DP Chemistry (last assessment 2024)

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Question 23M.3.SL.TZ2.2

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Date May 2023 Marks available [Maximum mark: 8] Reference code 23M.3.SL.TZ2.2
Level SL Paper 3 Time zone TZ2
Command term Calculate, Determine, Outline, State, Suggest Question number 2 Adapted from N/A
2.
[Maximum mark: 8]
23M.3.SL.TZ2.2

To investigate how much kale would supply the daily recommended intake of iron a student:

  1. weighed 79.6 g of kale leaves and blended with 500 cm3 of water
  2. boiled, filtered and cooled
  3. pipetted 10.0 cm3 of the filtrate into 20.0 cm3 of 2.00 mol dm−3 sulfuric acid in a flask
  4. titrated with 0.00100 mol dm−3 potassium manganate (VII).

The reaction taking place is:

5Fe2+ (aq) + MnO4− (aq) + 8H+ (aq) → 5Fe3+ (aq) + Mn2+ (aq) + 4H2O (l)

(a)

All species are almost colourless except for MnO4−, which has an intense purple colour, though the kale extract is coloured by the chlorophyll present.

(a.i)

State the colour change at the end point.

 

From:  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 


To:   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

[1]

Markscheme

green to purple
OR
green to brown
OR
green to purple-green ✓

 

Accept “colourless to purple”.
Accept “green to grey/blueish”.
Do not accept “clear” for “colourless”.
Do not accept “purple to “brown”.
Do not accept blue as final colour.

(a.ii)

Outline how the addition of distilled water to the 10.0 cm3 aliquot before titration will affect the titrant volume at the end point.

[1]

Markscheme

none / no effect ✓

(b)

State the class of errors that always affect results in a particular direction.

[1]

Markscheme

systematic ✓

 

Class must be stated, not specific examples.

(c)

The end point occurred when 3.1 ± 0.1 cm3 of the titrant had been added.

(c.i)

Calculate the percentage uncertainty associated with the titre.

[1]

Markscheme

«0.13.1 × 100 =» 3 «%» ✓

(c.ii)

Suggest one procedural modification which would reduce the percentage uncertainty for a single titration, other than using a burette with greater precision.

[1]

Markscheme

using more dilute potassium manganate(VII)
OR
using more dilute titrant
OR
larger aliquot/volume of filtrate ✓

 

Accept “using a pipette with more precision”
OR
“using a volumetric flask”
OR
“using a better balance/scale”
OR
“determining a more accurate mass”.

Do not accept “weight” for “mass”.

(c.iii)

The solution in the titration flask contained 8.66 × 10−4 g of iron. Determine, to three significant figures, the percentage of iron, by mass, in the kale leaves.

[2]

Markscheme

ALTERNATIVE 1
mass Fe in the 79.6 g kale «=8.66x10−4 × 50010.0»
= 0.0433 «g» ✓
percent by mass «= 0.043379.6 × 100»
= 0.0544«%» ✓

 

ALTERNATIVE 2
mass of kale in titration flask «=79.6 × 10.0500»
= 1.592 «g» ✓
percent by mass «= 8.66×10-41.592 × 100»
= 0.0544«%» ✓

 

Award [2] for correct final answer.

For ALTERNATIVE 2:
Award M1 for either 1.59 «g» OR 1.592 «g» and
Award M2 for 0.0545«%» OR 0.0544«%».

M2 must be to 3 sig. fig.

Award [1 max] for 0.00109«%».

(d)

The value obtained is about 30 times greater than published values for the percentage of iron in kale. Suggest one reason, other than human error, why there might be such a large discrepancy.

[1]

Markscheme

other substances in the leaves «as well as iron» react with the manganate(VII) «ion»
OR
kale modified to have more iron/Fe
OR
iron/Fe in water/pipes/container used for boiling
OR
manganate(VII) oxidized/reacted with other ions/substances/metals
OR
manganate(VII) concentration changes over time ✓

 

Accept “different species of kale can result in more iron/Fe «content»”
OR
“sample of kale not representative”
OR
“sample of kale grown in different soils”
OR
“kale sample being dry/dehydrated”.