2.3. Formulas
A subsection of Chemistry, 9701, through 2. Atoms, molecules and stoichiometry
Listing 10 of 88 questions
Compound A is an organic compound which contains carbon, hydrogen and oxygen. When 0.240 g of the vapour of A is slowly passed over a large quantity of heated copper(oxide, CuO, the organic compound A is completely oxidised to carbon dioxide and water. Copper is the only other product of the reaction. The products are collected and it is found that 0.352 g of CO2 and 0.144 g of H2O are formed. In this section, give your answers to three decimal places. Calculate the mass of carbon present in 0.352 g of CO2. Use this value to calculate the amount, in moles, of carbon atoms present in 0.240 g of A. Calculate the mass of hydrogen present in 0.144 g of H2O. Use this value to calculate the amount, in moles, of hydrogen atoms present in 0.240 g of A. Use your answers to calculate the mass of oxygen present in 0.240 g of A. Use this value to calculate the amount, in moles, of oxygen atoms present in 0.240 g of A. Use your answers to to calculate the empirical formula of A. When a 0.148 g sample of A was vapourised at 60oC, the vapour occupied a volume of 67.7 cm3 at a pressure of 101 kPa. Use the general gas equation pV = nRT to calculate Mr of A. Mr = Hence calculate the molecular formula of A. Compound A is a liquid which does not react with 2,4-dinitrophenylhydrazine reagent or with aqueous bromine. Suggest two structural formulae for A. Compound A contains only carbon, hydrogen and oxygen. Explain how the information on the opposite page about the reaction of A with CuO confirms this statement.
9701_w11_qp_22
THEORY
2011
Paper 2, Variant 2
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Zinc is an essential trace element which is necessary for the healthy growth of animals and plants. Zinc defi ciency in humans can be easily treated by using zinc salts as dietary supplements. One salt which is used as a dietary supplement is a hydrated zinc sulfate, ZnSO4.xH2O, which is a colourless crystalline solid. Crystals of zinc sulfate may be prepared in a school or college laboratory by reacting dilute sulfuric acid with a suitable compound of zinc. Give the formulae of two simple compounds of zinc that could each react with dilute sulfuric acid to produce zinc sulfate. and A simple experiment to determine the value of x in the formula ZnSO4.xH2O is to heat it carefully to drive off the water. ZnSO4.xH2O→ ZnSO4+ xH2OA student placed a sample of the hydrated zinc sulfate in a weighed boiling tube and reweighed it. He then heated the tube for a short time, cooled it and reweighed it when cool. This process was repeated four times. The fi nal results are shown below. mass of empty tube / g mass of tube + hydrated salt / g mass of tube + salt after fourth heating / g 74.25 77.97 76.34 Why was the boiling tube heated, cooled and reweighed four times? Calculate the amount, in moles, of the anhydrous salt produced. Calculate the amount, in moles, of water driven off by heating. Use your results to and to calculate the value of x in ZnSO4.xH2O. For many people, an intake of approximately 15 mg per day of zinc will be suffi cient to prevent defi ciencies. Zinc ethanoate crystals, (CH3CO2)2Zn.2H2O, may be used in this way. What mass of pure crystalline zinc ethanoate (Mr = 219.4) will need to be taken to obtain a dose of 15 mg of zinc? If this dose is taken in solution as 5 cm3 of aqueous zinc ethanoate, what would be the concentration of the solution used? Give your answer in mol dm–3.
9701_w12_qp_21
THEORY
2012
Paper 2, Variant 1
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Zinc is an essential trace element which is necessary for the healthy growth of animals and plants. Zinc defi ciency in humans can be easily treated by using zinc salts as dietary supplements. One salt which is used as a dietary supplement is a hydrated zinc sulfate, ZnSO4.xH2O, which is a colourless crystalline solid. Crystals of zinc sulfate may be prepared in a school or college laboratory by reacting dilute sulfuric acid with a suitable compound of zinc. Give the formulae of two simple compounds of zinc that could each react with dilute sulfuric acid to produce zinc sulfate. and A simple experiment to determine the value of x in the formula ZnSO4.xH2O is to heat it carefully to drive off the water. ZnSO4.xH2O→ ZnSO4+ xH2OA student placed a sample of the hydrated zinc sulfate in a weighed boiling tube and reweighed it. He then heated the tube for a short time, cooled it and reweighed it when cool. This process was repeated four times. The fi nal results are shown below. mass of empty tube / g mass of tube + hydrated salt / g mass of tube + salt after fourth heating / g 74.25 77.97 76.34 Why was the boiling tube heated, cooled and reweighed four times? Calculate the amount, in moles, of the anhydrous salt produced. Calculate the amount, in moles, of water driven off by heating. Use your results to and to calculate the value of x in ZnSO4.xH2O. For many people, an intake of approximately 15 mg per day of zinc will be suffi cient to prevent defi ciencies. Zinc ethanoate crystals, (CH3CO2)2Zn.2H2O, may be used in this way. What mass of pure crystalline zinc ethanoate (Mr = 219.4) will need to be taken to obtain a dose of 15 mg of zinc? If this dose is taken in solution as 5 cm3 of aqueous zinc ethanoate, what would be the concentration of the solution used? Give your answer in mol dm–3.
