1 3 f calculations involving gas volumes

8/10/2019 1 3 f Calculations Involving Gas Volumes

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Q1.

For drivers in the UK, the legal limit of the concentration of ethanol (molar mass46 g mol1) in the blood is 80 mg per 100 cm3. This is equivalent to a concentration of

A 17.4 mol dm3 B 1.74 mol dm3

C 0.0174 mol dm3 D 0.00174 mol dm3

(Total for Question = 1 mark)Q2.Hydrogen and oxygen react according to the following equation.

If all volumes are measured at 110 C and one atmospherepressure, the volume of steam produced after 50 cm3of

hydrogen react completely with 25 cm3of oxygen is

A 25 cm3 B 50 cm3 C 75 cm3 D 100 cm3(Total for question = 1 mark)

Q3.The equation for the complete combustion of octane is

(a) The mass of 10 mol of octane is(1)

A 0.66 kg B 1.14 kg C 2.10 kg D 2.28 kg

(b) The volume of 1 mol of any gas (measured at room temperature and pressure) is 24 dm3. Hence thevolume of oxygen (measured at room temperature and pressure) required for the complete combustion of10 mol of octane is

(1)

A 240 dm3 B 300 dm3 C 3000 dm3 D 6000 dm3

(Total for question = 2 marks)

Q4.Complete combustion of 50 cm3of a hydrocarbon vapour gave 350 cm3of carbon dioxide, both gasvolumes being measured at the same temperature and pressure. The formula of the hydrocarbon could

be

A C8H18 B C7H16 C C6H14 D C5H12

(Total for question = 1 mark)Q5.Which of the following gas samples occupies the greatest volume at the sametemperature and pressure?

A 1 gram of ethane B 1 gram of oxygen

C 1 gram of fluorine D 1 gram of neon

(Total for question = 1 mark)

Q6.The overall equation for the reaction between sulfur and oxygen to form sulfur trioxide

1.3.f Formulae, equations and amounts of substance

Students will be assessed on their ability to:

f. use chemical equations to calculate volumes of gases and vice versa using the concepts of amount of substanceand molar volume of gases, eg calculation of the mass or volume of CO2 produced by combustion of a hydrocarbon(given a molar volume for the gas)


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is shown below.

0.9 mol of O2(g) reacted completely with excess sulfur. What volume, in dm3, of

sulfur trioxide would form?

[Assume the molar gas volume = 24 dm3mol-1]

A (0.9 3/2) 24 B (0.9 3/2) 24 C (0.9 2/3) 24 D (0.9 2/3) 24

(Total for question = 1 mark)Q7.0.400 g of magnesium ribbon reacted with exactly 22.2 cm3of hydrochloric acid ofconcentration 1.50 mol dm3.

400 cm3of hydrogen gas was formed, the volume being measured at room temperatureand pressure.

In the calculations that follow, use the following molar masses:

Mg = 24.0 g mol1Cl = 35.5 g mol1

(a) Calculate the amount (in moles) of magnesium used.(1)

(b) Calculate the amount (in moles) of hydrochloric acid used.(1)

(c) Calculate the amount (in moles) of hydrogen produced.[Molar volume of any gas at room temperature and pressure = 24 000 cm3mol1]

(1)

(d) Show that the calculated amounts of magnesium, hydrochloric acid and hydrogen are consistent withthe following equation for the reaction

(1)(e) Calculate the maximum mass of magnesium chloride that would be formed in this reaction. Give youranswer to threesignificant figures.

(3)

(Total for Question = 7 mark)


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Q8.Hydrogen has three isotopes, 1H, known as protium, 2H, deuterium, and 3H, tritium.(a) In terms of sub-atomic particles, give the similarities and differences between atoms of these threeisotopes of hydrogen.

(3)

..............................................................................................................................................

..............................................................................................................................................

..............................................................................................................................................(b) When a nitrogen atom collides with a high energy neutron, one atom of tritium

and one atom of another element are formed. Complete the equation below.(1)

(c) Tritium-deuterium gas, consisting of molecules each containing one deuterium atom and one tritiumatom, is used in some nuclear warheads. Typically, each warhead has about 4.0 g of the gas added.

