Cambridge International AS & A Level

CHEMISTRY 9701/35
Paper 3 Advanced Practical Skills 1 October/November 2020
MARK SCHEME
Maximum Mark: 40

Published

This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the
examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the
details of the discussions that took place at an Examiners’ meeting before marking began, which would have
considered the acceptability of alternative answers.

Mark schemes should be read in conjunction with the question paper and the Principal Examiner Report for
Teachers.

Cambridge International will not enter into discussions about these mark schemes.

Cambridge International is publishing the mark schemes for the October/November 2020 series for most
Cambridge IGCSE™, Cambridge International A and AS Level and Cambridge Pre-U components, and some
Cambridge O Level components.

This document consists of 10 printed pages.

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9701/35 Cambridge International AS & A Level – Mark Scheme October/November 2020
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Generic Marking Principles

These general marking principles must be applied by all examiners when marking candidate answers. They should be applied alongside the
specific content of the mark scheme or generic level descriptors for a question. Each question paper and mark scheme will also comply with these
marking principles.

GENERIC MARKING PRINCIPLE 1:

Marks must be awarded in line with:

• the specific content of the mark scheme or the generic level descriptors for the question
• the specific skills defined in the mark scheme or in the generic level descriptors for the question
• the standard of response required by a candidate as exemplified by the standardisation scripts.

GENERIC MARKING PRINCIPLE 2:

Marks awarded are always whole marks (not half marks, or other fractions).

GENERIC MARKING PRINCIPLE 3:

Marks must be awarded positively:

• marks are awarded for correct/valid answers, as defined in the mark scheme. However, credit is given for valid answers which go beyond the
scope of the syllabus and mark scheme, referring to your Team Leader as appropriate
• marks are awarded when candidates clearly demonstrate what they know and can do
• marks are not deducted for errors
• marks are not deducted for omissions
• answers should only be judged on the quality of spelling, punctuation and grammar when these features are specifically assessed by the
question as indicated by the mark scheme. The meaning, however, should be unambiguous.

GENERIC MARKING PRINCIPLE 4:

Rules must be applied consistently, e.g. in situations where candidates have not followed instructions or in the application of generic level
descriptors.

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GENERIC MARKING PRINCIPLE 5:

Marks should be awarded using the full range of marks defined in the mark scheme for the question (however; the use of the full mark range may
be limited according to the quality of the candidate responses seen).

GENERIC MARKING PRINCIPLE 6:

Marks awarded are based solely on the requirements as defined in the mark scheme. Marks should not be awarded with grade thresholds or
grade descriptors in mind.

Science-Specific Marking Principles

1 Examiners should consider the context and scientific use of any keywords when awarding marks. Although keywords may be present, marks
should not be awarded if the keywords are used incorrectly.

2 The examiner should not choose between contradictory statements given in the same question part, and credit should not be awarded for any
correct statement that is contradicted within the same question part. Wrong science that is irrelevant to the question should be ignored.

3 Although spellings do not have to be correct, spellings of syllabus terms must allow for clear and unambiguous separation from other syllabus
terms with which they may be confused (e.g. ethane / ethene, glucagon / glycogen, refraction / reflection).

4 The error carried forward (ecf) principle should be applied, where appropriate. If an incorrect answer is subsequently used in a scientifically
correct way, the candidate should be awarded these subsequent marking points. Further guidance will be included in the mark scheme where
necessary and any exceptions to this general principle will be noted.

5 ‘List rule’ guidance

For questions that require n responses (e.g. State two reasons …):

• The response should be read as continuous prose, even when numbered answer spaces are provided.
• Any response marked ignore in the mark scheme should not count towards n.
• Incorrect responses should not be awarded credit but will still count towards n.
• Read the entire response to check for any responses that contradict those that would otherwise be credited. Credit should not be
awarded for any responses that are contradicted within the rest of the response. Where two responses contradict one another, this
should be treated as a single incorrect response.
• Non-contradictory responses after the first n responses may be ignored even if they include incorrect science.

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6 Calculation specific guidance

Correct answers to calculations should be given full credit even if there is no working or incorrect working, unless the question states ‘show
your working’.

