7.2. Brønsted–Lowry theory of acids and bases
A subsection of Chemistry, 9701, through 7. Equilibria
Listing 10 of 133 questions
Silver sulfide, Ag2S, is very insoluble in water. Write an expression for the solubility product, Ksp, of Ag2S. Ksp =  The solubility of Ag2Sin water at 298 K is 1.16 × 10–17 mol dm–3. Calculate the numerical value of the solubility product, Ksp, of Ag2Sat 298 K.  Ksp = Calculate the minimum volume of water needed to dissolve 1.00 g of Ag2Sunder standard conditions.  volume = dm3 Bromic(acid, HOBr, is a weak acid. Its Ka is 2.0 × 10–9 mol dm–3. Calculate the pH of 0.20 mol dm–3 HOBr.  pH = 5.0 cm3 of 0.20 mol dm–3 potassium hydroxide, KOH, are added to 20.0 cm3 of 0.20 mol dm–3 HOBr. Calculate the pH of the buffer solution produced.  pH = 
9701_w19_qp_43
THEORY
2019
Paper 4, Variant 3
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Benzoic acid, C6H5COOH, is a weak acid. The Ka of benzoic acid is 6.31 × 10–5 mol dm–3 at 298 K. A 1.00 dm3 buffer solution is made at 298 K containing 1.00 g of C6H5COOH and a slightly greater mass of sodium benzoate, C6H5COO–Na+. This buffer solution has a pH of 4.15. Define buffer solution. Write equations to show how this solution acts as a buffer solution when the named substances are added to it: dilute aqueous sodium hydroxide dilute aqueous nitric acid. Calculate the H+ concentration and the C6H5COOH concentration in the buffer solution described. Use the expression for the Ka of C6H5COOH to calculate the concentration of C6H5COO–Na+ in the buffer solution. Show your working and give each answer to a minimum of three significant figures. [H+] = mol dm–3 = mol dm–3 [C6H5COO–Na+] = mol dm–3 A 10.0 cm3 sample of the buffer solution is mixed with 10.0 cm3 of 1.00 mol dm–3 KOH. Both solutions are at 298 K. A reaction is allowed to occur without stirring. Two observations are recorded: ● the temperature, after the reaction is complete, is fractionally above 298 K ● the pH, after the reaction, is greater than 13. Explain these two observations. Magnesium benzoate, Mg(C6H5COO)2, has a solubility in water of less than 1.00 g dm–3 at 298 K. Ksp = [Mg2+][C6H5COO–]2 = 1.76 × 10–7 at 298 K Calculate the solubility of Mg(C6H5COO)2 in water at 298 K. Give your answer in g dm–3. Show your working. [Mr: Mg(C6H5COO)2 , 266.3] solubility = g dm–3 An excess of Mg(C6H5COO)2 is added to a sample of 0.50 mol dm–3 MgSO4 at 298 K. State whether the equilibrium concentration of Mg(C6H5COO)2 is higher than, the same as, or lower than your answer to . Explain your answer. The concentration is the concentration in . explanation
9701_w23_qp_42
THEORY
2023
Paper 4, Variant 2
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A student uses thin-layer chromatography (TLC) to analyse a mixture containing different metal cations. The student repeats the experiment using different solvents. shows the chromatogram obtained by the student using water as a solvent. solvent front cm baseline M Suggest a compound that could be used as the stationary phase in this experiment. Table 6.1 shows the Rf values for different metal cations when separated by TLC using water as a solvent. Table 6.1 cation Rf value Cd2+0.40 Co2+0.77 Cu2+0.32 Fe3+0.12 Hg2+0.23 Ni2+0.75 Suggest the identity of the cation that causes the spot at M in . Explain your answer. The student repeats the experiment using butan-1-ol as a solvent. The metal cations do not travel as far up the TLC plate in this experiment. Suggest why the metal cations do not move as far up the TLC plate with butan-1-ol as a solvent. The student sprays the TLC plate in with KSCN. The colour of some of the spots changes, as some of the metal cations undergo a ligand exchange reaction. Identify the ligands involved in the ligand exchange reaction. exchanges with In a third experiment, the pH of the mixture of metal ions is kept constant using a buffer solution. The student prepares the buffer solution by mixing 20.0 cm3 of 0.150 mol dm–3 KOHand 50.0 cm3 of 0.100 mol dm–3 C8H5O4K. C8H5O4K is a weak carboxylic acid that has pKa = 5.40. OH O–K+ O O C8H5O4K Complete the equation for the reaction of C8H5O4Kwith KOH. C8H5O4K + Calculate the pH of the buffer solution. Show all your working. pH =
9701_m23_qp_42
THEORY
2023
Paper 4, Variant 2
