the john fisher school summer revision calculations for as

The John Fisher School

Summer revision calculations for AS chemists September 2016

Q1.Dilute nitric acid reacts with potassium hydroxide solution.

The equation for the reaction is:

HNO3 + KOH KNO3 + H2O

A student investigated the temperature change in this reaction.

This is the method the student used.

Step 1 Put 25 cm3 of dilute nitric acid in a polystyrene cup. Step 2 Use a thermometer to measure the temperature of the dilute nitric acid. Step 3 Use a burette to add 4 cm3 of potassium hydroxide solution to the dilute nitric

acid and stir the mixture. Step 4 Use a thermometer to measure the highest temperature of the mixture. Step 5 Repeat steps 3 and 4 until 40 cm3 of potassium hydroxide solution have been

added.

The dilute nitric acid and the potassium hydroxide solution were both at room temperature.

(a) Figure 1 shows part of the thermometer after some potassium hydroxide solution had been added to the dilute nitric acid.

What is the temperature shown on the thermometer?

The temperature shown is .................... °C (1)

(b) Errors are possible in this experiment.

(i) Suggest two causes of random error in the experiment.


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(ii) Another student used a glass beaker instead of a polystyrene cup.

This caused a systematic error.

Why does using a glass beaker instead of a polystyrene cup cause a systematic error?

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(c) The results of the student using the polystyrene cup are shown in Figure 2.

(i) How do the results in Figure 2 show that the reaction between dilute nitric acid and potassium hydroxide solution is exothermic?

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(ii) Explain why the temperature readings decrease between 28 cm3 and 40 cm3 of potassium hydroxide solution added.

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(iii) It is difficult to use the data in Figure 2 to find the exact volume of potassium hydroxide solution that would give the maximum temperature.

Suggest further experimental work that the student should do to make it easier to find the exact volume of potassium hydroxide solution that would give the maximum temperature

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(d) The student did further experimental work and found that 31.0 cm3 of potassium hydroxide solution neutralised 25.0 cm3 of dilute nitric acid.

The concentration of the dilute nitric acid was 2.0 moles per dm3.

HNO3 + KOH KNO3 + H2O

Calculate the concentration of the potassium hydroxide solution in moles per dm3.

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Concentration = ............................ moles per dm3

(3)


(e) The student repeated the original experiment using 25 cm3 of dilute nitric acid in a polystyrene cup and potassium hydroxide solution that was twice the original concentration.

She found that:

• a smaller volume of potassium hydroxide solution was required to reach the maximum temperature

• the maximum temperature recorded was higher.

Explain why the maximum temperature recorded was higher.

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(Total 14 marks)

Q2.Methane (CH4) is used as a fuel.

(a) The displayed structure of methane is:

Draw a ring around a part of the displayed structure that represents a covalent bond. (1)

(b) Why is methane a compound?

Tick ( ) one box.

Methane contains atoms of two elements, combined chemically.

Methane is not in the periodic table.


Methane is a mixture of two different elements.

(1)

(c) Methane burns in oxygen.

(i) The diagram below shows the energy level diagram for the complete combustion of methane.

Draw and label arrows on the diagram to show:

• the activation energy • the enthalpy change, ΔH.

(2)

(ii) Complete and balance the symbol equation for the complete combustion of methane.

CH4 + .......... CO2 + .......... (2)

(iii) Explain why the incomplete combustion of methane is dangerous.

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(iv) Explain why, in terms of the energy involved in bond breaking and bond making, the combustion of methane is exothermic.

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(d) Methane reacts with chlorine in the presence of sunlight.

The equation for this reaction is:

Some bond dissociation energies are given in the table.

Bond Bond dissociation

energy in kJ per mole

C−H 413

C−Cl 327

Cl−Cl 243

H−Cl 432

(i) Show that the enthalpy change, ΔH, for this reaction is −103 kJ per mole.

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(ii) Methane also reacts with bromine in the presence of sunlight.

This reaction is less exothermic than the reaction between methane and chlorine.

The enthalpy change, ΔH, is −45 kJ per mole.

What is a possible reason for this?

Tick ( ) one box.

CH3Br has a lower boiling point than CH3Cl

The C−Br bond is weaker than the C−Cl bond.

The H−Cl bond is weaker than the H−Br bond.

Chlorine is more reactive than bromine.

(1) (Total 15 marks)

Q3.A student investigated the rate of reaction of magnesium and hydrochloric acid.

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

The student studied the effect of changing the concentration of the hydrochloric acid.

She measured the time for the magnesium to stop reacting.