9701_w12_qp_22
THEORY
2012
Paper 2, Variant 2
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Cobalt is a transition element and forms compounds with oxidation numbers +2 and +3. Complete the electronic structures for a cobalt atom, 1s22s22p6 cobalt in the +3 oxidation state. 1s22s22p6 In an aqueous solution of cobalt(sulfate the cobalt forms complex ions. What is meant by the term complex ion? State two chemical properties of cobalt, other than the formation of complexes, that are not shown by a typical s-block element. Cobalt(ions, Co2+, show some chemical properties similar to those of copper(ions, Cu2+. Use this information and the Data Booklet to suggest the formula of the cobalt species formed in each of the following reactions. State the type of reaction taking place in each case. formula of cobalt species formed type of reaction Co2++ an excess of NH3Co2++ OH–Co2++ S2O8 2– Some transition elements are present in superconductors. These are materials that conduct electricity with little or no resistance. Compound Q is a superconductor and contains 13.4% yttrium, 41.2% barium, 28.6% copper and 16.8% oxygen by mass. Show that the empirical formula of Q is YBa2Cu3O7. Show all your working. The table shows the oxidation numbers of yttrium, barium and oxygen in Q. element oxidation number yttrium +3 barium +2 oxygen –2 Calculate the average oxidation number of copper in Q. Hence deduce the oxidation number of each of the three copper atoms in Q.
9701_w15_qp_43
THEORY
2015
Paper 4, Variant 3
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Copper is a transition element and has atomic number 29. Complete the electronic configuration for the copper atom and the copper ion in the +2 oxidation state. • copper atom • copper ion in the +2 oxidation state The following equilibrium exists between two complex ions of copper in the +2 oxidation state. [Cu(H2O)6]2+ + 4Cl – [CuCl 4]2– + 6H2O Name the type of reaction occurring here. State the colours of these two complex ions. [Cu(H2O)6]2+ [CuCl 4]2– State the shape of the [CuCl 4]2– ion. Write the expression for the stability constant, Kstab, for this equilibrium. Kstab = Copper also forms the complex ions [Cu(NH3)2(H2O)4]2+ and [Cu(H2O)4]2+ where en is the bidentate ligand ethane-1,2-diamine, H2NCH2CH2NH2. [Cu(H2O)6]2+ + 2NH3 [Cu(NH3)2(H2O)4]2+ + 2H2O equilibrium 1 [Cu(H2O)6]2+ + en [Cu(H2O)4]2+ + 2H2O equilibrium 2 What is meant by the term bidentate ligand? The table lists the values of stability constants for these two complexes. stability constant, Kstab [Cu(NH3)2(H2O)4]2+ 7.94 × 107 [Cu(H2O)4]2+ 3.98 × 1010 What do these Kstab values tell us about the relative positions of equilibria 1 and 2? Nickel forms the complex ion 2+ in which it is surrounded octahedrally by six nitrogen atoms. Name the type of stereoisomerism displayed by 2+. Draw three-dimensional diagrams to show the two stereoisomers of 2+. Ethane-1,2-diamine is a useful reagent in organic chemistry. Explain how the amino groups in ethane-1,2-diamine allow the molecule to act as a Brønsted-Lowry base. Write an equation for the reaction of ethane-1,2-diamine with an excess of hydrochloricacid. Under certain conditions, ethane-1,2-diamine reacts with ethanedioic acid, HO2CCO2H, to form the polymer Z. Draw the structure of this polymer, Z, showing two repeat units. Name the type of reaction occurring during this polymerisation. Polymer Z is an example of a biodegradable polymer. Name a polymer that is non-biodegradable.