(i) Calculate the number of moles of tritium-deuterium in 4.0 g. (2)

(ii) Calculate the volume, in cm3, of 4.0 g of tritium-deuterium gas.

[Molar volume of a gas under these conditions = 24 000 cm3mol1](1)


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(d) Tritium is not usually included in calculations of therelative atomic mass of hydrogen, because it isradioactive and has a relatively short half-life. Calculatethe relative atomic mass of hydrogen with the followingisotopic composition. Give your answer to four decimalplaces.

(2)

(e) The electronic energy levels in hydrogen are shownbelow.

(i) Mark on the energy level diagram, with an arrow, thetransition that

represents the ionization energy of hydrogen.(1)

(ii) In some versions of the Periodic Table, hydrogen isplaced in the same group

as sodium. Give the electronic configurations for both ahydrogen atom and a

sodium atom, using the sand pnotation.

Use these electronic configurations to suggest why thisis a reasonable

grouping.(2)

H........................................................................................................................................................................

Na

........................................................................................................................................................................

*(f) Which element in the Periodic Table has the highest first ionization energy? Justifyyour answer.

(3)

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..............................................................................................................................................(Total for question = 15 marks)


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Q9.Sodium burns in oxygen to give a pale yellow solid X.(a) (i) 1.73 g of sodium reacts with 1.20 g of oxygen.

Calculate the empirical formula of X.(2)

(ii) The molar mass of Xis 78 g mol1. Give the molecular formula of X.(1)

..............................................................................................................................................(iii) Write the equation, including state symbols, for the reaction of sodium with oxygen to produce X.

(2)

(iv) Calculate the volume of oxygen in dm3(at room temperature and pressure) which reacts with 1.73 gof sodium. (The molar volume of any gas at room temperature and pressure is 24 dm3mol1.)

(2)

(v) Calculate the number of oxygen moleculesthat react with 1.73 g of sodium.(The Avogadro constant = 6.02 1023mol1.)

(1)

>(b) If sodium is burnt in air, compound Xis not the only product. Suggest why this is so.(1)

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Q10.This question is about hydrocarbons.(a) Liquefied petroleum gas (LPG) is a fuel sold as an alternative to petrol. It is a

mixture of liquefied C3and C4alkanes.(i) Suggest a reason why the alkanes are liquefied.

(1)..............................................................................................................................................

..............................................................................................................................................


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(ii) There are two C4alkanes.

Draw skeletalformulae of each ofthe C4alkanes in thespacesprovided.Name eachalkane.

(4)

(iii) Complete thefollowing sentence.

(1)Compounds with thesame molecularformula but differentstructural formula arecalled:

.............................

(b) Propane, C3H8, reacts with chlorine, Cl2, in asubstitution reaction.

The mechanism for this reaction is described in three stages.(i) Give the initiation stepfor this reaction and state the condition necessary for

this step to occur.(2)

Initiation step

Condition........................................................................................................................................................................

(ii) Give the TWO propagation stepsfor this reaction.(2)

(iii) Give a possible termination stepfor this reaction.(1)


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(c) Myrcene, C10H16, is a naturally occurring compoundwhich is used in perfumes.

(i) Name the functional group in myrcene.(1)

............................................................................................

(ii) What colour change would you observe when bromine, dissolved in an organicsolvent, is added to myrcene?

(1)From ........................................................................ To ...............................................................................

(iii) Classify the type and mechanism of the reaction that occurs when myrcenereacts with bromine, Br2.

(2)

..............................................................................................................................................

..............................................................................................................................................

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(iv) In an experiment, 1.36 g of myrcene (molar mass: 136 g mol1) was found to react with 0.72 dm3ofhydrogen, H2, in the presence of a nickel catalyst.

Use this informationto draw the structural formula of the product of the reaction between myrcene andhydrogen.

[Assume the molar volume of H2under the conditions of the experiment is dm3mol1.]

