2. Atoms, molecules and stoichiometry
A section of Chemistry, 9701
Listing 10 of 208 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_21
THEORY
2011
Paper 2, Variant 1
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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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Questions Discovered
208