For questions in which the number of significant figures required is not stated, credit should be awarded for correct answers when rounded by
the examiner to the number of significant figures given in the mark scheme. This may not apply to measured values.

For answers given in standard form (e.g. a × 10n) in which the convention of restricting the value of the coefficient (a) to a value between 1
and 10 is not followed, credit may still be awarded if the answer can be converted to the answer given in the mark scheme.

Unless a separate mark is given for a unit, a missing or incorrect unit will normally mean that the final calculation mark is not awarded.
Exceptions to this general principle will be noted in the mark scheme.

7 Guidance for chemical equations

Multiples / fractions of coefficients used in chemical equations are acceptable unless stated otherwise in the mark scheme.

State symbols given in an equation should be ignored unless asked for in the question or stated otherwise in the mark scheme.

Rounding errors (RE) and transcription errors (TE) are penalised only once in the paper.

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Question Answer Marks

1(a) I Clearly shows initial and final mass and both recorded to the same precision and with correct units. 1
Units: (g) or / g or in g or g by every entry (including mass used if given)
Reject weight

II The following data must be shown 1

• burette readings and titre for rough titration
• 2 × 2 ‘box’ showing both accurate burette readings

III Headings and units correct for accurate titration table and headings match readings. 1
• initial / start and (burette) reading / volume
• final / end and (burette) reading / volume
• titre or volume / FA 2 and used / added
Units: (cm3) or / cm3 or in cm3 or cm3 by every entry

Allow vol for volume, value for reading

Allow change in volume
Reject difference, total or amount

IV All accurate burette readings are recorded to 0.05 cm3 (including 0.00) 1
Reject if 50(.00) is used as an initial burette reading;
Reject if more than one final burette reading is 50(.00)
Reject if any burette reading is > 50(.00)

V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1
Ignore any titre labelled ‘rough’
Reject if any ‘accurate’ burette reading (apart from an initial 0 cm3) is given to zero dp.

Check and correct titre and mass subtractions where necessary.

Examiner selects the best mean titre.
Apply hierarchy: 2 identical, titres within 0.05 cm3, titres within 0.10 cm3, etc.Examiner calculates supervisor’s corrected average
titre × supervisor’s corrected mass to 1 dp.
Examiner calculates candidate’s corrected average titre × candidate’s corrected mass to 1 dp.
Subtract the candidate product value from that of the supervisor: δ

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Question Answer Marks

1(a) Award VI if 4.0 < δ ⩽ 6.0 g cm3 1

Award VI and VII if 2.0 < δ ⩽ 4.0 g cm3 1

Award VI, VII and VIII if δ ⩽ 2.0 g cm3 1

If there is only one accurate titration award accuracy marks based on that titration without further penalty.
If only a rough titration is shown award accuracy marks based on this value but cancel one accuracy mark.
Apply spread penalty as follows:if titres selected (by examiner) differ ⩾ 1.00 cm3 then cancel one accuracy mark.
If Supervisor’s value ⩽ 10.00 cm3 then halve tolerances
(3 marks: δ ⩽ 1.0; 2 marks: δ ⩽ 2.; 1 mark: δ ⩽ 3.0 g cm3)

1(b) Candidate must average two (or more) titres that are all within 0.20 cm3. 1
Working must be shown or ticks must be put next to the two (or more) accurate titres selected.

1(c)(i) Answers for (ii) and both parts of (iii) are quoted to 3−4 sf. 1

1(c)(ii) Correctly calculates 1

1.25 × 10–3

1(c)(iii) Correctly uses 1

2.50 × 10−3
AND
250  0.625 
2.50 × 10−3 × = 
(b )  (b ) 
1(c)(iv) Correctly uses 1
correctly calculated mass from
(a)
ans (iii )

Display of
( ANS − 158.2 ) 1
18

1(c)(iv) Uses values to calculate x to the nearest integer 1

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Question Answer Marks

1(d)(i) Uncertainty in a single reading: 1

for 1 dp balance allow the uncertainty given to be ± 0.1 or 0.05
for 2 dp balance allow the uncertainty given to be ± 0.01 or 0.005, etc.