(2)Calculation

Hence structural formula of the product

(d) Myrcene is one of a group of compounds related to 2-methylbut-2-ene shown below.

2-methylbut-2-ene undergoes addition polymerization in a similar way to ethene.

Draw the structural formula of the repeat unit of the polymer formed.


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(2)

(Total for question = 19 marks)Q11.Sulfamic acid is a white solid used by plumbers as a limescale remover.

(a) Sulfamic acid contains 14.42% by mass of nitrogen, 3.09% hydrogen and 33.06% sulfur. Theremainder is oxygen.

(i) Calculate the empirical formula of sulfamic acid.

(3)

(ii) The molar mass of sulfamic acid is 97.1 g mol1. Use this information to deduce the molecular formulaof sulfamic acid.

(1)

(b) A solution of sulfamic acid contains hydrogen ions. The hydrogen ions react with magnesium toproduce hydrogen gas. In an experiment, a solution containing 5.5 x 103moles of sulfamic acid wasreacted with excess magnesium. The volume of hydrogen produced was 66 cm3, measured at roomtemperature and pressure.

(i) Draw a labelled diagram of the apparatus you would use to carry out this experiment, showing how youwould collect the hydrogen produced and measure its volume.

(2)

(ii) Calculate the number of moles of hydrogen, H2, produced in this reaction

[The molar volume of a gas is 24 dm3mol1at room temperature and pressure]

(1)


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(iii) Show that the data confirms that each mole of sulfamic acid produces one mole of hydrogen ions insolution.

(2)

(c) Plumbers use sulfamic acid powder for descaling large items such as boilers. Sulfamic acid acts as adescaler because the hydrogen ions react with carbonate ionsin limescale.

(i) Write an ionic equation for the reaction of hydrogen ions with carbonate ions. State symbols are notrequired.

(1)

(ii) Suggest ONE reason why sulfamic acid is considered less hazardous than hydrochloric acid as a

descaler.

(1)..............................................................................................................................................



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Q1.No Examiner's Report available for this question






Q7.(a)

Most candidates found the calculations in this question straightforward and a higherproportion were able to round their final answer in (e) correctly than has been the casewith similar questions in earlier papers in this series. However, a significant number ofcandidates still expressed their answers to the first three parts of the question incorrectlywith excessive and incorrect rounding being the common errors. While the stoichiometricratio question seemed well understood by most candidates, the mark was often lost byeither failing to make the necessary link between the calculation and the equation or lookingat only two figures, usually the magnesium and the acid.


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Results Plus: Examiner CommentsThis candidate clearly understands how to do this question but in (d) fails to complete the answer,only considering the stoichiometric ratio of magnesium to hydrogen. Note also the use of a roundedvalue of the number of moles in (e).

Results Plus: Examiner TipIt is good practice to retain in your calculator unrounded values obtained in intermediate steps of acalculation and to use these rather than the rounded values that you write down.


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one mark.In part (ii), they should have got an answer of 31920, but the use of the original fraction from part (i)was not penalised.

Results Plus: Examiner TipWith calculations always make working clear.

Results Plus: Examiner CommentsNotice that the candidate has indicated the molar mass in part (i), so even though it is incorrect theyget the second mark.Unfortunately they divided by the molar volume in (ii) so do not gain any credit.

Results Plus: Examiner TipIt is worth practising this very common type of calculation, calculating a volume of a gas from itsmass, without using the density.

(d)

Candidates needed to show their working clearly if they got the wrong answer, which manydid because they were unable to give their answer to four decimal places. A few lost a markfor incorrect units, though the unit was not required.

Results Plus: Examiner CommentsA well set out correct answer.

Results Plus: Examiner TipAlways show your working - it may get you credit even if you make an arithmetic error.

Results Plus: Examiner CommentsAlthough the candidate has the correct expression for calculating the relative atomic mass whichgained the first mark, the answer is not given to four decimal places.

Results Plus: Examiner TipEnsure you understand the difference between significant figures and decimal places.

