volumetric analysis

Question 1

back titration

Answer 1

method where an excess pf reagent is reacted with a sample. the unreacted reagent is then determined by titration

Question 2

oxidising agents for iodine thiosulfate titration

Answer 2

1. hydrogen peroxide
2. iodate ions IO3-

Question 3

iodate ions half equation

Answer 3

2IO3- +12H+ +103- -> I2 + 6H2O
2I- -> I2 + 2e-

Question 4

ionic equation for iodate ions

Answer 4

IO3- + 6H+ + 5I- -> 3I2 + 3H2O

Question 5

equations for sodium thiosulfate being oxidised to sodium tetrathionate (NA2S4O6)

Answer 5

I2 + Na2S2O3 -> Na2S4O6 +2NaI
I2 + 2S2O3 2- -> 2I- + S4O6 2-

Question 6

equations for the titration of liberated iodine

Answer 6

I2 + 2e- -> 2I-
2S2O3 2- -> S4O6 2- +2e-

ionic equation
I2 + 2S2O3 2- —> 2I- + S4O6 2-

Question 7

half equations for hydrogen peroxide

Answer 7

2I- —> I2 + 2e-
H2O2 + 2H+ +2e- —> 2H2O

Question 8

ionic equation for H2O2

Answer 8

H2O2 + 2I- + 2H+ —> I2 + 2H2O

Question 9

summary of colour changes in iodine-thiosulfate titration

Answer 9

• during addition of KI (COLOURLESS > BROWN)
• during titration with standard thiosulfate (BROWN > STRAW YELLOW)
• on addition of starch indicator (STRAW YELLOW > BLUE-BLACK)
• at end point (BLUE-BLACK > COLOURLESS)

Question 10

equations for titration of iron(ii) by manganate(vii)

Answer 10

half equations
Fe2+ > Fe3+ + e- (x5)
MnO4- + 8H+ + 5e- > Mn2+ + 4H2O

Ionic equation
5Fe2+ +MnO4- 8H+ > 5Fe3+ + Mn2+ + 4H2O
purple> colourless > pale permanent pink

Question 11

when is a back titration used

Answer 11

when substance under analysis is insoluble in water