NSAA 2020 Chemistry PART Y: Questions & Worked Solutions

Question 21

The table gives standard enthalpy change data measured at 25 °C.

What is the standard enthalpy change for the following reaction at 25 °C?

H⁺(g) + Br⁻(g) → H⁺(aq) + Br⁻(aq)

A. $-510 \text{ kJ mol}^{-1}$
B. $+510 \text{ kJ mol}^{-1}$
C. $-694 \text{ kJ mol}^{-1}$
D. $+694 \text{ kJ mol}^{-1}$
E. $-1242 \text{ kJ mol}^{-1}$
F. $+1242 \text{ kJ mol}^{-1}$
G. $-1426 \text{ kJ mol}^{-1}$
H. $+1426 \text{ kJ mol}^{-1}$

Answer: G

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Question 22

Alpha-linolenic acid is a polyunsaturated straight-chain carboxylic acid.

0.001 \text{ mol}

of the acid reacts exactly with

15 \text{ cm}^3

of

0.2 \text{ mol dm}^{-3}

aqueous bromine.

Alpha-linolenic acid contains 18 carbon atoms per molecule.

Which of the following is a formula for the acid?

A. $C_{17}H_{35}COOH$
B. $C_{17}H_{34}COOH$
C. $C_{17}H_{33}COOH$
D. $C_{17}H_{32}COOH$
E. $C_{17}H_{31}COOH$
F. $C_{17}H_{30}COOH$
G. $C_{17}H_{29}COOH$
H. $C_{17}H_{28}COOH$

Answer: G

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Question 23

25.0 \text{ g}

of compound X contains

9.75 \text{ g}

of potassium,

0.25 \text{ g}

of hydrogen and

12.0 \text{ g}

of oxygen.

When heated strongly, compound X produces a gas that turns limewater cloudy.

The chemical formula of X is the same as its empirical formula.

Compound X is the only product formed between substance Y and two further substances. One of these is the fourth most abundant gas in clean dry air, and the other changes the colour of anhydrous copper(II) sulfate from white to blue.

Which of the following could be the identity of substance Y?

(

A_r

values: H = 1; C = 12; O = 16; K = 39)

A.K
B. $K_2CO_3$
C. $K_2O_3$
D. $KHCO_3$
E. $CH_3COOK$

Answer: B

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Question 24

Strontium (Sr) is a Group 2 metal.

Strontium hydride is an ionic compound made up of strontium and hydrogen only.

When a small mass of strontium hydride is added to

180 \text{ g}

of water in an insulated container and stirred, the temperature of the water rises by 25 °C and hydrogen gas is given off.

Assume that:
• the standard enthalpy change of the reaction is

-360 \text{ kJ mol}^{-1}

of strontium hydride.
• the specific heat capacity of water (and the solution formed) is

4 \text{ J g}^{-1} \text{°C}^{-1}

. Ignore any change in mass of the water due to the reaction.
• all heat is transferred to the water only.
• all measurements are made at atmospheric pressure, with all reactants and products in their standard states.
• one mole of a gas occupies

24 \text{ dm}^3

at room temperature and pressure.

What is the maximum volume of hydrogen (measured at room temperature) that could be released?

A. $1.20 \text{ dm}^3$
B. $1.80 \text{ dm}^3$
C. $2.40 \text{ dm}^3$
D. $240 \text{ dm}^3$
E. $480 \text{ dm}^3$
F. $2400 \text{ dm}^3$

Answer: C

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Question 25

Element X forms the most soluble hydroxide of the hydroxides of the Group 2 elements in the range Mg to Ba.

50 \text{ cm}^3

of a

0.2 \text{ mol dm}^{-3}

aqueous solution of an acid Y is exactly neutralised by

400 \text{ cm}^3

of a

0.05 \text{ mol dm}^{-3}

aqueous solution of potassium hydroxide.

100 \text{ cm}^3

of a

0.1 \text{ mol dm}^{-3}

aqueous solution of Y is titrated with an aqueous solution of the hydroxide of X until the acid is exactly neutralised.

Which statement about one product of the titration could be correct?

