ESAT Challenge Chemistry Esat-chem-challenge-2

10 questions10 marks20Updated August 2026

The ESAT Challenge Chemistry Esat-chem-challenge-2 paper in full: all 10 questions, each with its answer. ESAT is the Engineering and Science Admissions Test. Sit it cold under exam timing, mark it, then work back through anything you missed using the solutions below.

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Question 1

1 mark

A scrap dealer assays a batch of copper and zinc alloy. A

5.00\,\text{g}

turning is dropped into an excess of dilute hydrochloric acid; only the zinc reacts, and

1.20\,\text{dm}^3

of hydrogen is collected at room conditions. What percentage of the alloy, by mass, is copper? (

A_r

: Zn

=65

)

A. $35.0\%$
B. $26.0\%$
C. $40.0\%$
D. $55.0\%$
E. $65.0\%$

Answer: A

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Question 2

1 mark

Complete combustion of

0.150\,\text{g}

of a compound of carbon, hydrogen and oxygen only gives

0.220\,\text{g}

of carbon dioxide and

0.090\,\text{g}

of water. The relative molecular mass of the compound is

60

. What is its molecular formula? (

A_r

: C

=12

, H

=1

, O

=16

)

A. $\text{CH}_2\text{O}$
B. $\text{C}_3\text{H}_8\text{O}$
C. $\text{C}_2\text{H}_4\text{O}_2$
D. $\text{C}_2\text{H}_4\text{O}$
E. $\text{C}_3\text{H}_4\text{O}_2$

Answer: C

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Question 3

1 mark

An iron supplement tablet is dissolved in excess dilute sulfuric acid and the solution made up to

250\,\text{cm}^3

. A

25.0\,\text{cm}^3

portion is titrated against

0.0100\,\text{mol dm}^{-3}

potassium manganate(VII), of which

20.0\,\text{cm}^3

is required. All the iron is present as

\text{Fe}^{2+}

, and

\text{MnO}_4^-

is reduced to

\text{Mn}^{2+}

. What mass of iron does one tablet contain? (

A_r

: Fe

=56

)

A. $56\,\text{mg}$
B. $112\,\text{mg}$
C. $280\,\text{mg}$
D. $560\,\text{mg}$
E. $5.60\,\text{g}$

Answer: D

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Question 4

1 mark

20\,\text{cm}^3

of a gaseous hydrocarbon is mixed with

150\,\text{cm}^3

of oxygen, an excess, and the mixture is sparked. After cooling to the original temperature and pressure the volume is

110\,\text{cm}^3

; after passing the residue through concentrated potassium hydroxide solution,

30\,\text{cm}^3

remains. All volumes are measured at the same temperature and pressure. What is the hydrocarbon?

A. $\text{C}_4\text{H}_6$
B. $\text{C}_4\text{H}_{10}$
C. $\text{C}_5\text{H}_{10}$
D. $\text{C}_3\text{H}_8$
E. $\text{C}_4\text{H}_8$

Answer: E

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Question 5

1 mark

10.0\,\text{g}

mixture of calcium carbonate and calcium oxide is heated strongly until there is no further change in mass, leaving

7.80\,\text{g}

of solid. What percentage of the original mixture, by mass, was calcium carbonate? (

A_r

: Ca

=40

, C

=12

, O

=16

)

A. $22.0\%$
B. $44.0\%$
C. $50.0\%$
D. $56.0\%$
E. $78.0\%$

Answer: C

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Question 6

1 mark

In a calorimetry experiment,

0.92\,\text{g}

of ethanol,

\text{C}_2\text{H}_5\text{OH}

, is burned completely. All of the energy released is transferred to

200\,\text{g}

of water, whose temperature rises from

18.0\,^\circ\text{C}

47.0\,^\circ\text{C}

.

The specific heat capacity of water is

4.2\,\text{J g}^{-1}\,\text{K}^{-1}

. Relative atomic masses:

\text{H} = 1

\text{C} = 12

\text{O} = 16

.

What is the enthalpy change of combustion of ethanol calculated from this experiment?

