iGCSE Chemistry (0620) Core:3.1 Formulae: Exam Style Questions Paper 1- New Syllabus
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Detailed solution:
By carefully counting the carbon and hydrogen atoms in each diagram, the first, third, and fourth molecules all have four carbon atoms and ten hydrogen atoms, giving the molecular formula $C_4H_{10}$. The second molecule stands out because it has five carbon atoms and twelve hydrogen atoms, making it $C_5H_{12}$. So three of the four molecules share the exact same molecular formula, while one is different, which is exactly what option D states.
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Detailed solution:
Sodium carbonate has the correct formula Na₂CO₃ because sodium ions are Na⁺ and carbonate ions are CO₃²⁻, so two sodium ions are needed to balance the charge. Option A is wrong: cobalt(II) chloride should be CoCl₂, not Cu₂Cl (copper is a different element altogether). Xenon is a monatomic noble gas, so its formula is simply Xe, not Xe₂. Ammonium sulfate contains NH₄⁺ and SO₄²⁻ ions, so the correct formula is (NH₄)₂SO₄, not NH₄SO₄.
▶️ Answer/Explanation
Detailed solution:
Magnesium reacts with hydrochloric acid to form magnesium chloride (which is soluble, so it’s aqueous) and hydrogen gas. The correct formula for magnesium chloride is $\text{MgCl}_2$ because Mg has a 2+ charge and Cl has a 1– charge. Hydrochloric acid is an aqueous solution, so its state symbol is (aq), not (l). Option A shows the correct balanced equation with the right state symbols: $\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$.