A. $0.005 \text{ mol}$ of barium chloride is formed.
B. $0.05 \text{ mol}$ of barium chloride is formed.
C. $0.005 \text{ mol}$ of magnesium chloride is formed.
D. $0.05 \text{ mol}$ of magnesium chloride is formed.
E. $0.01 \text{ mol}$ of barium sulfate is formed.
F. $0.1 \text{ mol}$ of barium sulfate is formed.
G. $0.01 \text{ mol}$ of magnesium sulfate is formed.
H. $0.1 \text{ mol}$ of magnesium sulfate is formed.

Answer: E

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Question 26

In the presence of a suitable catalyst,

30.0 \text{ cm}^3

of a

0.10 \text{ mol dm}^{-3}

aqueous solution of

Na_2C_2O_4

reacts exactly with

40.0 \text{ cm}^3

of a

0.15 \text{ mol dm}^{-3}

aqueous solution of

Ce(SO_4)_2

.

The only products of the reaction are another cerium salt, a different water-soluble salt and carbon dioxide.

What is the cerium ion formed by this reaction?

A. $Ce^+$
B. $Ce^{2+}$
C. $Ce^{3+}$
D. $Ce^{4+}$
E. $Ce^{5+}$

Answer: C

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Question 27

Consider all of the structural isomers of

C_4H_9Br

and the ways that they could react with hydroxide ions from aqueous sodium hydroxide.

How many different organic products (including all structural and E/Z isomers only) containing four carbons could be made?

A.4
B.5
C.6
D.7
E.8
F.9

Answer: E

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Question 28

In which of the following conversions does the stated bond angle decrease?

1. hydrogen-nitrogen-hydrogen bond angle in the conversion from

NH_3

to

NH_4^+

2. fluorine-iodine-fluorine bond angle in the conversion from

IF_4^-

to

IF_6^+

3. chlorine-aluminium-chlorine bond angle in the conversion from

AlCl_3

to

AlCl_4^-

A.none of them
B.1 only
C.2 only
D.3 only
E.1 and 2 only
F.1 and 3 only
G.2 and 3 only
H.1, 2 and 3

Answer: D

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Question 29

Ethanol combines with ethanoic acid to form ethyl ethanoate according to the following reaction.

C_2H_5OH(l) + CH_3COOH(l) \rightleftharpoons CH_3COOC_2H_5(l) + H_2O(l) \quad \Delta H = -6 \text{ kJ mol}^{-1}

at 25 °C

A mixture of

9.2 \text{ g}

ethanol,

12 \text{ g}

ethanoic acid and

8.8 \text{ g}

ethyl ethanoate is allowed to react and reach equilibrium.

The resulting equilibrium mixture is found to contain

4.8 \text{ g}

ethanoic acid at 25 °C and

K_c

was calculated.

What will happen to the value of

K_c

when the temperature is increased from 25 °C to 45 °C?

(

M_r

values:

C_2H_5OH = 46

;

CH_3COOH = 60

;

CH_3COOC_2H_5 = 88

;

H_2O = 18

)

A.It increases from an initial value of 0.242
B.It decreases from an initial value of 0.242
C.It increases from an initial value of 1.833
D.It decreases from an initial value of 1.833
E.It increases from an initial value of 2.750
F.It decreases from an initial value of 2.750
G.It increases from an initial value of 4.125
H.It decreases from an initial value of 4.125

Answer: H

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Question 30

Compound Z is known to have percentage composition by mass C: 80.0%, H: 6.7%, O: 13.3%.
The mass spectrum of compound Z is given showing the mass-to-charge ratio (m/z) of the molecular ion and its fragments.

Which of the following could be the structure of Z?

(

A_r

values: C = 12; H = 1; O = 16)

A.Acetophenone (a benzene ring attached to a carbonyl group, which is attached to a methyl group)
B.Phenylacetaldehyde (a benzene ring attached to a CH₂ group, which is attached to a CHO group)
C. $CH_3CH_2CH_2CH(OH)CH(OH)CH_3$
D. $CH_2=CHCH=CHCH=CH-CH=C=O$
E.1-Phenylethanol (a benzene ring attached to a CH(OH)CH₃ group)
F.2-Phenyloxirane (a benzene ring attached to an epoxide ring with one other carbon atom)

Answer: A

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Question 31

The time it takes the ion

^1_1H^+

to travel through the flight tube in a time-of-flight mass spectrometer is

t

.

