Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 7.1 — The characteristic properties of acids and bases
• Topic 4.1 — Electrolysis
• Topic 6.4 — Redox
(a)(i) $H^{+}$ (The hydrogen ion).
(a)(ii) Nitric acid is a strong acid because it undergoes complete dissociation in aqueous solution (all molecules release $H^{+}$). Ethanoic acid is a weak acid because it undergoes only partial dissociation (it forms an equilibrium heavily favoring the un-dissociated molecules).
(a)(iii) Ethanoic acid.
Explanation: Because ethanoic acid only partially dissociates, it has the lowest concentration of hydrogen ions ($[H^{+}]$) among the three 1 molar solutions, which mathematically corresponds to the highest pH.
(a)(iv) Colourless. (Thymolphthalein is only blue in alkaline conditions above ~pH 9.3).
(a)(v) Sulfuric acid anion: $SO_{4}^{2-}$. Ethanoic acid anion: $CH_{3}COO^{-}$.
(a)(vi) Hydrogen (formed at the cathode as $H^{+}$ gains electrons) and Oxygen (formed at the anode as $OH^{-}$ loses electrons).
(a)(vii) Calcium ethanoate. (Calcium replaces the acidic hydrogen of the ethanoic acid).
(b)(i) A base is defined as a proton acceptor.
(b)(ii) It tells us that $Al(OH)_{3}$ is insoluble in water. (By definition, an alkali is a base that is soluble in water).
(b)(iii) Aluminium oxide. (Aluminium carbonate or pure aluminium metal would also be correct).
(b)(iv)
Working: The compound $Al(NO_{3})_{3}$ is a neutral salt, so the sum of all oxidation states equals 0.
We are given $Al = +3$ and $O = -2$. There are 3 nitrogen atoms and 9 oxygen atoms.
$3 + 3(N) + 9(-2) = 0$
$3N – 15 = 0$
$3N = +15$
$N =$ +5