Concentration of hydrochloric acid in moles per dm3

0.5 1.0 1.5 2.0

(a) The student changed the concentration of the hydrochloric acid.

Give two variables that the student should control.

1 ....................................................................................................................

2 .................................................................................................................... (2)

(b) (i) The rate of reaction increased as the concentration of hydrochloric acid increased.

Explain why.

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(ii) Explain why increasing the temperature would increase the rate of reaction.

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(c) (i) The student had a solution of sodium hydroxide with a concentration of 0.100 moles per dm3.

She wanted to check the concentration of a solution of hydrochloric acid.

She used a pipette to transfer 5.00 cm3 of the hydrochloric acid into a conical flask.

She filled a burette with the 0.100 moles per dm3 sodium hydroxide solution.

Describe how she should use titration to obtain accurate results.

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(ii) Sodium hydroxide neutralises hydrochloric acid as shown in the equation:

NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)

The student found that 27.20 cm3 of 0.100 moles per dm3 sodium hydroxide neutralised 5.00 cm3 of hydrochloric acid.

Calculate the concentration of the hydrochloric acid in moles per dm3.

Give your answer to three significant figures.

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Concentration of hydrochloric acid = .............................. moles per dm3

(3) (Total 14 marks)

Q4.A spacecraft landed on Mars in 2008.

© Stocktrek Images

(a) Probes on the spacecraft analysed some soil.

The probes found a compound in the soil that scientists named compound X.

(i) Compound X had the following percentage composition by mass:

10.8% magnesium, 31.8% chlorine and 57.4% oxygen.

Relative atomic masses: Mg = 24; Cl = 35.5; O = 16

Calculate the empirical formula of compound X.

You must show all of your working to get full marks.

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Empirical formula = ............................................................ (4)

(ii) Compound X was shown to be ionic. Compound X had a high melting point.

Use your knowledge of the structure and bonding of ionic compounds to explain why compound X had a high melting point.

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(b) Hydrazine and hydrogen were both carried to be used as fuels on the spacecraft.

Hydrazine is stored as a liquid but hydrogen is stored as a gas.

Suggest three advantages of using a liquid fuel rather than a gaseous fuel.

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(Total 10 marks)

Q5. Chlorine, bromine and iodine are all halogens. They belong to Group 7 of the periodic


table.

(a) A teacher demonstrated the reactivity of the halogens to some students.

Halogen vapour was passed over heated iron wool in a fume cupboard.

The teacher’s observations are shown in the table below.

Observations

During the reaction After the reaction

Bromine The iron wool glowed A red–brown solid had been produced

Chlorine The iron wool glowed brightly A dark brown solid had been produced

Iodine The iron wool did not glow A black solid had been produced

(i) What is the order of reactivity of these three halogens?

Order of reactivity: most reactive halogen 1 ....................................

2 ....................................

least reactive halogen 3 .................................... (1)

(ii) Explain how you used the teacher’s observations to decide your order of reactivity.

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............................................................................................................... (2)

(iii) The students were asked to comment on the teacher’s observations.

One student said that the observations were not valid because not all the halogens are vapours – chlorine is a gas, bromine is a liquid and iodine is a solid.

Are the student’s comments justified?

Give reasons for your answer.

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(b) Chlorine reacts with hot sodium hydroxide solution.

A sample of a compound formed in the reaction was found to contain 10.8 g of sodium atoms, 16.7 g of chlorine atoms and 22.5 g of oxygen atoms.

Calculate the empirical formula of the compound formed.

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(Total 9 marks)

Q6.Scientists found that a compound contained:

22.8% sodium; 21.8% boron; and 55.4% oxygen.


Use the percentages to calculate the empirical formula of the compound.

Relative atomic masses (A r): B = 11; O = 16; Na = 23

To gain full marks you must show all your working.

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Empirical formula = ............................................................ (Total 5 marks)

Q7.(a) A student had a colourless solution.

The student thought the solution was dilute hydrochloric acid.

(i) The student added universal indicator to this solution.

What colour would the universal indicator change to if the solution is hydrochloric acid?

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(ii) Describe how the student could show that there are chloride ions in this solution.

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(b) The results of a titration can be used to find the concentration of an acid.

Describe how to use the apparatus to do a titration using 25 cm3 of dilute hydrochloric acid.

In your answer you should include:

• how you will determine the end point of the titration • how you will make sure the result obtained is accurate.

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(c) Hydrochloric acid is a strong acid.

Ethanoic acid is a weak acid.

What is meant by the term weak acid?

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(d) The displayed formula of ethanoic acid is:

(i) On the formula, draw a circle around the functional group in ethanoic acid. (1)

(ii) Ethanoic acid and ethanol react together to make the ester ethyl ethanoate.

