2. Atoms, molecules and stoichiometry
A section of Chemistry, 9701
Listing 10 of 208 questions
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
View
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
View
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
View
[MnCl 4]2– is a complex ion. Deduce the oxidation state of manganese in [MnCl 4]2–.  oxidation state = The [MnCl 4]2– complex does not contain any 180° bond angles. Draw a three-dimensional diagram to show the shape of the [MnCl 4]2– complex. State one bond angle on your diagram. Mn  A solution of cobalt(sulfate contains the complex ion [Co(H2O)6]2+. A solution containing [Co(H2O)6]2+ is reacted separately with an excess of each of NaOH, NH3and NaCl . Write an equation for each of these reactions. State one observation that can be made immediately after the reaction, include the colour and state of the cobalt-containing product. [Co(H2O)6]2+ and an excess of NaOHequation observation  [Co(H2O)6]2+ and an excess of NH3equation observation  [Co(H2O)6]2+ and an excess of NaCl equation observation  Name the type of reaction that occurs in . Cobalt forms the complex ion [Co(NH3)22]2+. The abbreviation en is used for the bidentate ligand 1,2-diaminoethane, H2NCH2CH2NH2. The complex ion shows both geometrical and optical isomerism. Define the term bidentate ligand. Draw three-dimensional diagrams for the two optical isomers of [Co(NH3)22]2+. Each en ligand can be represented using N N . Co Co  
9701_w21_qp_41
THEORY
2021
Paper 4, Variant 1
View
Transition elements form complexes. Molybdenum, Mo, forms an octahedral complex consisting of one Mo atom surrounded by carbon monoxide, CO, molecules. CO is a monodentate ligand. Iron forms an octahedral complex consisting of one Fe3+ and a number of cyanide, CN–, ions. CN– is a monodentate ligand. Define the term monodentate ligand. Complete the table by stating the formulae and charges of the complexes described. formula charge molybdenum complex iron(complex  Draw a three-dimensional diagram to show the shape of this iron(complex. Label one 180° bond angle on your diagram. Fe  An excess of aqueous ammonia is added to dilute copper(sulfate solution. A dark blue complex, [Cu(NH3)4(H2O)2]2+, is formed. Write an ionic equation for this reaction. Explain the origin of colour in copper(complexes. An excess of concentrated hydrochloric acid is added to the dark blue solution of [Cu(NH3)4(H2O)2]2+. A new complex, Z, is formed. The colour of the solution changes. Write an equation for the formation of Z from the solution of [Cu(NH3)4(H2O)2]2+. Include the formula and charge of Z. Name the type of reaction when Z forms from [Cu(NH3)4(H2O)2]2+. State the geometry of Z. State the colour of a solution of Z. Explain why the colour of a solution of [Cu(NH3)4(H2O)2]2+ is different from the colour of a solution of Z. You should refer to the energies of the orbitals involved in your answer. 
9701_w21_qp_42
THEORY
2021
Paper 4, Variant 2
View
Questions Discovered
208