Acids and bases: Grade 12 Past Paper Questions

An antacid tablet contains a mixture of Al(OH)₃ and Mg(OH)₂ only. This tablet is reacted with 50 cm³ of a 0,5 mol·dm⁻³ HCl solution. The balanced equations for the two reactions that take place are: Al(OH)₃(s) + 3HCl(aq) → AlCl₃(aq) + 3H₂O(l); Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l). The resulting solution is completely neutralised by 265 cm³ of a 0,378 mol·dm⁻³ NaOH solution: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l). Calculate the number of moles of hydroxide ions present in the tablet. If the tablet contains an equal number of moles of aluminium hydroxide and magnesium hydroxide, calculate the mass of Al(OH)₃ in the tablet.

Oxalic acid (H₂C₂O₄) is an organic diprotic acid commonly found in plants such as spinach. It is used in various industrial and laboratory applications, including cleaning, bleaching, and as a standard solution in acid-base titrations. When oxalic acid ionises in water, it follows the steps given below: STEP 1: H₂C₂O₄(aq) + H₂O(l) ⇌ HC₂O₄⁻(aq) + H₃O⁺(aq); STEP 2: HC₂O₄⁻(aq) + H₂O(l) ⇌ C₂O₄²⁻(aq) + H₃O⁺(aq). Define a weak acid. Identify the acid-base conjugate pair in STEP 1. Give a reason why oxalic acid is referred to as a diprotic acid. The oxalate ion (HC₂O₄⁻) can act as an ampholyte — give a reason for this statement. In a volumetric flask, 2,25 g of oxalic acid is added to water to make up a standard solution to 250 cm³. Calculate the concentration of the oxalic acid solution. 25 cm³ of the oxalic acid solution is titrated against sodium hydroxide; the average volume of NaOH required for neutralisation is 28,60 cm³: H₂C₂O₄(aq) + 2NaOH(aq) → Na₂C₂O₄(aq) + 2H₂O(l). Calculate the concentration of the sodium hydroxide. Explain why phenolphthalein would be a suitable indicator for this reaction. The reaction between excess magnesium hydroxide (Mg(OH)₂), a slightly soluble base, and nitric acid (HNO₃) occurs in aqueous solution, producing magnesium nitrate (Mg(NO₃)₂) and water: Mg(OH)₂(aq) + 2HNO₃(aq) → Mg(NO₃)₂(aq) + 2H₂O(l). 0,05 dm³ of the Mg(OH)₂ solution, with a concentration of 0,115 mol·dm⁻³, is added to 0,025 dm³ of a 0,095 mol·dm⁻³ HNO₃ solution. Calculate the pH of the final solution.

Water ionises according to the balanced equation: 2H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq). The equilibrium constant for the ionisation of water, Kw, varies with temperature: Kw = 1,00 × 10⁻¹⁴ at 25 °C, rising to Kw = 5,48 × 10⁻¹⁴ at 50 °C. Is the ionisation of water EXOTHERMIC or ENDOTHERMIC? Explain the answer. Calculate the concentration of H₃O⁺ ions in water at 50 °C. How will the pH of water be affected when the temperature increases? Choose from INCREASES, DECREASES or STAYS THE SAME, and give a reason by referring to the formula used to calculate pH. Water is an ampholyte. Write down the balanced equation for the reaction of water with HCO₃⁻(aq) where water acts as an acid, and where water acts as a base. A table shows the equilibrium constants, Ka, of four acids (including acetic acid, CH₃COOH, with Ka = 1,8 × 10⁻⁵), each at a concentration of 0,1 mol·dm⁻³ at 25 °C, used as electrolytes. Define the term electrolyte. Which ONE of the acids is the most effective electrolyte? Give a reason for the answer.

According to the Arrhenius theory, an acid is: A. a source of H⁺ ions in water. B. a source of OH⁻ ions in water. C. a substance that can accept a proton. D. a substance that can donate a proton. A property of sodium nitrate solution, NaNO₃(aq), is that it is: A. acidic. B. basic. C. ampholytic. D. neutral. An iron rod, Fe(s), is placed in a beaker containing chromium(III) ions, Cr³⁺(aq). Which ONE of the following is the CORRECT combination of products formed? A. Fe²⁺(aq) and Cr(s). B. Fe²⁺(aq) and Cr²⁺(aq). C. Fe³⁺(aq) and Cr²⁺(aq). D. Fe³⁺(aq) and Cr(aq).

The expression for the equilibrium constant (Kc) of a hypothetical reaction is given as Kc = [PQ₃]² / ([P₂][Q₂]³). Which of the following equations for a reaction at equilibrium matches this expression? A. P₂(g) + 3Q₂(g) ⇌ 2PQ₃(aq). B. 2PQ₃(l) ⇌ P₂(g) + 3Q₂(g). C. 2PQ₃(aq) ⇌ P₂(g) + 3Q₂(g). D. P₂(g) + 3Q₂(g) ⇌ 2PQ₃(g). Which statement best describes the difference between the endpoint and the equivalence point in a titration? A. The endpoint occurs when the acid or base has completely reacted with each other, while the equivalence point is when the indicator changes colour. B. The equivalence point occurs when the acid or base has completely reacted with each other, while the endpoint is when the indicator changes colour. C. The endpoint and equivalence point occur at different times and have no connection in a titration. D. The equivalence point and endpoint are always exactly the same in every titration.

Consider the energy profile graph for a REVERSIBLE REACTION shown below. The following statements are given: (i) a − c represents the activation energy for the reverse reaction; (ii) ΔH for the reverse reaction is a − b; (iii) a catalyst lowers the activation energy for both forward and reverse reactions. Identify the CORRECT statement(s): A. (ii) and (iii) only. B. (iii) only. C. (i), (ii) and (iii). D. (i) only. The reaction of an acid-base indicator (HIn), represented as HIn(aq), with H₂O(l), reaches equilibrium: HIn(aq) + H₂O(l) ⇌ H₃O⁺(aq) + In⁻(aq), ΔH < 0. Which of the following will change the colour of the solution from red to yellow? A. Increasing the concentration of H₃O⁺ ions. B. Increasing the temperature of the solution. C. Adding a base. D. Adding an acid.