Draw the displayed formula of ethyl ethanoate.

(2)

(Total 11 marks)


Q8.(a) Ethanol (C2H5OH) is an alcohol.

(i) Draw the displayed structure of ethanol.

(1)

(ii) Complete combustion of ethanol produces carbon dioxide and water.

Complete the balanced symbol equation for this reaction.

C2H5OH + ............................................................................................ (3)

(iii) Explain, in terms of bond breaking and bond forming, why the combustion of ethanol is exothermic.

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(b) A group of students investigated the amount of energy given out when different alcohols are burned. The students used the apparatus shown in the figure below.


In one experiment the temperature of 50 g of water increased from 22.5 °C to 38.3 °C. The mass of alcohol burned was 0.85 g.

(i) The energy used to heat the water, Q, can be found using the equation:

Q = m × c × ΔT

Calculate the heat energy, in joules, given out by burning 0.85 g of the alcohol.

Assume that all of the heat energy given out by burning the alcohol is used to heat the water (specific heat capacity of water = 4.2 J / g / °C).

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Heat energy = .................................................. J (2)

(ii) Use your answer to part (b)(i)and the fact that 0.85 g of alcohol were burned to calculate the heat energy that would be given out by 1 g of alcohol.

If you could not answer part (b)(i), use 3000 J as the heat energy given out by burning 0.85 g of the alcohol. This is not the correct answer to part (b)(i).

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Heat energy = .................................................. J (1)

(c) Another group of students investigated the amount of heat energy given out when different alcohols are burned. They used a different, better, set of apparatus than the first group of students.

They used the results from their investigation to calculate the heat energy given out by burning 1 g of each alcohol.

They used a data book to find the theoretical amount of heat energy 1 g of each


alcohol should have given out when burned completely.

The students recorded their experimental results and the theoretical values in the table below.

Name of alcohol

Number of carbon atoms

in one molecule

of alcohol

Experimental amount of heat energy given

out when 1 g is burned in kJ

Theoretical amount of

heat energy given out when 1 g is burned

completely in kJ

Methanol 1 11.4 22.7

Ethanol 2 14.5 29.7

Propanol 3 16.0 33.7

Butanol 4 16.8 36.2

Pentanol 5 17.2 37.8

Hexanol 6 17.4 39.1

(i) What is the relationship between the number of carbon atoms in one molecule of the alcohol and the heat energy given out when the alcohol is burned?

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(ii) Suggest one reason why the students’ experimental results are lower than the theoretical values.

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(iii) The students observed that as the number of carbon atoms in one molecule of the alcohol increased:

• the flame was more orange

• more carbon was left on the bottom of the calorimeter.

Suggest why.


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(iv) The heat energy given out when 1 g of ethanol (C2H5OH) is burned is 29.7 kJ.

Calculate the heat energy, in kilojoules, that will be given out when 1 mole of ethanol is burned.

Give your answer to three significant figures.

Relative atomic masses: H = 1; C = 12; O = 16

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Heat energy = .................................................. kJ (3)

(Total 16 marks)

Q9.Printed pictures can be made using etchings.


© Eduardo Jose Bernardino/iStock

An etching can be made when a sheet of brass reacts with iron chloride solution.

(a) Brass is a mixture of two metals, copper and zinc.

(i) A mixture of two metals is called ............................................................ . (1)

(ii) Draw a ring around the correct answer to complete the sentence.

Copper and zinc atoms are different sizes.

harder

This makes brass more flexible than the pure metals.

softer

(1)

(b) Iron chloride has the formula FeCl3

Relative atomic masses (Ar): Cl = 35.5; Fe = 56.

(i) Calculate the relative formula mass (Mr) of iron chloride (FeCl3).

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Relative formula mass (Mr) of iron chloride = ...................................... (2)

(ii) Calculate the percentage of iron in iron chloride (FeCl3).

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Percentage of iron in iron chloride = .................................................% (2)

(Total 6 marks)


M1.(a) 31 1

(b) (i) any two from: • incorrect reading of thermometer / temperature • incorrect measurement of volume of acid • incorrect measurement of volume of alkali (burette).