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The sketch graph for the titration of ethanoicacid, CH3CO2H, with sodiumhydroxide is shown. volume of sodium hydroxide / cm3 pH equivalence point In the region circled on the graph, identify the two organic species that are present in the solution. Explain why the pH of the mixture only changes slowly and gradually in this region when sodium hydroxide is being added. two species present The equivalence point in this acid-base titration is where the two solutions have been mixed in exactly equal molar proportion. Suggest why the pH is greater than 7 at the equivalence point in this titration. An impure sample of ammoniumvanadate(, NH4VO3, with mass 0.150 g, is dissolved in an excess of dilute acid. In this solution all vanadium is present as VO2 + ions. An excess of zinc powder is added to the solution and all the VO2 + ions are reduced to V2+ ions. The mixture is filtered to remove any remaining zinc powder. VO2 + + 4H+ + 3e– → V2+ + 2H2O When the resulting solution is titrated, 20.10 cm3 of 0.0250 mol dm–3 acidified MnO4 – oxidises all V2+ ions back to VO2 + ions. MnO4 – + 8H+ + 5e– Mn2+ + 4H2O Calculate the percentage by mass of NH4VO3 in the 0.150 g impure sample of NH4VO3. Give your answer to three significant figures. [Mr: NH4VO3, 116.9]  percentage by mass of NH4VO3 = % 
9701_s21_qp_41
THEORY
2021
Paper 4, Variant 1
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The sketch graph for the titration of ethanoicacid, CH3CO2H, with sodiumhydroxide is shown. volume of sodium hydroxide / cm3 pH equivalence point In the region circled on the graph, identify the two organic species that are present in the solution. Explain why the pH of the mixture only changes slowly and gradually in this region when sodium hydroxide is being added. two species present The equivalence point in this acid-base titration is where the two solutions have been mixed in exactly equal molar proportion. Suggest why the pH is greater than 7 at the equivalence point in this titration. An impure sample of ammoniumvanadate(, NH4VO3, with mass 0.150 g, is dissolved in an excess of dilute acid. In this solution all vanadium is present as VO2 + ions. An excess of zinc powder is added to the solution and all the VO2 + ions are reduced to V2+ ions. The mixture is filtered to remove any remaining zinc powder. VO2 + + 4H+ + 3e– → V2+ + 2H2O When the resulting solution is titrated, 20.10 cm3 of 0.0250 mol dm–3 acidified MnO4 – oxidises all V2+ ions back to VO2 + ions. MnO4 – + 8H+ + 5e– Mn2+ + 4H2O Calculate the percentage by mass of NH4VO3 in the 0.150 g impure sample of NH4VO3. Give your answer to three significant figures. [Mr: NH4VO3, 116.9]  percentage by mass of NH4VO3 = % 
9701_s21_qp_43
THEORY
2021
Paper 4, Variant 3
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Complete the following electronic confi guration of the Cu2+ ion. 1s2 2s2 2p6 In a free, gas-phase transition metal ion, the d-orbitals all have the same energy, but when the ion is in a complex the orbitals are split into two energy levels. Explain why this happens. How does this splitting help to explain why transition metal complexes are often coloured? Why does the colour of a transition metal complex depend on the nature of the ligands surrounding the transition metal ion? Draw a fully-labelled diagram of the apparatus you could use to measure the E o of a cell composed of the Fe3+/Fe2+ electrode and the Cu2+/Cu electrode. The E o for Cu2+/Cu is +0.34 V. When NH3is added to the electrode solution, the Eelectrode changes. Describe the type of reaction taking place between Cu2+and NH3. Write an equation for the reaction. Describe the change in the colour of the solution. Predict and explain how the Eelectrode might change on the addition of NH3. Fehling’s reagent is an alkaline solution of Cu2+ ions complexed with tartrate ions. It is used in organic chemistry to test for a particular functional group. Name the functional group involved. Describe the appearance of a positive result in this test. Write an equation for the reaction between Cu2+ and OH– ions and a two-carbon compound containing the functional group you named in . A solution containing a mixture of tartaric acid and its sodium salt is used as a buffer in some pre-prepared food dishes. Calculate the pH of a solution containing 0.50 mol dm–3 of tartaric acid and 0.80 mol dm–3 sodium tartrate. [Ka(tartaric acid) = 9.3 × 10–4 mol dm–3] pH =
9701_w12_qp_43
THEORY
2012
Paper 4, Variant 3
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Questions Discovered
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