Volumetric Analysis

Question 1

state the ionic equation for iodine-thiosulfate

Answer 1

I.2 + 2S.2O.3^2- —> 2I- + S.4O.6^2-

Question 2

why is iodine-thiosulfate widely used in volumetric analysis

Answer 2

it is a self indicating reaction as iodine in solution is red-brown but iodide ion solution is colourless

Question 3

what is iodine-thiosulfate used for

Answer 3

to determine conc of oxidising agent by its reaction with excess acidified potassium iodine

Question 4

what is in the pipette, conical flask and burrette (iodine thiosulfate)

Answer 4

• pipette: 25cm^3 standard potassium (V) solution
• conical flask: 25cm^3 standard KIO.3 solution & 1.5g KI in excess H.2SO.4 solution
• burette: sodium thiosulfate

Question 5

colour changes in iodine-thiosulfate titration

Answer 5

• red-brown (I- —> I.2)
• yellow/straw (I- form in titration of thiosulfate)
• blue-black (starch added before colourless … I.2 still present)
• colourless (I- solution)

Question 6

oxidising agents in iodine-thiosulfate

Answer 6

hydrogen peroxide (H.2O.2)
iodate(V) ions (IO.3-)

Question 7

ratios for oxidising agents (iodine-thiosulfate)

Answer 7

• H.2O.2 : I.2 (1 : 1)
• IO.3- : I.2 (1 : 3)
• I.2 : S.2O.3^2- (1 : 2)

Question 8

oxidation states of iodine and sulfur (iodine-thiosulfate)

Answer 8

iodine: 0 —> -1
sulfur: +2 —> +2.5

Question 9

potassium manganate (VII) alternative name

Answer 9

potassium permanganate

Question 10

KMnO.4 appearance (standard and solution)

Answer 10

standard: purple solid
solution: purple

Question 11

what is KMnO.4 used for

Answer 11

a powerful oxidising agent (reduced to manganese ions)
used to determine conc of reducing agents

Question 12

half equation of manganate ions being reduced

Answer 12

MnO.4- + 8H+ + 5e- —> Mn^2+ + 4H.2O

Question 13

what is in the pipette, conical flask and burette (potassium manganate (VII)

Answer 13

• pipette: 25cm^3 reducing agent
• conical flask: 25cm^3 reducing agent & excess sulfuric acid
• burette: standard solution of potassium manganate (VII)

Question 14

colour changes in potassium manganate (VII) titration

Answer 14

• purple (manganate (VII) solution)
• almost colourless (each drop of potassium manganate solution decolourises manganate (II)
• pink (reducing agent used up…last drop turns solution pink)

Question 15

reducing agents in potassium manganate titration

Answer 15

iron (II) ions (Fe^2+]
oxalate ions (C.2O.4^2-)

Question 16

potassium manganate titration ratios

Answer 16

• MnO.4^- : Fe^2+ (1 : 5)
• MnO.4^- : C.2O.4^2- (2 : 5)