2

(ii) glass is a (heat) conductor or polystyrene is a (heat) insulator

answer needs to convey idea that heat lost using glass or not lost using polystyrene

accept answers based on greater thermal capacity of glass (such as “glass absorbs more heat than polystyrene”)

1

(c) (i) temperature increases 1

(ii) no reaction takes place or all acid used up or potassium hydroxide in excess

1

cool / colder potassium hydroxide absorbs energy or lowers temperature

ignore idea of heat energy being lost to surroundings 1

(iii) take more readings

ignore just “repeat” 1

around the turning point or between 20 cm3 and 32 cm3

accept smaller ranges as long as no lower than 20 cm3 and no higher than 32 cm3

1

(d) 1.61 or 1.6(12903)

correct answer with or without working scores 3

if answer incorrect, allow a maximum of two from:

moles nitric acid = (2 × 25 / 1000) = 0.05 for 1 mark

moles KOH = (moles nitric acid) = 0.05 for 1 mark

concentration KOH = 0.05 / 0.031

answer must be correctly rounded (1.62 is incorrect) 3


(e) same amount of energy given out 1

which is used to heat a smaller total volume or mixture has lower thermal capacity or number of moles reacting is the same but the total volume / thermal capacity is less

if no other marks awarded award 1 mark for idea of reacting faster

1

[14]

M2.(a) circle round any one (or more) of the covalent bonds

any correct indication of the bond − the line between letters 1

(b) Methane contains atoms of two elements, combined chemically 1

(c) (i) activation energy labelled from level of reagents to highest point of curve

ignore arrowheads 1

enthalpy change labelled from reagents to products

arrowhead must go from reagents to products only 1

(ii) 2 O2

1

2 H2O

if not fully correct, award 1 mark for all formulae correct.

ignore state symbols 1


(iii) carbon monoxide is made 1

this combines with the blood / haemoglobin or prevents oxygen being carried in the blood / round body or kills you or is toxic or poisonous

dependent on first marking point 1

(iv) energy is taken in / required to break bonds

accept bond breaking is endothermic 1

energy is given out when bonds are made

accept bond making is exothermic 1

the energy given out is greater than the energy taken in

this mark only awarded if both of previous marks awarded 1

(d) (i) energy to break bonds = 1895

calculation with no explanation max = 2 1

energy from making bonds = 1998 1

1895 − 1998 (= −103) or energy to break bonds = 656 energy from making bonds = 759 656 − 759 (= −103)

allow:

bonds broken − bonds made =

413 + 243 − 327 − 432 = -103 for 3 marks. 1

(ii) The C — Br bond is weaker than the C — Cl bond 1

[15]

M3.(a) any two from:

• temperature (of the HCl) • mass or length of the magnesium • surface area of the magnesium • volume of HCl

2


(b) (i) (a greater concentration has) more particles per unit volume

allow particles are closer together 1

therefore more collisions per unit time or more frequent collisions. 1

(ii) particles move faster

allow particles have more (kinetic) energy 1

therefore more collisions per unit time or more frequent collisions 1

collisions more energetic (therefore more collisions have energy greater than the activation energy) or more productive collisions

1

(c) (i) add (a few drops) of indicator to the acid in the conical flask

allow any named indicator 1

add NaOH (from the burette) until the indicator changes colour or add the NaOH dropwise

candidate does not have to state a colour change but penalise an incorrect colour change.

1

repeat the titration 1

calculate the average volume of NaOH or repeat until concordant results are obtained

1

(ii) moles of NaOH


0.10 × 0.0272 = 0.00272 moles

correct answer with or without working gains 3 marks 1

Concentration of HCl

0.00272 / 0.005 = 0.544

allow ecf from mp1 to mp2 1

correct number of significant figures 1

[14]

M4.(a) (i) 10.8 / 24 31.8 / 35.5 57.4 / 16

division by atomic mass 1

0.45 0.90 3.59

proportion 1

1 : 2 : 8

ratio 1

MgCl2O8

formula

if the candidate has just written down the correct answer with no working allow 3 marks. If working is shown award 4 marks.

allow 2 marks if the candidate has the initial fractions upside down and gets the answer Mg8Cl4O as long as working has been shown

allow symbols in any order but do not allow incorrect capitals or subscript and superscript numbers, eg mGCl2O8 (final mark)

allow ecf after step 2 provided a sensible attempt has been made in step 1

1


(ii) because it has electrostatic forces of attraction or attraction between oppositely charged ions

any mention of covalent bonds or molecules or intermolecular forces max. 2 marks

1

giant structure / lattice or many bonds

accept attraction between positive and negative ions do not accept atoms

1

that need lots of energy to overcome / break 1

(b) any three from:

• more can be stored in a given volume or takes up less space

could be answered in terms of comparing liquid with gas, but not simply the disadvantages of storing a gaseous fuel

• (liquid has) greater energy density • (liquid) does not require high pressure containers • (liquid is) less likely to leak

allow easier to detect a leak 3

[10]

M5. (a) (i) chlorine > bromine > iodine

accept symbols in place of names (Cl2 or Cl, etc) 1

(ii) any two suitable comparisons about the extent to which the iron wool glowed

eg chlorine is more reactive than bromine because iron glowed morebrightly with chlorine than bromine

eg bromine is more reactive than iodine because iron glowed withbromine but not with iodine

eg iodine is the least reactive because it is the only one that did not glow


eg chlorine is the most reactive as it was the only one that glowed brightlyor glowed most brightly

2 max.

