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CIE iGCSE Co-Ordinated Science C7.2 Oxides Exam Style Questions Paper 4

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Question

Fig. 6.1 shows the structure of magnesium oxide.
(a) Deduce the formula of magnesium oxide.
(b) Oxides can be classified as acidic, basic or amphoteric.
Classify magnesium oxide.
Explain your answer.
(c) Magnesium is in Group II and period 3 of the Periodic Table.
Determine the electronic configuration of a magnesium atom.
(d) Magnesium occurs naturally in three stable isotopes, \(^{24}\text{Mg}\), \(^{25}\text{Mg}\) and \(^{26}\text{Mg}\).
Describe the similarity and the difference between the three isotopes.
(e) The compound magnesium sulfate, MgSO₄, is found in sea water.
Calculate the amount (mol) of magnesium sulfate in a 3.05 g sample of magnesium sulfate where all the magnesium atoms are the isotope \(^{26}\text{Mg}\).
\([A_r: \text{O}, 16; \text{S}, 32]\)
(f) A student measures the boiling point of a sample of sea water.
The boiling point is 102 °C.
Describe how the student knows that the sea water is not pure water.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic C2.4 — Ions and ionic bonds (Part (a))
• Topic C7.2 — Oxides (Part (b))
• Topic C2.2 — Atomic structure and the Periodic Table (Part (c))
• Topic C2.3 — Isotopes (Part (d))
• Topic C3.2 — Relative masses of atoms and molecules / C3.3 — The mole and the Avogadro constant (Part (e))
• Topic C10.1 — Water (Part (f))

▶️ Answer/Explanation

(a) Formula = MgO.
The diagram shows equal numbers of Mg²⁺ and O²⁻ ions in a 1:1 ratio. The charges balance (+2 and -2), so the formula is MgO.

(b) Classification: Basic oxide.
Explanation: Magnesium is a metal. Metal oxides react with acids to form salts and water. Magnesium oxide is a basic oxide because it neutralises acids.

(c) Electronic configuration = 2.8.2.
Magnesium has 12 protons/electrons. The first shell holds 2, the second holds 8, and the remaining 2 electrons go into the third shell.

(d) Similarity: Same number of protons (atomic number = 12).
Difference: Different number of neutrons / different mass numbers (24, 25, 26).

(e) Calculation:
\(M_r\) of MgSO₄ = 26 + 32 + (4 × 16) = 122
\(\text{moles} = \frac{\text{mass}}{M_r} = \frac{3.05}{122} = 0.025\text{ mol}\)
Note: The \(^{26}\text{Mg}\) isotope has mass number 26, so the relative atomic mass used for Mg is 26 in this calculation.

(f) Pure water boils at 100°C at standard atmospheric pressure.
The sea water sample boils at 102°C, which is higher than 100°C. This elevation in boiling point indicates the presence of dissolved impurities (salts), so the water is not pure.

Question

A sodium atom is represented with numbers next to its chemical symbol, as shown in Fig. 6.1.
(a) Complete Table 6.1 to show the structure of a sodium atom.
(b) Fig. 6.2 shows an outline of the Periodic Table. The letter E shows the position of an element in the Periodic Table. The letter E is not the chemical symbol of the element.
Predict the electronic configuration of element E. Tick \((\checkmark)\) one box.
☐ 2.8
☐ 2.8.2
☐ 2.8.8
☐ 2.8.8.1
(c) Carbon-12 and carbon-13 are two isotopes of the element carbon. These isotopes of carbon have the same chemical properties. Explain why.
(d) State the type of oxide formed when carbon, a non-metal, reacts with oxygen to produce carbon dioxide, \(\text{CO}_2\).
(e) Carbon dioxide is a greenhouse gas and causes global warming. Complete the sentences to describe how carbon dioxide causes global warming. Use words from the list.
Each word can be used once, more than once, or not at all.
absorbed       reflected       refracted       stored
Energy from the Sun reaches the Earth’s surface. Some energy is ______________ back into space. Most of the energy is ______________ by the Earth’s surface, causing an increase in temperature. The warm Earth emits energy. Some of this emitted energy is then ______________ by greenhouse gases. When this energy is re-emitted, it can be transferred back to the Earth’s surface.
(f) Some coal burns to make \(11000\text{ g}\) of carbon dioxide gas. Calculate the volume occupied by \(11000\text{ g}\) of carbon dioxide gas. The volume of one mole of any gas is \(24\text{ dm}^3\) at room temperature and pressure (r.t.p.).
[\(A_r: \text{C}, 12; \text{O}, 16\)]

Topic codes:

• Topic C2.2 — Atomic structure and the Periodic Table (Part (a) & (b))
• Topic C2.3 — Isotopes (Part (c))
• Topic C7.2 — Oxides (Part (d))
• Topic C10.2 — Air quality and climate / Global warming (Part (e))
• Topic C3.3 — The mole and the Avogadro constant (Part (f))

▶️ Answer/Explanation

(a) Complete Table 6.1:

Sodium has 11 protons (atomic number = 11), 11 electrons (neutral atom), and 23 − 11 = 12 neutrons.

(b) ☑ 2.8.2

Element E is in Period 3 (three electron shells) and Group II (two outer-shell electrons). Therefore, the electronic configuration is 2,8,2.

(c) Isotopes of carbon have the same chemical properties because they have the same number of electrons / the same electronic configuration. Chemical properties depend on the number and arrangement of electrons in the atom.

(d) Acidic oxide — carbon dioxide is an acidic oxide because it reacts with bases to form salts and water.

(e) Energy from the Sun reaches the Earth’s surface. Some energy is reflected back into space. Most of the energy is absorbed by the Earth’s surface, causing an increase in temperature. The warm Earth emits energy. Some of this emitted energy is then absorbed by greenhouse gases. When this energy is re-emitted, it can be transferred back to the Earth’s surface.

(f) \(M_r\) of CO₂ = 12 + (2 × 16) = 44

Moles of CO₂ = 11000 ÷ 44 = 250 mol

Volume of CO₂ = 250 × 24 = 6000 dm³

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