Unit 1 – Amount of Substance (2010)
Q1-5 (Jan 2010), Q6 – 11 (Jun 2010)
Unit 1 - Jan 2010
Q1.
 The human body contains around 0.025 g of iodine molecules, I2. Which of the following shows the number of iodine atoms in 0.025 g of I2?

 The Avogadro constant is 6.02 × 1023 mol-1.

(Total for question = 1 mark)

Q2.
 Chloroethane, C2H5Cl, can be made from either ethane or ethene.
(a) (i) What reagent and condition would be used to make chloroethane from ethane?
(2)
Reagent
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Condition
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(ii) State the type of reaction and mechanism by which this reaction occurs.
(2)
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(b) (i) What reagent would be used to make chloroethane from ethene?
(1)
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(ii) Show, in full, the mechanism for this reaction in which ethene is converted to chloroethane.
(3)
(c) Which method of making chloroethane has
(3)
•   a higher atom economy?
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•   a higher percentage yield?
 ............................................................................................................................................. 
 Explain your answers.
Higher atom economy
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Higher percentage yield
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(d) The compound chloroethene, CH2=CHCl, forms an addition polymer.
(i) Draw a diagram, using dots or crosses, to show the arrangement of electrons in chloroethene. Only the outer shell electrons need be shown.
(2)

(ii) Chloroethene can form an addition polymer. Write the displayed formula of poly(chloroethene) showing two repeat units.
(1)

*(iii) Poly(chloroethene) is commonly known as PVC. Almost a quarter of the PVC which is manufactured is used to make water pipes, which were formerly made of metal.

                     Give TWO factors which have to be considered when deciding which material, PVC or metal, contributes to more sustainable uses of resources in the long term.
(2)
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(Total for question = 16 marks)
Q3.
 Which of the following mixtures could not form when octane, C8H18, is cracked?
   A      propane + pentene
   B      butane + butene
   C      pentane + propene
   D      heptane + ethene
(Total for question = 1 mark)

Q4.
 Magnesium chloride can be made by reacting solid magnesium carbonate, MgCO3, with dilute hydrochloric acid.
(a) Write an equation for the reaction, including state symbols.
(2)

(b) Give TWO observations you would make when the reaction is taking place.
(2)
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(c) In an experiment to make crystals of hydrated magnesium chloride, MgCl2.6H2O, magnesium carbonate was added to 25 cm3 of hydrochloric acid with concentration 2.0 mol dm-3. The molar mass of magnesium carbonate is 84.3 g mol-1.
(i) How many moles of acid are used in the reaction?
(1)

(ii) What mass of magnesium carbonate, in grams, reacts with this amount of acid?
(1)

(iii) Suggest why slightly more than this mass of magnesium carbonate is used in practice.
(1)
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(iv) How would you separate the magnesium chloride solution from the reaction mixture in (iii)?
(1)
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(v) The magnesium chloride solution was left to crystallise. The crystals were separated and dried carefully. A sample of 3.75g of hydrated crystals, MgCl2.6H2O, which have molar mass 203.3 g mol-1, was obtained. Calculate the percentage yield of this reaction.
(2)

(vi) Give ONE reason why the yield of crystals is less than 100%, even when pure compounds are used in the preparation.
(1)
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(d) Lattice energies can be measured using the Born-Haber cycle, or calculated from electrostatic theory. Lattice energies of magnesium chloride and magnesium iodide are shown below.

(i) What does this data indicate about the bonding in magnesium chloride?
(1)
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*(ii) Explain why there is a greater difference between the experimental (Born-Haber) and theoretical lattice energies for magnesium iodide, MgI2, compared with magnesium chloride.
(2)
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 (e) Blood plasma typically contains 20 parts per million (ppm) of magnesium, by mass.
(i) Calculate the mass of magnesium, in grams, present in 100 g of plasma.
(1)

(ii) Magnesium chloride can be used as a supplement in the diet to treat patients with low amounts of magnesium in the blood. Suggest ONE property which makes it more suitable for this purpose than magnesium carbonate.
(1)
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 (Total for question = 16 marks)

Q5.
 Sulfamic acid is a white solid used by plumbers as a limescale remover.
(a) Sulfamic acid contains 14.42% by mass of nitrogen, 3.09% hydrogen and 33.06% sulfur. The remainder is oxygen.
(i) Calculate the empirical formula of sulfamic acid.
(3)

(ii) The molar mass of sulfamic acid is 97.1 g mol−1. Use this information to deduce the molecular formula of sulfamic acid.
(1)

(b) A solution of sulfamic acid contains hydrogen ions. The hydrogen ions react with magnesium to produce hydrogen gas. In an experiment, a solution containing 5.5 x 10−3 moles of sulfamic acid was reacted with excess magnesium. The volume of hydrogen produced was 66 cm3, measured at room temperature and pressure.
(i) Draw a labelled diagram of the apparatus you would use to carry out this experiment, showing how you would collect the hydrogen produced and measure its volume.
(2)

(ii) Calculate the number of moles of hydrogen, H2, produced in this reaction. [The molar volume of a gas is 24 dm3 mol−1 at room temperature and pressure]
(1)

(iii) Show that the data confirms that each mole of sulfamic acid produces one mole of hydrogen ions in solution.
(2)

