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Chemistry entrance exam, 2005 E.C. (2013)

Real questions from the Grade 12 university entrance exam in Chemistry, as students sat it in 2005 E.C. Every question comes with the correct answer and a worked explanation.

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Natural science stream
Who sits it
2005 E.C. (2013)
Exam year
74 questions with answers
In this set

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Answer each question yourself before opening the answer. The explanations point to the exact textbook section, so you know which page to reread when you miss one.

Question 1

Which of the following is NOT a basic SI unit?

A.Candela
B.Gram
C.Mole
D.Second

Show the answer and explanation

Answer: B Gram

Why

The seven SI base units include the candela for luminous intensity, the mole for amount of substance and the second for time. The base unit of mass is the kilogram, so the gram is a fraction of a base unit and not a base unit itself. The mole tempts a lot of students because it feels like a counting label rather than a measurement, but amount of substance is one of the seven base quantities.

Question 2

Which of the following statements is TRUE?

A.Ultraviolet light has longer wavelength than visible light.
B.The energy of radiation decreases as the wavelength decreases.
C.The frequency of radiation increases as the wavelength decreases.
D.Wave number of an electromagnetic radiation increases as wavelength increases.

Show the answer and explanation

Answer: C The frequency of radiation increases as the wavelength decreases.

Why

Wavelength and frequency are tied together by \(c = \nu\lambda\), and the speed of light \(c\) is fixed. Rearranged, \(\nu = c/\lambda\), so when the wavelength gets shorter the frequency must get higher. Option B tempts because energy and wavelength sound like they should rise together, but \(E = hc/\lambda\) shows the energy goes up when the wavelength goes down.

Question 3

Which of the following ionic compounds is formed from the reaction between magnesium and nitrogen?

A.\(\mathrm{MgN_2}\)
B.\(\mathrm{Mg_2N_2}\)
C.\(\mathrm{Mg_3N_2}\)
D.\(\mathrm{Mg_2N_3}\)

Show the answer and explanation

Answer: C \(\mathrm{Mg_3N_2}\)

Why

Magnesium is in group IIA and loses two electrons to give \(\mathrm{Mg^{2+}}\). Nitrogen gains three electrons to give \(\mathrm{N^{3-}}\). The compound must be electrically neutral, so three \(\mathrm{Mg^{2+}}\) ions carrying \(+6\) pair with two \(\mathrm{N^{3-}}\) ions carrying \(-6\), which gives \(\mathrm{Mg_3N_2}\). Option A tempts if you copy the 2 from the \(\mathrm{N_2}\) gas, but the subscripts in an ionic formula come from the charges.

Question 4

Which of the following molecules represents a non-polar covalent bond?

A.B-Cl
B.C-Cl
C.Cl-Cl
D.Mg-Cl

Show the answer and explanation

Answer: C Cl-Cl

Why

A bond is non-polar when both atoms pull on the shared pair with the same strength. Two chlorine atoms have identical electronegativity, so the difference is zero and the electron pair sits in the middle. C-Cl is the tempting choice because carbon and chlorine are both non-metals, but their electronegativities differ by about 0.5, so that bond is polar covalent.

Question 5

Which of the following groups in the periodic table has paramagnetic atoms?

A.Group zero
B.Group IIA
C.Group IIB
D.Group IVA

Show the answer and explanation

Answer: D Group IVA

Why

Paramagnetic means the atom holds at least one unpaired electron. Group IVA atoms end in \(ns^2np^2\), and by Hund's rule those two p electrons go into separate orbitals with parallel spins, so two electrons stay unpaired. Group zero, IIA and IIB all finish with completely filled subshells, so every electron is paired and they are diamagnetic. Group IIB tempts because of its d electrons, but that \(d^{10}\) subshell is full.

Question 6

How many types of cubic unit cells are known?

Show the answer and explanation

Answer: B 3

Why

A cubic lattice can arrange its points in three ways: at the eight corners only (simple cubic), corners plus one point at the centre of the cube (body centred), or corners plus one point at the centre of each face (face centred). No other arrangement keeps full cubic symmetry. Four is a common guess from students who add an edge centred cell, but that pattern is the same as one of the three already counted.

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Chemistry papers from other years

2018 E.C. (2026)2017 E.C. (2025)2016 E.C. (2024)2015 E.C. (2023)2014 E.C. (2022)2013 E.C. (2021)

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Are these the real 2005 E.C. Chemistry entrance exam questions?

Yes. They come from the national university entrance exam in Chemistry that students sat in 2005 E.C. (2013). Our set holds 74 of its questions, each with its answer and an explanation that points back to the textbook.

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