This question is about copper and copper compounds.
(a) Describe how you could prepare a pure sample of crystals of hydrated copper(II) sulfate using dilute sulfuric acid and an excess of copper(II) oxide. [3]
(b) Anhydrous copper(II) sulfate is used to test for water.
$\begin{aligned} & \mathrm{CuSO}_4+5 \mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O} \\ & \text{anhydrous hydrated} \\ & \text{copper(II) sulfate copper(II) sulfate} \end{aligned}$
(i) What is meant by the symbol $\rightleftharpoons$? [1]
(ii) How can hydrated copper(II) sulfate be changed into anhydrous copper(II) sulfate? [1]
(c) Complete the table to calculate the relative formula mass of anhydrous copper(II) sulfate, $\mathrm{CuSO}_4$. Use your Periodic Table to help you.
relative formula mass = …………………………. [2]
(d) Complete the table to show the number of electrons, protons and neutrons in the sulfur atom and copper ion shown.
(e) Alloys of copper are used to make coins.
(i) What is meant by the term alloy? [1]
(ii) Suggest why an alloy of copper is used to make coins instead of using pure copper. [1] [Total: 13]
▶️ Answer/Explanation
(a) Ans: Heat copper(II) oxide with dilute sulfuric acid, filter the excess oxide, then evaporate the filtrate to obtain hydrated copper(II) sulfate crystals.
(b)(i) Ans: The symbol $\rightleftharpoons$ means the reaction is reversible.
(b)(ii) Ans: Heat the hydrated copper(II) sulfate to remove water and convert it to anhydrous form.
(c) Ans: Relative formula mass of $\mathrm{CuSO}_4$ is 160 (Cu = 64, S = 32, O = 16 × 4).
(d) Ans: Sulfur atom: 16 electrons, 16 protons, 18 neutrons. Copper ion ($\mathrm{Cu}^{2+}$): 27 electrons, 29 protons, 34 neutrons.
(e)(i) Ans: An alloy is a mixture of a metal with other elements to enhance properties.
(e)(ii) Ans: Copper alloys are harder and more wear-resistant, making them suitable for coins.
(a) The table shows the percentage by mass of the elements on Earth and in the Universe.
Answer these questions using only the information in the table.
(i) Deduce the percentage by mass of other elements present on Earth.$\%[1]$
(ii) Which non-metallic element is present on Earth in the greatest percentage by mass?[1]
(iii) Give two major differences in the percentage by mass of the elements on Earth and in the Universe.[2]
(b) Complete the diagram to show the electron arrangement in an oxygen atom.
(c) Helium, neon and argon are noble gases.
(i) Explain, in terms of the electronic structure, why neon is unreactive.[1]
(ii) State one use of argon.[1] [Total: 7]
▶️ Answer/Explanation
(a)(i) Ans: 5.0%
Sum the given percentages on Earth: 47 + 28 + 8 + 5 + 4 + 3 = 95%. The remaining percentage is 100% – 95% = 5%.
(a)(ii) Ans: oxygen
Oxygen (47%) is the highest among non-metals (others: silicon, magnesium, etc., are metals).
(a)(iii) Ans: Any two differences
1. The Universe has more hydrogen (75%) compared to Earth (1%).
2. Earth has more oxygen (47%) compared to the Universe (1%).
(b) Ans: Electron arrangement for oxygen
Oxygen has 8 electrons: 2 in the first shell and 6 in the second shell.
(c)(i) Ans: Neon has a complete outer electron shell
Neon’s outer shell is full (8 electrons), making it stable and unreactive.
(c)(ii) Ans: Used in lamps or as an inert atmosphere
Argon is used in light bulbs to prevent filament oxidation or in welding.