9701_w16_qp_41
THEORY
2016
Paper 4, Variant 1
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Cobalt is a transition element that forms complex ions with oxidation states +2 and +3. Explain what is meant by the term transition element. The following scheme shows some reactions of [Co(H2O)6]2+. precipitate A solution of C solution of B [Co(H2O)6]2+ OH–excess Cl –excess NH3State the formula of each of the following. A B C State the colour of the following solutions. [Co(H2O)6]2+ solution of B solution of C Define the term standard electrode potential. An electrochemical cell was set up to measure the standard electrode potential, , of a cell made of a Co2+ / Co half-cell and a Fe3+ / Fe2+ half-cell. Complete the table with the substance used to make the electrode in each of these half-cells. half-cell electrode Co2+ / Co Fe3+ / Fe2+ Write the equation for the overall cell reaction. Use the Data Booklet to calculate the . = V The electrochemical cell in was set up again but this time the concentration of Co2+was 0.050 mol dm–3. The Nernst equation can be used to calculate the value of an electrode potential at different concentrations. E = E o + (0.059 / z) log [Co2+] Nernst equation Use the Data Booklet and the Nernst equation to calculate the value of E for the Co2+ / Co half-cell in this experiment. E for Co2+ / Co = V Suggest how this change will affect the overall cell potential, Ecell, compared to in . Circle your answer. less positive no change more positive Iron(ions can oxidise vanadium metal. Construct an equation for the reaction of an excess of iron(ions with vanadium metal. Use of the Data Booklet will be helpful.
9701_w16_qp_41
THEORY
2016
Paper 4, Variant 1
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Copper is a transition element and has atomic number 29. Complete the electronic configuration for the copper atom and the copper ion in the +2 oxidation state. • copper atom • copper ion in the +2 oxidation state The following equilibrium exists between two complex ions of copper in the +2 oxidation state. [Cu(H2O)6]2+ + 4Cl – [CuCl 4]2– + 6H2O Name the type of reaction occurring here. State the colours of these two complex ions. [Cu(H2O)6]2+ [CuCl 4]2– State the shape of the [CuCl 4]2– ion. Write the expression for the stability constant, Kstab, for this equilibrium. Kstab = Copper also forms the complex ions [Cu(NH3)2(H2O)4]2+ and [Cu(H2O)4]2+ where en is the bidentate ligand ethane-1,2-diamine, H2NCH2CH2NH2. [Cu(H2O)6]2+ + 2NH3 [Cu(NH3)2(H2O)4]2+ + 2H2O equilibrium 1 [Cu(H2O)6]2+ + en [Cu(H2O)4]2+ + 2H2O equilibrium 2 What is meant by the term bidentate ligand? The table lists the values of stability constants for these two complexes. stability constant, Kstab [Cu(NH3)2(H2O)4]2+ 7.94 × 107 [Cu(H2O)4]2+ 3.98 × 1010 What do these Kstab values tell us about the relative positions of equilibria 1 and 2? Nickel forms the complex ion 2+ in which it is surrounded octahedrally by six nitrogen atoms. Name the type of stereoisomerism displayed by 2+. Draw three-dimensional diagrams to show the two stereoisomers of 2+. Ethane-1,2-diamine is a useful reagent in organic chemistry. Explain how the amino groups in ethane-1,2-diamine allow the molecule to act as a Brønsted-Lowry base. Write an equation for the reaction of ethane-1,2-diamine with an excess of hydrochloricacid. Under certain conditions, ethane-1,2-diamine reacts with ethanedioic acid, HO2CCO2H, to form the polymer Z. Draw the structure of this polymer, Z, showing two repeat units. Name the type of reaction occurring during this polymerisation. Polymer Z is an example of a biodegradable polymer. Name a polymer that is non-biodegradable.
9701_w16_qp_43
THEORY
2016
Paper 4, Variant 3
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Cobalt is a transition element that forms complex ions with oxidation states +2 and +3. Explain what is meant by the term transition element. The following scheme shows some reactions of [Co(H2O)6]2+. precipitate A solution of C solution of B [Co(H2O)6]2+ OH–excess Cl –excess NH3State the formula of each of the following. A B C State the colour of the following solutions. [Co(H2O)6]2+ solution of B solution of C Define the term standard electrode potential. An electrochemical cell was set up to measure the standard electrode potential, , of a cell made of a Co2+ / Co half-cell and a Fe3+ / Fe2+ half-cell. Complete the table with the substance used to make the electrode in each of these half-cells. half-cell electrode Co2+ / Co Fe3+ / Fe2+ Write the equation for the overall cell reaction. Use the Data Booklet to calculate the . = V The electrochemical cell in was set up again but this time the concentration of Co2+was 0.050 mol dm–3. The Nernst equation can be used to calculate the value of an electrode potential at different concentrations. E = E o + (0.059 / z) log [Co2+] Nernst equation Use the Data Booklet and the Nernst equation to calculate the value of E for the Co2+ / Co half-cell in this experiment. E for Co2+ / Co = V Suggest how this change will affect the overall cell potential, Ecell, compared to in . Circle your answer. less positive no change more positive Iron(ions can oxidise vanadium metal. Construct an equation for the reaction of an excess of iron(ions with vanadium metal. Use of the Data Booklet will be helpful.
9701_w16_qp_43
THEORY
2016
Paper 4, Variant 3
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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_41
THEORY
2017
Paper 4, Variant 1
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Questions Discovered
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