(e) (i)Though this was the easiest question of this type, it was rather poorly done with arrowsfrom and to the wrong energy levels, and both up and down.

Results Plus: Examiner CommentsThough this starts from the correct level and goes in the correct direction, the arrow only reachesn=2 which is not enough.

Results Plus: Examiner TipTry out lots of these types of questions with different elements, for different ionization energies.

Results Plus: Examiner CommentsHere the energy levels are correct but the arrow is in the wrong direction for ionization.

Results Plus: Examiner TipThink about what ionization means when answering questions like this.

(e) (ii)


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Lower scoring candidates gave the electronic configuration for lithium but could still scorethe second mark.

In the second part it was essential to mention the term 'outer', so answers like 'both have ahalf filled s shell' or 'same electron configuration were insufficient'.

Results Plus: Examiner CommentsA typical one mark answer for the correct electron configurations.

Results Plus: Examiner TipTry to be critical of your own answers. Have you said everything you know which is relevant?

Results Plus: Examiner CommentsThis answer secures the second mark at the end with the words 'and their outer shells'. It alsobenefits from extremely clear writing.

Results Plus: Examiner TipWrite clearly in black ink making lower case letters large enough to be read easily.

(f)Lower scoring candidates selected elements other than helium. Though hydrogen couldscore justification marks, it was not possible to credit justifications for any other elements ofwhich neon was the most common.

Some who correctly gave helium only made generalisations of ionization energies risingacross periods or down groups when something more concrete relating to helium wasneeded. Similarly reference to a filled electron sub-shell was insufficient.

Results Plus: Examiner Comments

Although true statements about helium are given, they are insufficient explanations to justify theanswer.

Results Plus: Examiner TipJustifications should always attempt to answer the question 'why' as fully as possible.

Results Plus: Examiner CommentsHelium and more (electron) shielding are worth 2 marks.The structure of the second sentence can be construed as 'it' refers to an electron which is close tothe nucleus, so a mark was awarded on this occasion.

Results Plus: Examiner TipAlways try to be precise in answers.


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Q9.(a)(i)

Many candidates calculated the empirical formula correctly as NaO.

Results Plus: Examiner Comments1:1 ratio of Na:O had not been made explicit, but the correct answer NaO was given so both markswere awarded.

Results Plus: Examiner TipAlways show every step in your working.

(a)(ii)

A surprising number of candidates gave a molecular formula for a compound for which the molar masswas not 78 g mol1.

(a)(iii)

Many candidates were awarded both marks, but a significant number gave the state symbol for Na2O2as(aq) rather than (s).

Results Plus: Examiner CommentsThis is a correctly balanced equation, with the state symbols which are also correct, the answerscored both marks.

Results Plus: Examiner TipAlways consider state symbols carefully when asked for them in an equation.

(a)(iv)

Most candidates gained the second scoring point which required the calculated moles of oxygen, O2, tobe multiplied by 24 dm3mol1in order to obtain the volume of gas in units of dm3. There was someconfusion, however, between calculating moles of oxygen atoms, O(g), and moles of oxygen molecules,O2(g).


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Results Plus: Examiner CommentsThis is a well laid-out answer which scored both marks.

Results Plus: Examiner Tip

Set out your answers to calculation questions in an orderly way as illustrated in the above response.

(a)(v)

This question was generally well answered, although sometimes candidates chose to calculate thenumber of oxygen atoms instead of molecules.

Results Plus: Examiner CommentsThis was the correct response, which scored the mark available.

Results Plus: Examiner Tip

Always check which type of particle (atom, ion or molecule) is being referred to in the question.

(b )

This proved difficult for the majority of candidates. A significant number of responses included incorrectreferences to the presence of hydrogen gas, H2, in air.


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Results Plus: Examiner CommentsThis response scored a mark as it acknowledged that other oxides of sodium may have been formedin the reaction described.

Results Plus: Examiner TipBe aware of the gases present in air.


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Q10.In (a)(i), many candidates suggested answers relating to either the ease of combustion of alkanes orsafetymeasures rather than ease of transport or storage.

