, the ionic half-equations involved are

Answer 7

MgO4_(aq) + 8H+(aq) +5e- → Mn2+(aq) + 4H2O(l)

Fe2+(aq) → Fe3+(aq) + e-

Question 8

the overall equation for the reaction between iron(II) ions and potassium manganate(VII) is:

Answer 8

MgO4-(aq) + 8H+(aq) +5Fe2+(aq) → Mn2+(aq) +4H2O(l) + 5Fe3+(aq)

Question 9

an aqueous solution of containing MnO4- ions is………in colour

Answer 9

purple

Question 10

in concentrated solutions, the following ions have which colours?
Fe2+(aq)
Fe3+(aq)
Mn2+(aq)

Answer 10

Fe2+(aq) = pale green
Fe3+(aq) = yellow
Mn2+(aq)= pale pink

not that in titrations, the solutions are usually so dilute that they appear to be colourless

Question 11

the method for performing the titration of potassium manganate(VII) with iron(II) ions is:

Answer 11

• pipette 25.0cm3 of the iron(II) solution into a conical flask and then add a small volume of dilute sulfuric acid (this is so the MnO4- ions produce Mn2+ ions, not manganese(IV) oxide)
• slowly add potassium manganate(VII) solution from the burette and swirl the flask
• the potassium manganate(VII) solution will turn colourless until all of the iron(II) ions have been oxidised
• the addition of one drop of potassium manganate(VII) solution will turn the mixture pale pink

Question 12

the formula of ethanedioic acid is:

Answer 12

HOOCCOOH