7. Equilibria
A section of Chemistry, 9701
Listing 10 of 344 questions
Define Kw mathematically by completing the expression. Kw = Two solutions, V and W, are described. • V is HCl . • W is NaOH. • The concentration of HCl in V is the same as the concentration of NaOH in W. • The pH values of V and W differ by exactly 11.00 at 298 K. Calculate the concentration of HCl in V. concentration of HCl in V = mol dm−3 Equal volumes of the two solutions V and W are mixed, giving solution X. Name solution X and state its pH. solution X pH A 1 cm3 sample of 1.0 mol dm−3 HNO3 is added to 100 cm3 of solution X, forming mixture Y. A 1 cm3 sample of 1.0 mol dm−3 KOH is added to 100 cm3 of solution X, forming mixture Z. Estimate the pH of mixtures Y and Z. No calculations are required. mixture Y mixture Z CH3CH2COOH, CH3CCl2COOH and H2SO4 are all acidic. Suggest the trend in the relative acid strength of these three compounds. Explain your answer. strongest acid weakest acid explanation When concentrated H2SO4 is added to water a series of acid-base reactions occurs. There are three conjugate acid-base pairs that can be identified during this series of reactions. Write the formulae of these three conjugate acid-base pairs. conjugate acid 1 conjugate base 1 conjugate acid 2 conjugate base 2 conjugate acid 3 conjugate base 3 The partition coefficient, Kpc, of a substance, Q, between hexane and water is 7.84 at 298 K. Q is more soluble in hexane than it is in water. Define partition coefficient, Kpc. 5.00 g of Q is shaken with a mixture of 100.0 cm3 of water and 100.0 cm3 of hexane at 298 K and left until there is no further change in concentrations. Calculate the mass of Q dissolved in the water. mass of Q = g A sample of Q is shaken with a different mixture of water and hexane and left until there is no further change in concentrations. It is found that the mass of Q dissolved in each solvent is the same. Use the Kpc value to suggest possible values for the volume of water used and the volume of hexane used. volume of water = cm3 volume of hexane = cm3 Q is more soluble in hexane than it is in water. It is suggested that Q is one of KCl, CH3(CH2)4OH or HCOOH. Identify Q. Explain your answer.
9701_w23_qp_41
THEORY
2023
Paper 4, Variant 1
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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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Define Kw mathematically by completing the expression. Kw = Two solutions, V and W, are described. • V is HCl . • W is NaOH. • The concentration of HCl in V is the same as the concentration of NaOH in W. • The pH values of V and W differ by exactly 11.00 at 298 K. Calculate the concentration of HCl in V. concentration of HCl in V = mol dm−3 Equal volumes of the two solutions V and W are mixed, giving solution X. Name solution X and state its pH. solution X pH A 1 cm3 sample of 1.0 mol dm−3 HNO3 is added to 100 cm3 of solution X, forming mixture Y. A 1 cm3 sample of 1.0 mol dm−3 KOH is added to 100 cm3 of solution X, forming mixture Z. Estimate the pH of mixtures Y and Z. No calculations are required. mixture Y mixture Z CH3CH2COOH, CH3CCl2COOH and H2SO4 are all acidic. Suggest the trend in the relative acid strength of these three compounds. Explain your answer. strongest acid weakest acid explanation When concentrated H2SO4 is added to water a series of acid-base reactions occurs. There are three conjugate acid-base pairs that can be identified during this series of reactions. Write the formulae of these three conjugate acid-base pairs. conjugate acid 1 conjugate base 1 conjugate acid 2 conjugate base 2 conjugate acid 3 conjugate base 3 The partition coefficient, Kpc, of a substance, Q, between hexane and water is 7.84 at 298 K. Q is more soluble in hexane than it is in water. Define partition coefficient, Kpc. 5.00 g of Q is shaken with a mixture of 100.0 cm3 of water and 100.0 cm3 of hexane at 298 K and left until there is no further change in concentrations. Calculate the mass of Q dissolved in the water. mass of Q = g A sample of Q is shaken with a different mixture of water and hexane and left until there is no further change in concentrations. It is found that the mass of Q dissolved in each solvent is the same. Use the Kpc value to suggest possible values for the volume of water used and the volume of hexane used. volume of water = cm3 volume of hexane = cm3 Q is more soluble in hexane than it is in water. It is suggested that Q is one of KCl, CH3(CH2)4OH or HCOOH. Identify Q. Explain your answer.
9701_w23_qp_43
THEORY
2023
Paper 4, Variant 3
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Questions Discovered
344