AND
 2 × uncertainty given 
Display of   × 100
 candidate mass 

1(d)(ii) Correctly uses 1

 (100 − ( d)(i ) ) 
  × candidate’s Mr
 100 
 

OR

(mass ( a ) − 2 × ( single ) uncertainty )

ans ( c )(iii )

Allow numerator as (mass (a) – single uncertainty) if lack of doubling already penalised in (d)(i).

1(e) Titre value is less because concentration of thiosulfate is greater 1

1(f) The indicator colour change is easier to see blue-black to colourless. 1

OR
The dark colour of the aqueous iodine makes the burette harder to read.

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Question Answer Marks

2(a) I Appropriate headings and units for data given. 1

• volume of FA 5
• volume of water
• time
• rate
Volumes in cm3 or / cm3 or (cm3). Time in seconds or / s or (s), rate in s–1 or (s-1). Ignore 1000.
Allow ecf for display of units if penalised in 1(a).

II Both times recorded to the nearest second. 1

Examiner corrects times to the nearest second.

time for experiment 2
Examiner calculates ratio: to 1 dp.
time for experiment 1

III Award if this ratio is between 1.9 and 2.1. 1

1000 1
IV Both rates correctly calculated using and recorded to a minimum of 2 sf.
time

2(b) Agree because ratio is almost 2. 1

OR
Disagree because ratio is not 2.

Allow answers in terms of time

2(c) Records volumes of FA 4, FA 5 and water. 1

3 additional volumes of FA 5 chosen with intervals not less than 4.0 cm3 and all volumes of FA 5 ⩾ 24.0 cm3 and ⩽
36.0 cm3

In all 3 additional experiments water is added to make a total volume of FA 5 + water = 40.0 cm3 1

Allow for intervals of less than 4.0 cm3 for FA 5.

Allow if FA 4 omitted.
Reject if FA 4 ≠ 20(.0) cm3

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Question Answer Marks

2(a) I table of data 5

must show all of the following:
• (mass of) crucible + lid
• (mass of) crucible + lid + FA 5
• (mass of) crucible + lid + residue / contents / MgX
• (mass of) FA 5
• (mass of) water lost / mass lost

II recording of data
• unit / g, (g) or in grams is covering each entry
• all three balance readings are recorded to the same number of decimal points (2 or 3 dp).

III correctly calculates

• mass of FA 5
• mass of water lost

IV, V
award IV if  ⩽ 0.20
award V if  ⩽ 0.10
mass of FA 5
where d is the difference between the supervisor’s and candidate’s mass ratio = and calculated correct
mass of water lost
to 2 dp
the theoretical value of 1.96 is used if there is no supervisor’s value available

2(b)(i) correctly calculates 1

mass of water lost
amount of H2O lost = (mol)
18
AND
answer to 2 – 4 sf

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Question Answer Marks

2(b)(ii) correctly uses 2

M1: amount of Mg = (b)(i) / 7 (mol)

M2: percentage of Mg
= [{(b)(i)  24.3} / (7  mass of FA 5)]  100 (%)
AND
answer to 2 – 4 sf

2(c) M1: all the water is lost. 2

OR
heating was for long enough to reach constant mass. (owtte)

M2: MgX / residue is thermally stable / does not decompose.

OR
the mass loss is due to the water only.

Question Answer Marks

num
FA 6 is NH4Fe(SO4)2(aq); FA 7 is NaBr(aq); FA 8 is Na2CO3(aq); FA 9 is NaCl(aq).

3(a)(i) M1: selects NaOH OR NH3 2

AND
red-brown / brown / rust / orange-brown ppt on adding NaOH or NH3

M2: warms solution with NaOH

AND
ammonia / gas (given off on warming) that turns (damp red) litmus blue.

3(a)(ii) M1: selects (acidified) potassium manganate(VII) / KMnO4 (with or without barium salt) 2
OR
selects BaCl 2 or Ba(NO3)2 AND HCl or HNO3 (in either order) (or full names)

M2: (KMnO4) solution stays purple / no change / no reaction

OR
white ppt AND insoluble in (excess) acid

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