A. $-2440\,\text{kJ mol}^{-1}$
B. $-1970\,\text{kJ mol}^{-1}$
C. $-1220\,\text{kJ mol}^{-1}$
D. $-24.4\,\text{kJ mol}^{-1}$
E. $-5.60\,\text{kJ mol}^{-1}$
F. $-1.22\,\text{kJ mol}^{-1}$
G. $+1220\,\text{kJ mol}^{-1}$

Answer: C

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Question 7

1 mark

Acidified dichromate ions oxidise iodide ions to iodine. The unbalanced equation is

\text{Cr}_2\text{O}_7^{\,2-} + \text{I}^- + \text{H}^+ \rightarrow \text{Cr}^{3+} + \text{I}_2 + \text{H}_2\text{O}

Which row gives the number of moles of

\text{I}_2

formed per mole of

\text{Cr}_2\text{O}_7^{\,2-}

, and the coefficient of

\text{H}^+

in the balanced equation?

A.moles of $\text{I}_2$ : $1$ , coefficient of $\text{H}^+$ : $7$
B.moles of $\text{I}_2$ : $2$ , coefficient of $\text{H}^+$ : $14$
C.moles of $\text{I}_2$ : $3$ , coefficient of $\text{H}^+$ : $7$
D.moles of $\text{I}_2$ : $3$ , coefficient of $\text{H}^+$ : $8$
E.moles of $\text{I}_2$ : $3$ , coefficient of $\text{H}^+$ : $14$
F.moles of $\text{I}_2$ : $6$ , coefficient of $\text{H}^+$ : $7$
G.moles of $\text{I}_2$ : $6$ , coefficient of $\text{H}^+$ : $14$
H.moles of $\text{I}_2$ : $7$ , coefficient of $\text{H}^+$ : $14$

Answer: E

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Question 8

1 mark

Ammonia is manufactured according to

\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3

A reaction vessel is charged with

42\,\text{g}

\text{N}_2

and

6.0\,\text{g}

\text{H}_2

. When the reaction has finished,

25.5\,\text{g}

\text{NH}_3

has been collected.

Relative atomic masses:

\text{H} = 1

\text{N} = 14

.

Which row gives the percentage yield and the mass of the reactant in excess that is left unreacted, assuming the limiting reactant is fully consumed?

A.percentage yield: $50\%$ , mass left: $14\,\text{g}$
B.percentage yield: $50\%$ , mass left: $28\,\text{g}$
C.percentage yield: $75\%$ , mass left: $6.0\,\text{g}$
D.percentage yield: $75\%$ , mass left: $14\,\text{g}$
E.percentage yield: $75\%$ , mass left: $28\,\text{g}$
F.percentage yield: $75\%$ , mass left: $42\,\text{g}$

Answer: D

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Question 9

1 mark

An element X has exactly two stable isotopes, with mass numbers

69

and

71

. An atom of X contains

31

protons. The relative atomic mass of X is

69.7

.

Which row gives the percentage abundance of the heavier isotope, and the number of neutrons in an atom of the lighter isotope?

A.abundance of heavier isotope: $35\%$ , neutrons: $31$
B.abundance of heavier isotope: $35\%$ , neutrons: $38$
C.abundance of heavier isotope: $35\%$ , neutrons: $40$
D.abundance of heavier isotope: $65\%$ , neutrons: $38$
E.abundance of heavier isotope: $65\%$ , neutrons: $40$
F.abundance of heavier isotope: $70\%$ , neutrons: $38$

Answer: B

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Question 10

1 mark

Ethene reacts with bromine to form 1,2-dibromoethane:

\text{CH}_2{=}\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{BrCH}_2\text{Br}

Mean bond energies in

\text{kJ mol}^{-1}

\text{C}=\text{C}

612

;

\text{C}-\text{C}

347

;

\text{C}-\text{H}

413

;

\text{Br}-\text{Br}

193

;

\text{C}-\text{Br}

276

.

What is the enthalpy change of this reaction?

A. $-287\,\text{kJ mol}^{-1}$
B. $-94\,\text{kJ mol}^{-1}$
C. $+94\,\text{kJ mol}^{-1}$
D. $+182\,\text{kJ mol}^{-1}$
E. $+253\,\text{kJ mol}^{-1}$

Answer: B

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