(iii) NO (no marks)

accept YES (no marks)

because the observations were made using the halogen vapours

because the observations were made using the halogen vapours

1

so the fact that the three elements are naturally solid, liquid or gas isnot relevant

so the iodine (and bromine) must have been heated 1

(b) division of masses by Ar values

if division inverted (Ar / mass) then award zero marks 1

correct answers of division

if one slip in Ar values used, do not award first mark, but award remainder consequentially

1

simplification by division by smallest 1

correct formula (NaClO3)

accept elements in any order (such as ClNaO3)

correct answer alone with or without working = 4 marks 1

[9]

M6.Divide by Ar:


Na = 22.8 / 23

B = 21.8 / 11

O = 55.4 / 16

if student has calculated moles upside down they can score mp 3 mp 4 and mp 5 as follows:

Na 23 / 22.8

B 11 / 21.8

O 16 / 55.4 1

Values

0.991

1.01

1.98

0.505

3.46

0.289 1

Divide by the smallest

1 : 2 : 3.5

Divide by the smallest (1)

3.5 : 1.75 : 1 1

Whole number ratio

2 : 4 : 7

Whole number ratio (1)

14 : 7 : 4 1

Empirical formula

Na2B4O7

Empirical formula (1)

Na14B7O4

if no working shown allow 4 marks for Na2B4O7

1

[5]


M7.(a) (i) red

ignore pink 1

(ii) add silver nitrate (solution) 1

white precipitate

dependent on addition of silver nitrate

ignore addition of another acid

if hydrochloric acid added max 1 mark 1

(b) suitable named alkali / sodium hydroxide solution in burette 1

add alkali solution until (indicator) becomes pink / red 1

if acid to acid titration described, first two marking points not available

any two from:

• wash / rinse equipment • add dropwise or slowly (near end point) • swirl / mix • read (meniscus) at eye level • white background • read start and final burette levels / calculate the volume needed • repeat

2

(c) does not ionise / dissociate completely

allow for acids of the same concentration, weak acids have a higher pH or fewer hydrogen ions

1


(d) (i) ring round COOH

1

(ii)

if not fully correct, allow 1 mark for correct ester group − minimum

2

[11]

M8.(a) (i)

allow –OH 1

(ii) (C2H5OH +) 3O2 ➔ 2CO2 + 3H2O

formulae

all 3 formulae gain 2 marks;

2 formulae gain 1 mark


2

balanced

dependent on formulae

accept multiples 1

(iii) energy is required to break bonds 1

energy is released when bonds are formed 1

exothermic because more energy is released than taken in 1

(b) (i) 3318 or 3320 (J)

correct answer with or without working gains 2 marks

if answer incorrect, allow 1 mark for correct working, eg:

temperature change = (38.3 – 22.5 = ) 15.8 or Q = (50 x 4.2 x 15.8 =)

allow ecf from temp change 2

(ii) if answer from part (i) used:

(3318 / 0.85) = 3904

allow ecf from part (i)

if value of 3000 used:

3000 / 0.85 = 3529

accept 2 sig figs up to calculator value 1

(c) (i) the more carbon atoms in one molecule, the more heat energy is given out

do not accept proportional 1


(ii) heat loss (to surroundings or to the can or to the air)

accept not all energy used to heat water

allow incomplete combustion

allow not all burned

ignore thermometer or balance errors 1

(iii) incomplete combustion (occurs when more C atoms in molecule) 1

(iv) 1370


(Mr ethanol =) 46 gains 1 mark

46 x 29.7 gains 2 marks

max 2 marks if incorrect sig figs (1366.2, 1366, 1400)

final answer ecf on incorrect Mr max 2 marks 3

[16]

M9.(a) (i) an alloy 1

(ii) harder 1

(b) (i) 162.5


if no answer or incorrect answer then evidence of correct working [56 + (3x35.5)] gains 1 mark

2

(ii) 34.46

accept rounding from 34 - 34.5


accept ecf from (b)(i) correctly calculated for 2 marks

if no answer or incorrect answer then evidence of 56 / 162.5


or 56 / answer to (b)(i) gains 1 mark

2

[6]

the john fisher school summer revision calculations for as

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