Results Plus: Examiner CommentsThis candidate scored the mark, thereference to safety being ignored.

In (a)(ii), it was clear that some candidates were unfamiliar with the concept of skeletal formulae. Namingof the two isomers was generally very good and some credit was awarded if a candidate drew two

correctstructural formulae instead of two skeletal formulae. Part (a)(iii) proved to be very straightforward.

In (b)(i), both the initiation step and the requirement for UV light were recalled accurately.


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However, in (b)(ii), candidates found it more diffi cult to write two correct propagation steps. Thereappeared to be a lack of understanding that the propagation steps lead to the formation of more chlorinefree radicals which keep the reaction going.

There was, in the vast majority of cases, a suitable use of dots to show free radicals. As shown in theresponse below, a sizeable number of candidates chose to write the propagation steps for the

chlorinationof methanerather than propane. One out of two marks was awarded if two correct steps were shown formethane reacting with chlorine.


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Results Plus: Examiner CommentsThe candidate has not completed thetermination step in (iii).

In (c)(iii), whilst many appreciated that the reaction of bromine with an alkene is an addition reaction, onlyrelatively few candidates classifi ed the process as electrophilic. In (c)(iv), calculating the number of

moles

in 0.72 dm3 of hydrogen gas proved to be far more problematic than finding the number of moles ofalkenein 1.36 g of myrcene.


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Results Plus: Examiner CommentsThis candidate has shown a meticulous approach so as to ensure all units are correct.

In a surprising number of cases, structural formulae of the product showed either carbon atoms that werenot tetravalent or structures with not all methyl groups in the correct place, as illustrated below:-


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A minority of candidates obviated this difficulty by showing the correct skeletal formula of the product.

In (d), the majority of candidates were able to draw the repeat unit, although some drew a section of thepolymer chain without then indicating what the repeat unit was.


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Q11.Calculating an empirical formula in (a) was a familiar task and well done. A number of candidates forgotto include the oxygen, but could earn the mark in (ii) by doubling their empirical formula, showing thatits mass was close to half of the molar mass. Another error was to use atomic numbers instead of atomicmasses.

Results Plus: Examiner TipAll the atoms in the compound must be included, so the percentage of oxygen has to be calculated.In (ii) the question asks for information to be used, so the answer should show that this has beendone.

Results Plus: Examiner CommentsThis answer has omitted the oxygen so loses the third mark in (i). However the empirical formulamass has been calculated and shown to be close to half the molar mass, so the mark was given in(ii) for using information to deduce a molecular formula. In a perfect answer there would have beenwords or abbreviations showing what the numbers referred to.

The diagrams in (b)(i) had to show a workable method of collecting hydrogen, and a suitable method ofmeasuring its volume. The diagram had to be good enough to show another person what to do, buttubing,bungs etc were not expected to be drawn to text-book standard. A number of diagrams showed hydrogenbeing collected in test tubes with no way of measuring the volume. More major errors suggested a lack of

practical experience. Collection over water often showed worrying disregard for water levels or indeed theneed for water at all! There were diagrams showing a gas being produced in a sealed container (otherthana syringe) connected to a sealed flask, or with gas being produced in an open container from which it


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Candidates found the ionic equation in (c)(i) challenging. Candidates need to learn to balance both atomsand charges when writing these. Three possible answers were allowed. The equation could show eitherformation of water and carbon dioxide, formation of carbonic acid, or formation of hydrogen carbonateions.

Results Plus: Examiner TipBoth atoms and charges must be balanced in the equation.

Results Plus: Examiner CommentsThis doesn't get the mark because the charge on the carbonate ion should be 2-. If the products areknown, it is possible to work out that another negative charge is needed for balance.

In (c)(ii) few candidates thought about the advantages of a descaling agent being a solid, and mostfocussedon the acid strength or corrosive properties of the two acids.


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1.3F MARKSCHEME

Q1.C Q2.B Q3.(a) B (b) C Q4.B Q5.D Q6.C

Q7.


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Q8.


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Q9.


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Q11.


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1 3 f calculations involving gas volumes

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