How long would it take the ion

^2_1H^+

to travel through the same flight tube?

A. $rac{1}{4}t$
B. $rac{1}{2}t$
C. $rac{1}{\sqrt{2}}t$
D. $\sqrt{\frac{2}{3}}t$
E. $\sqrt{\frac{3}{2}}t$
F. $\sqrt{2}t$
G. $2t$
H. $4t$

Answer: F

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Question 32

A paper coffee cup is lined with a thin layer of plastic to make it waterproof. This plastic makes up 5% of the mass of the cup and has the following molecular structure.

960 \text{ dm}^3

of a gaseous hydrocarbon monomer (measured at room temperature and pressure) was used to make a certain number of plastic-lined cups.

It is proposed that a more environmentally-friendly, biodegradable plastic, poly(lactic acid), could be used instead in the same proportions by mass (5%). Assume that any polymerisation reaction has a 100% yield.

What mass of lactic acid (

M_r = 90

) is required to make the same number of biodegradable cups?

(

A_r

values: H = 1; C = 12; O = 16. Assume that one mole of a gas occupies

24 \text{ dm}^3

at room temperature and pressure.)

A.896 g
B.1120 g
C.1200 g
D.1400 g
E.1500 g
F.3000 g
G.3600 g

Answer: D

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Question 33

In which of the following pairs do both molecules possess permanent dipoles in the gaseous state?

1. methylamine (

CH_3NH_2

) and dichlorine monoxide (

Cl_2O

)
2. difluoroethyne (

C_2F_2

) and sulfur hexafluoride
3. phosphorus trichloride and xenon tetrafluoride

A.none of them
B.1 only
C.2 only
D.3 only
E.1 and 2 only
F.1 and 3 only
G.2 and 3 only
H.1, 2 and 3

Answer: B

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Question 34

At high temperatures, carbon dioxide reacts partially with solid carbon and forms the following equilibrium.

CO_2(g) + C(s) \rightleftharpoons 2CO(g)

In an experiment,

1.0 \text{ mol}

of carbon dioxide is placed in a flask of capacity

V \text{ dm}^3

containing no air and an excess of carbon. It is then heated to a high temperature where an equilibrium between carbon dioxide and carbon monoxide is established. At this temperature,

x \text{ mol}

of carbon dioxide converts to carbon monoxide and the value of the equilibrium constant,

K_c

, is

2.0 \text{ mol dm}^{-3}

.

As it is a solid, carbon is not included in the equilibrium constant expression.

What is the relationship between the value of

x

and

V

?

A. $x = \frac{-V+\sqrt{V^2+32V}}{16}$
B. $x = \frac{-V+\sqrt{V^2+8V}}{4}$
C. $x = \frac{-2+\sqrt{4V^2+8V}}{4}$
D. $x = \frac{-2V+\sqrt{4V^2-2V}}{2}$
E. $x = \frac{-2+\sqrt{4-32V}}{4V}$

Answer: B

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Question 35

A student carried out an experiment to find the mass of the active ingredient calcium carbonate (

M_r = 100

) in an indigestion tablet. The student crushed the tablet and placed it in a beaker.

The student added

20.0 \text{ cm}^3

of

2.00 \text{ mol dm}^{-3}

hydrochloric acid, an excess, to the tablet. When the reaction was complete, the reaction mixture was all transferred to a volumetric flask and deionised water added to make the solution up to

250 \text{ cm}^3

.

25.0 \text{ cm}^3

samples of the solution in the volumetric flask were titrated against

0.100 \text{ mol dm}^{-3}

sodium hydroxide solution. The mean titre was

26.40 \text{ cm}^3

of sodium hydroxide solution.

Assuming that all other ingredients in the tablet are inert, what is the mass of calcium carbonate in the tablet?

A.0.264 g
B.0.680 g
C.1.36 g
D.1.87 g
E.2.72 g

Answer: B

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Question 36

Ellingham diagrams show how the quantity

\Delta G

for a reaction varies over a range of temperatures.

