7. Equilibria
A section of Chemistry, 9701
Listing 10 of 344 questions
Ammonium chloride, NH4Cl , dissolves in water to form an acidic solution. This is due to the dissociation of the ammonium ions. NH4 +H++ NH3The ammonium ion is a weak acid. The pH of a 0.300 mol dm–3 solution of ammonium chloride is 4.89 under standard conditions. Calculate the [H+] in a 0.300 mol dm–3 solution of ammonium chloride.  [H+] = mol dm–3 Calculate the value of pKa of the ammonium ion.  pKa = A buffer solution can be made by mixing ammonium chloride with ammonia solution. Explain, with the aid of an equation, how this solution can behave as a buffer when a small amount of a strong acid is added. Explain, with the aid of an equation, how this solution can behave as a buffer when a small amount of a strong base is added. Use the value of Kw to calculate [H+] in pure water under standard conditions. Show your working.  [H+] = mol dm–3 The pH of pure water at 50 °C is 6.64. Calculate the numerical value of Kw at 50 °C.  Kw = 
9701_w19_qp_42
THEORY
2019
Paper 4, Variant 2
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Ethoxyethane, C2H5OC2H5, can dissolve both in water and in octan-1-ol. The expression and numerical value for the partition coefficient of ethoxyethane between water and octan-1-ol are given. Water and octan-1-ol are immiscible. Kpc = concentration of C2H5OC2H5 in octan-1-ol = 6.760 at 20 °C concentration of C2H5OC2H5 in water In an experiment, octan-1-ol at 20 °C is added to a solution of ethoxyethane in water at 20 °C. The mixture is analysed immediately and a value of Kpc is calculated. The calculation is performed correctly; the value calculated is 5.625. Explain why the value calculated is less than 6.760. A second experiment is performed and the value of Kpc is found to be 6.760. The concentration of ethoxyethane in the octan-1-ol layer is 7.62 g dm–3. Calculate the concentration, in g dm–3, of ethoxyethane in the aqueous layer.  g dm–3 100 cm3 of the octan-1-ol layer is taken and shaken with 100 cm3 of water. Calculate the maximum amount, in mol, of ethoxyethane that can be extracted into the water.  mol An aqueous solution of lead(nitrate is mixed with an aqueous solution of sodiumiodide. A yellow precipitate of lead(iodide is formed and is filtered out, leaving solutionX. The concentration of Pb2+ in solutionX is 5.68 × 10–3 mol dm–3. The concentration of I– in solutionX is 4.20 × 10–4 mol dm–3. Use these data to calculate a value for the solubility product, Ksp, of lead(iodide. State the units of Ksp.  Ksp =  units =  Potassiumiodide is very soluble in water. Describe and explain what is seen if a few drops of saturated potassiumiodide solution are added to a portion of solutionX.
9701_w21_qp_42
THEORY
2021
Paper 4, Variant 2
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The value of the solubility product, Ksp, of iron(hydroxide, Fe(OH)3, is given by the following expression. Ksp = [Fe3+][OH–]3 = 2.0 × 10–39 mol4 dm–12 Calculate the solubility of Fe(OH)3 in water.  solubility = mol dm–3 Calculate the solubility of Fe(OH)3 in 0.010 mol dm–3 bariumhydroxide, Ba(OH)2.  solubility = mol dm–3 Fe(OH)3 is less soluble in Ba(OH)2than it is in pure water. Name this effect. The numerical value of the Ka of HBrO is 2.00 × 10–9. X is a solution of HBrO which contains 4.00 × 10–3 mol of HBrO in 100 cm3 of solution. In this solution the following equilibrium is established in which there are two conjugate acid-base pairs. HBrO + H2O BrO– + H3O+ Define conjugate acid-base pair. Identify the two conjugate acid-base pairs shown in the equation above. pair one acid base pair two acid base  Calculate the pH of solution X. Show all your working.  pH = A solution containing 2.00 × 10–3 mol of NaOH is added to solution X. A buffer solution is formed. Calculate the pH of this buffer solution.  pH = 
9701_w22_qp_42
THEORY
2022
Paper 4, Variant 2
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Water is an amphoteric compound that also acts as a good solvent of polar and ionic compounds. Equation 1 shows water acting as a Brønsted–Lowry acid. equation 1 H2O + NO2 – HNO2 + OH– Identify the two conjugate acid–base pairs in equation 1. acid I H2O conjugate base of acid I acid II conjugate base of acid II Water also behaves as a Brønsted–Lowry acid when it dissolves CH3NH2. Explain the ability of CH3NH2 to act as a base. Write an equation to show water acting as a base with CH3COOH. The ionic product of water, Kw, measures the extent to which water dissociates. H2OH++ OH–shows how Kw varies with temperature. 0.00 1.00 2.00 3.00 4.00 5.00 6.00 temperature / °C Kw / 10–14 mol2 dm–6 Write an expression for Kw. Use information from to deduce whether the dissociation of water is an exothermic or an endothermic process. Explain your answer. An aqueous solution has pH = 7.00 at 30 °C. Use information from to explain why this solution can be considered to be alkaline at 30 °C. The three physical states of H2O have different standard entropies, S o , associated with them. Table 2.1 shows these S o values. Table 2.1 state of H2O standard entropy, S o / J K–1 mol–1 solid +48.0 liquid +70.1 gas +188.7 Explain the difference in the S o values of H2Oand H2O. Explain why the increase in S o is much greater when H2O boils than when it melts. The energy changes for H2O→ H2Oare shown. ΔG = 0.00 kJ mol–1 ΔH = +6.03 kJ mol–1 Use these data to show that the melting point of H2Ois 0 °C. Metal–air batteries are electrochemical cells that generate electrical energy from the reaction of metal anodes with air. The standard electrode potentials for the zinc–air battery are shown. [Zn(OH)4]2– + 2e– Zn + 4OH– E o = –1.22 V 2 O2 + H2O + 2e– 2OH– E o = +0.40 V Calculate the standard cell potential, E o cell, of the zinc–air battery. E o cell = V The zinc–air battery usually operates at pH 11 and 298 K. The overall cell potential is dependent on [OH–]. The Nernst equation shows how the electrode potential at the cathode changes with [OH–]. E = 0.40 – (0.059 z ) log([OH–]2) Calculate the electrode potential, E, at pH 11. E = V
9701_m24_qp_42
THEORY
2024
Paper 4, Variant 2
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Questions Discovered
344