2. Atoms, molecules and stoichiometry
A section of Chemistry, 9701
Listing 10 of 208 questions
Transition metal atoms and transition metal ions form complexes by combining with ligands. Explain why transition elements form complex ions. Co2+ ions form complex ion G. Each G ion contains two Co2+ ions, both of which are octahedrally coordinated. Each G ion contains one O2 molecule, which donates one pair of electrons to each Co2+ ion, and one NH2 – ion, which donates one pair of electrons to each Co2+ ion. The remaining ligands are NH3 molecules. Deduce the formula of complex ion G. Include its overall charge. formula of G The d-orbitals of the Co2+ ions present in complex ion G are split. State the number of d-orbitals that are at a higher energy level and the number of d-orbitals that are at a lower energy level in each Co2+ ion. number of d-orbitals at a higher energy level number of d-orbitals at a lower energy level Co2+ ions form a different complex ion, M. Each M ion contains two Co2+ ions, both of which are octahedrally coordinated, but the ligands are different from the ligands in G. Explain why G and M have different colours. Cadmium forms complex ion X, [Cd(NH3)4]2+. When a solution containing CN– ions is added to an aqueous solution of X, a ligand exchange reaction takes place, forming complex ion Y. Y contains no NH3 ligands and no H2O ligands. Y is in a much higher concentration in the mixture than X. The oxidation state and coordination number of cadmium do not change in this reaction. Write an ionic equation for this reaction, using the formulae of the complex ions. Cadmium forms complex ion Z in the same oxidation state and with the same coordination number as in X. All the ligands in Z are Cl – ions. When NaCl is added to a solution of X, very little Z forms. Write the three cadmium complexes, X, Y and Z, in order of increasing stability constant, Kstab. smallest value of Kstab largest value of Kstab Ethanedioate ions, C2O4 2–, form complexes with transition element ions. The concentration of C2O4 2– ions can be found by reaction with acidified Cr2O7 2– ions. C2O4 2– ions are protonated and form HOOCCOOH molecules which are oxidised by Cr2O7 2–. The half-equations are shown. Cr2O7 2– + 14H+ + 6e– 2Cr3+ + 7H2O 2CO2 + 2H+ + 2e– HOOCCOOH Construct an equation for the reaction between acidified Cr2O7 2– and HOOCCOOH. A 25.0 cm3 sample of a solution of Na2C2O4 reacts with exactly 16.20 cm3 of an acidified solution of 0.0500 mol dm–3 K2Cr2O7. Calculate the concentration of the solution of Na2C2O4. = mol dm–3
9701_w24_qp_43
THEORY
2024
Paper 4, Variant 3
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An aqueous solution of cobalt(chloride is a pink colour due to the presence of [Co(H2O)6]2+ complex ions. Complete Table6.1 to describe what is observed when the named reagent is added to an aqueous solution of cobalt(chloride. Table 6.1 reagent colour of cobalt-containing product state of cobalt-containing product NaOHan excess of conc. HCl  Write an equation for the reaction between [Co(H2O)6]2+ ions and NaOH. Write an equation for the reaction between [Co(H2O)6]2+ ions and an excess of conc.HCl. Define stability constant. Write an expression for the stability constant, Kstab, of the [Co(NH3)6]2+ complex ion. Kstab =  Give the units of the stability constant, Kstab, of the [Co(NH3)6]2+ complex ion.  units = The numerical value of the stability constant, Kstab, of the [Co(NH3)6]2+ complex ion is 7.7×104. In an aqueous solution the concentration of the [Co(NH3)6]2+ complex ion is 0.0740 mol dm–3 and the concentration of NH3 is 0.480 mol dm–3 at equilibrium. Calculate the concentration of [Co(H2O)6]2+ in this solution.  concentration = mol dm–3 
9701_w22_qp_42
THEORY
2022
Paper 4, Variant 2
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Combustion data can be used to calculate the empirical formula, molecular formula and relative molecular mass of many organic compounds. Combustion data cannot distinguish between different structural isomers. Define the term structural isomers. P is a hydrocarbon, CxHy. A gaseous sample of P occupied a volume of 25 cm3 at 37 °C and 100 kPa. The sample was completely burned in 200 cm3 of oxygen (an excess). The final volume, measured under the same conditions as the gaseous sample (so that the water produced is liquid and its volume can be ignored), was 150 cm3. Treating the remaining gaseous mixture with concentrated alkali, to absorb carbondioxide, decreased the volume to 50 cm3. The equation for the complete combustion of P can be represented as shown. CxHy + (x + y 4)O2 xCO2 + y 2H2O Use the data given to calculate the value of x. x = Use the data given to calculate the value of (x + y 4). (x + y 4) = If you were unable to calculate values in and then use the data in this box for the remaining parts of this question. These are not the correct values. x = 6 (x + y 4) = 9 Give the molecular formula and the empirical formula of P. molecular formula of P empirical formula of P P is unbranched. Give the skeletal formulae for two possible structures of P that are positional isomers of each other. Use the general gas equation to calculate the mass of P present in the original 25 cm3 gaseous sample, which was measured at 37 °C and 100 kPa. Give your answer to three significant figures. mass = g
9701_s17_qp_23
THEORY
2017
Paper 2, Variant 3
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Questions Discovered
208