The

\Delta G

values for different reactions can be combined using Hess's law.

For a reaction to be able to occur, the overall value of

\Delta G

must be less than zero.

Using the Ellingham diagram shown, which one of the following statements is correct?

A.Nickel can reduce tin(IV) oxide at 300 °C.
B.Carbon can reduce chromium(III) oxide at 1500 °C forming carbon dioxide and chromium.
C.Chromium(III) oxide and nickel oxide will decompose to their elements at all the temperatures shown.
D.Chromium will react with steam at all the temperatures shown.
E.Nickel oxide can be reduced by carbon at 300 °C forming carbon monoxide and nickel.

Answer: D

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Question 37

SO_3^{2-}(aq)

ions can cause the reduction of

VO_3^-(aq)

ions to

VO^{2+}(aq)

in acidic solution. During this process, the

SO_3^{2-}(aq)

ions are oxidised to

SO_4^{2-}(aq)

ions.

What is the minimum volume of

1.00 \text{ mol dm}^{-3}

sulfuric acid required to provide sufficient hydrogen ions to allow

Na_2SO_3(aq)

to reduce

40.0 \text{ cm}^3

of

0.100 \text{ mol dm}^{-3}

VO_3^-(aq)

ions completely to

VO^{2+}(aq)

?

A.no sulfuric acid is required
B. $4.0 \text{ cm}^3$
C. $6.0 \text{ cm}^3$
D. $6.6 \text{ cm}^3$
E. $8.0 \text{ cm}^3$
F. $12.0 \text{ cm}^3$
G. $13.2 \text{ cm}^3$

Answer: C

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Question 38

Sodium chloride is an ionic compound which forms highly ordered crystals of ions when in the solid state. This is called a lattice.

A lattice can be thought of as a stack of layered planes of ions.

In the following diagrams, the shaded and non-shaded circles represent sodium and chloride ions, respectively.

Which of the following is not a representation of a plane present in a sodium chloride lattice?

A.A square grid where each ion is surrounded by four ions of the opposite charge.
B.A hexagonal close-packed arrangement of ions of the same charge.
C.A square grid where rows alternate between containing only shaded ions and only non-shaded ions.
D.A single row of alternating shaded and non-shaded ions. Two such rows are shown separately.
E.A grid where each ion is surrounded by two ions of the opposite charge and two ions of the same charge, forming a checkerboard pattern.

Answer: E

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Question 39

When bromine reacts with (Z)-pent-2-ene, an intermediate called a bromonium ion forms. This is then attacked by the bromide ion to give the 2,3-dibromopentane addition product.

The mechanism is shown:

In a different reaction (E)-but-2-ene is reacted with bromine in an addition reaction. This reaction also occurs via a bromonium ion intermediate.

Which of the following structures show the final product of the reaction of (E)-but-2-ene with bromine?

A.structures 1 and 2 only
B.structures 1 and 3 only
C.structures 1 and 4 only
D.structures 2 and 3 only
E.structures 2 and 4 only
F.structures 3 and 4 only
G.structures 1, 2, 3 and 4

Answer: B

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Question 40

Both oxygen and carbon monoxide bind reversibly to haemoglobin.

For the purposes of this question, assume that only one molecule of either oxygen or carbon monoxide can bind to haemoglobin at any one time.

Within the human body, carbon monoxide is 200 times more effective than oxygen in binding to haemoglobin available in human blood. The binding efficiency is the equilibrium constant for this process and you may assume that the system is at equilibrium.

The effects of carboxy-haemoglobin in humans can typically be observed when it reaches 5% of the concentration of oxy-haemoglobin in their blood.

Assume that oxygen and carbon monoxide have the same molar solubility in blood.

What is the minimum proportion of carbon monoxide molecules in dry air, expressed as parts per million (ppm), that will result in a 5% ratio of carboxy-haemoglobin molecules to oxy-haemoglobin molecules in blood?

A.10 ppm
B.21 ppm
C.52.5 ppm
D.210 ppm
E.1000 ppm
F.5250 ppm

Answer: C

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