(c) Plumbers use sulfamic acid powder for descaling large items such as boilers. Sulfamic acid acts as a descaler because the hydrogen ions react with carbonate ions in limescale.
(i) Write an ionic equation for the reaction of hydrogen ions with carbonate ions. State symbols are not required.
(1)

(ii) Suggest ONE reason why sulfamic acid is considered less hazardous than hydrochloric acid as a descaler.
(1)
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Total for question = 11 marks)

Unit 1 - Jun 2010
Q6.
Which of these solutions does not contain the same total number of ions as the others?
   A     10.00 cm3 of 0.100 mol dm-3  NaCl(aq)
   B     20.00 cm3 of 0.050 mol dm-3  NaCl(aq)
   C     20.00 cm3 of 0.050 mol dm-3  MgCl2(aq)
   D     13.33 cm3 of 0.050 mol dm-3  MgCl2(aq)
(Total for question = 1 mark)

Q7.
This question is about methanol and the energy changes that accompany some of its reactions.
(a) Complete the diagram (using dots and crosses) to show the bonding in methanol, CH3OH. You should show outer electrons only.
(2)

(b) The Hess cycle below can be used to calculate the standard enthalpy change of combustion of methanol, using standard enthalpy changes of formation.

(i) Complete the cycle by filling in the empty box.
(2)
*(ii) Define the term standard enthalpy change of formation of a compound,
       making clear the meaning of standard in this context.
(3)
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 (iii) Use your cycle and the data below to calculate the standard enthalpy change of
       combustion of methanol, ΔHc.

(2)

(c) An experiment was carried out, using the apparatus below, to estimate the standard
       enthalpy change of combustion of methanol.

After burning the methanol for a few minutes, the temperature of water in the beaker had risen by 20.7 °C and the mass of methanol burnt was 0.848 g.

 (i) Calculate the amount of energy transferred to the water.

 Energy transferred (J) = mass of water × 4.18 × temperature change
(1)

(ii) Calculate the number of moles of methanol, CH3OH, burnt during the
       experiment.
(1)

(iii) Use your answers to (c)(i) and (ii) to calculate the experimental value for the standard enthalpy change of combustion. Include a sign and units in your answer, which should be given to three significant figures.
(1)

(iv) Compare your answers to (b)(iii) and (c)(iii) and give TWO reasons to explain
       any differences.
(2)
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 (Total for question = 14 marks)

Q8.
 This question is about the properties of ions and ionic compounds.
(a) Solid calcium carbonate, CaCO3, has a giant ionic structure.
(i) Draw a diagram (using dots or crosses) for a calcium ion. Show ALL the electrons and the charge on the ion.
(2)

(ii) Complete the electronic configuration for a calcium ion.
(1)
1s2
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(iii) Would you expect a calcium ion to be bigger, smaller or the same size as a calcium atom? Give TWO reasons to explain your answer.
(2)
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 (iv) Explain why ionic compounds have relatively high melting temperatures.
(2)
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(b) Changes in the concentration of ions in a solution can be estimated by measuring the electrical conductivity of the solution.
(i) Explain why solutions of ions are able to conduct electricity.
(1)
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(ii) Suggest why aqueous solutions of calcium chloride, CaCl2(aq), and barium chloride, BaCl2(aq), of the same molar concentration, have different electrical conductivities.
(1)
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(iii) 1 kg of a solution contains 0.100 mol of calcium ions, Ca2+.

      What is the concentration of the calcium ions by mass in parts per million (ppm)?

      [Assume the relative atomic mass of calcium is 40.]
(2)

.....................................ppm

*(c) Some buildings are made from limestone, which is mainly calcium carbonate. Gases in the atmosphere such as sulfur dioxide, SO2, and nitrogen dioxide, NO2, can be esponsible for damaging these buildings.

        Describe how these gases come to be present in the atmosphere and explain how they can damage a limestone building.
(3)
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 (d) The lattice energy of calcium chloride, CaCl2, is -2258 kJ mol-1 based on an experimental Born-Haber cycle and -2223 kJ mol-1 based on theoretical calculations.

      Would you expect its bonding to match the ionic model? Justify your answer.
(1)
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(Total for question = 15 marks)

Q9.
The equations below show some reactions of magnesium and its compounds.
A   2Mg(s) + O2(g)        → 2MgO(s)
B   Mg(NO3)2(s)           → MgO(s) + 2NO2(g) + O2(g)
C   MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(1)
D   Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)

(a) Which equation is not balanced?
(1)
   A
   B
   C
   D

(b) Which equation can be classified as a displacement reaction?
(1)
   A
   B
   C
   D
(Total for question = 2 marks)

Q10.
A sample of gas was prepared for use in helium-neon lasers. It contained 4 g of helium and 4 g of neon. What is the ratio of helium atoms to neon atoms in the sample?
   A       1 : 1
   B    2.5 : 1
   C       1 : 5
   D       5 : 1
(Total for question = 1 mark)

Q11.
The overall equation for the reaction between sulfur and oxygen to form sulfur trioxide is shown below.

0.9 mol of O2(g) reacted completely with excess sulfur. What volume, in dm3, of sulfur trioxide would form?
 [Assume the molar gas volume = 24 dm3 mol-1]
   A   (0.9 × 3/2) × 24
   B   (0.9 × 3/2) ÷ 24
   C   (0.9 × 2/3) × 24
   D   (0.9 × 2/3) ÷ 24
(Total for question = 1 mark)