2.3. Formulas
A subsection of Chemistry, 9701, through 2. Atoms, molecules and stoichiometry
Listing 10 of 88 questions
An electrochemical cell consists of a half-cell containing V3+and V2+ions and another half‑cell containing VO2 +and VO2+ions. Use data from the Data Booklet to calculate a value for the . = V Write the ionic equation for the cell reaction. Draw a fully labelled diagram of the apparatus you could use to measure the potential of this cell. Include the necessary chemicals. Use data from the Data Booklet to predict whether a reaction might take place when the following pairs of aqueous solutions are mixed. If a reaction occurs, write an equation for it and calculate the . ● V2+and Sn4+Does a reaction occur? equation ● VO2+and Fe3+Does a reaction occur? equation
9701_w17_qp_42
THEORY
2017
Paper 4, Variant 2
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Complete the following electronic structures. ● the iron atom, Fe 1s22s22p6 ● the iron(ion, Fe3+ 1s22s22p6 Solutions of iron(salts are acidic due to the equilibrium shown. [Fe(H2O)6]3+[Fe(H2O)5(OH)]2++ H+ Ka = 8.9 × 10–4 mol dm–3 Calculate the pH of a 0.25 mol dm–3 FeCl 3 solution. pH = The table shows numerical values of the stability constants for the following equilibrium where M can be one of the metal ions listed and L one of the ligands which replaces one H2O molecule. [M(H2O)6]n++ L–[M(H2O)5L](n–1)++ H2Ometal ion, M ligand, L stability constant, Kstab Fe3+ F– 1.0 × 106 Fe3+ Cl – 2.5 × 101 Fe3+ SCN– 9.0 × 102 Hg2+ Cl – 5.0 × 106 What is meant by the term stability constant, Kstab? Use the data in the table to predict the formula of the complex formed in the greatest amount when ● a solution containing equal concentrations of both F– and SCN– ions is added to Fe3+, ● a solution containing equal concentrations of both Fe3+ and Hg2+ ions is added to Cl –. Ethanedioate ions, –O2CCO2 –, are bidentate ligands. The abbreviation ed 2– can be used to represent ethanedioate ions. The complex [Fe(ed )2Cl 2]3– can be formed according to the equation shown. [Fe(H2O)4Cl 2]++ 2ed 2–[Fe(ed )2Cl 2]3–+ 4H2OWrite the expression for the equilibrium constant, Kstab, and state its units. Kstab = units [Fe(ed )2Cl 2]3– shows geometrical and optical isomerism. Complete the three-dimensional diagrams to show the three stereoisomers of [Fe(ed )2Cl 2]3–. You may use –O O– to represent ed 2–. Fe isomer A Fe isomer B Fe isomer C Give the letters of two isomers of [Fe(ed )2Cl 2]3– which are geometrical isomers of each other. Give the letters of the two isomers of [Fe(ed )2Cl 2]3– which show optical isomerism. Give the letter of the isomer which has no dipole moment.
9701_w17_qp_43
THEORY
2017
Paper 4, Variant 3
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Ethanedioate ions, C2O4 2–, are bidentate ligands. Explain what is meant by the term ligand. Cr3+and C2O4 2–ions form the complex ion [Cr(C2O4)2(H2O)2]–. Draw two stereoisomers of this complex ion. You may use O O to represent C2O4 2–. Cr Cr  The solubility of calcium ethanedioate, CaC2O4, is 6.65×10–3 g dm–3 at 298 K. Write an expression for the solubility product, Ksp, of CaC2O4. Include its units. Ksp =  units =  Calculate the numerical value of Ksp CaC2O4 at 298 K. Give your answer in standard form to two significant figures.  Ksp CaC2O4 = 
9701_w18_qp_41
THEORY
2018
Paper 4, Variant 1
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Ethanedioate ions, C2O4 2–, are bidentate ligands. Explain what is meant by the term ligand. Cr3+and C2O4 2–ions form the complex ion [Cr(C2O4)2(H2O)2]–. Draw two stereoisomers of this complex ion. You may use O O to represent C2O4 2–. Cr Cr  The solubility of calcium ethanedioate, CaC2O4, is 6.65×10–3 g dm–3 at 298 K. Write an expression for the solubility product, Ksp, of CaC2O4. Include its units. Ksp =  units =  Calculate the numerical value of Ksp CaC2O4 at 298 K. Give your answer in standard form to two significant figures.  Ksp CaC2O4 = 
9701_w18_qp_43
THEORY
2018
Paper 4, Variant 3
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An electrochemical cell is constructed using two half-cells. ● ● a Br2/Br – half-cell ● ● an Mn3+/Mn2+ half-cell State the material used for the electrode in each half-cell. Br2/Br – half-cell Mn3+/Mn2+ half-cell  The cell is operated at 298 K. The Br2/Br – half-cell has standard concentrations. The Mn3+/Mn2+ half-cell has [Mn3+] = 0.500 mol dm–3 and [Mn2+] = 0.100 mol dm–3. Use the Nernst equation to calculate the electrode potential, E, of the Mn3+/Mn2+ half‑cell under these conditions.  E = V Calculate the Ecell under these conditions.  Ecell = V Write an equation for the overall cell reaction that occurs. An aqueous solution of copper(sulfate is electrolysed using copper electrodes. A current of 1.50 A is passed for 3.00hours. 5.09 g of copper is deposited on the cathode. The charge on one electron is –1.60×10–19 C. The relative atomic mass of copper is 63.5. Use these data to calculate an experimentally determined value for the Avogadro constant, L. Give your answer to three significant figures.  L = mol–1 Explain why magnesium metal cannot be obtained by the electrolysis of dilute aqueous magnesium sulfate. Your answer should include data from the Data Booklet. 
9701_w19_qp_42
THEORY
2019
Paper 4, Variant 2
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Many copper compounds, such as CuSO4 and Cu(NO3)2 contain Cu2+ ions. Aqueous solutions of this ion contain the [Cu(H2O)6]2+ complex ion, in which water behaves as a monodentate ligand. Explain what is meant by the term monodentate ligand. Copper(sulfate solution, CuSO4, undergoes visible changes with certain reagents. Complete the table below. reagent added to a solution of CuSO4observations formula of the copper(compound or complex ion that is formed a few drops of NH3an excess of NH3an excess of NaOHan excess of concentrated HCl  When water is added to concentrated aqueous cobalt(chloride the colour of the solution changes from blue to pink. Explain this observation. No equation is needed, but you should include reference to electron movement between orbitals in your answer. When chlorine gas is bubbled into FeSO4the colour of the solution changes from pale green to yellow. Use data from the Data Booklet to explain this observation. Include an equation in your answer. Reference to electron movement between orbitals is not needed. If a solution of chloride ions is added to a solution containing [Fe(H2O)6]3+ an equilibrium is established. [Fe(H2O)6]3+ + 4Cl – [FeCl 4]– + 6H2O Write an expression for the stability constant of [FeCl 4]–, Kstab. Kstab =  For the above equilibrium the numerical value of Kstab = 0.080. Calculate the concentration of [Fe(H2O)6]3+ in a solution in which the concentration of Cl – is 2.0 mol dm–3 and the concentration of [FeCl 4]– is 0.10 mol dm–3.  concentration of [Fe(H2O)6]3+ = mol dm–3 
9701_w19_qp_42
THEORY
2019
Paper 4, Variant 2
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