Transition elements

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Question 26

Groundwater from a borehole near a disused iron mine is suspected to be contaminated with iron(II) and iron(III) ions.

A student determines the concentration of iron(II) ions in a sample of the groundwater by titration with potassium manganate(VII).

$cm325.0\text{ cm}^3$ portions of groundwater are acidified with dilute sulfuric acid. Each portion is titrated with $dm−30.00200\text{ mol dm}^{-3}$ potassium manganate(VII) until a permanent colour change is observed.

\text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) + 5\text{Fe}^{2+}(\text{aq}) \rightarrow \text{Mn}^{2+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l}) + 5\text{Fe}^{3+}(\text{aq})

State the colour change seen at the end point of the titration. from .................................... to ....................................

[1]

The student's titration results are shown in the table below. The trial titre has been omitted.

	1	2	3
Final volume / $cm3\text{cm}^3$	14.85	30.00	44.95
Initial volume / $cm3\text{cm}^3$	0.00	14.85	30.00
Titre volume / $cm3\text{cm}^3$

Complete the table above and calculate the mean titre that the student should use to determine the concentration of iron(II) ions in the groundwater.

mean titre = .................................... $cm3\text{cm}^3$

[2]

Determine the concentration, in $dm−3\text{mol dm}^{-3}$ , of iron(II) ions in the groundwater. Present your answer to three significant figures.

concentration = .................................... $dm−3\text{mol dm}^{-3}$

[3]

The student modifies the experiment to determine the combined concentration of iron(II) and iron(III) ions in the groundwater.

The student's method is shown below.

Step 1: Add excess zinc powder to a $cm3250.0\text{ cm}^3$ sample of groundwater and warm gently.
Step 2: Cool the solution and remove the excess zinc by filtration.
Step 3: Acidify $cm325.0\text{ cm}^3$ portions of the filtrate from Step 2. Then titrate each portion with $dm−30.00200\text{ mol dm}^{-3}$ potassium manganate(VII) until a colour change is seen.

The table below shows information about three redox systems.

Redox system	Half-equation	$EθE^\theta$ / $V\text{V}$
1	$Zn2+(aq)+2e−⇌Zn(s)\text{Zn}^{2+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Zn}(\text{s})$	$- 0.76$
2	$Fe3+(aq)+e−⇌Fe2+(aq)\text{Fe}^{3+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{Fe}^{2+}(\text{aq})$	$+ 0.77$
3	$MnO4−(aq)+8H+(aq)+5e−⇌Mn2+(aq)+4H2O(l)\text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) + 5\text{e}^- \rightleftharpoons \text{Mn}^{2+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l})$	$+ 1.51$

Use the information in the table above to explain the reasons for Step 1 and Step 2.

Reason(s) for Step 1: ....................................

Reason(s) for Step 2: ....................................

[4]

Final volume / $cm3\text{cm}^3$

14.85

30.00

44.95

Initial volume / $cm3\text{cm}^3$

0.00

14.85

30.00

Titre volume / $cm3\text{cm}^3$

Redox system

Half-equation

EθE^\theta

V\text{V}

Zn2+(aq)+2e−⇌Zn(s)\text{Zn}^{2+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Zn}(\text{s})

- 0.76

Fe3+(aq)+e−⇌Fe2+(aq)\text{Fe}^{3+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{Fe}^{2+}(\text{aq})

+ 0.77

MnO4−(aq)+8H+(aq)+5e−⇌Mn2+(aq)+4H2O(l)\text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) + 5\text{e}^- \rightleftharpoons \text{Mn}^{2+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l})

+ 1.51

Transition elements

Question 26

Transition elements Questions

Transition elements

Question 26

Transition elements Questions